Briggs Engine Failure Guide

Major Engine Failure Analysis

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MAJOR ENGINE FAILURE ANALYSIS

CUSTOMER EDUCATION’SMISSION STATEMENT

BRIGGS & STRATTON IS COMMITTED TO PROVIDE ITS SERVICEORGANIZATION WITH SUPERIOR TECHNICAL TRAININGPROGRAMS THROUGH WHICH PROFESSIONAL COMPETENCECAN BE ACQUIRED AND MAINTAINED ON ALL BRIGGS &STRATTON PRODUCTS, ASSURING ONLY THE HIGHESTSTANDARDS OF SERVICE SUPPORT FOR OUR CUSTOMERS.

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Table of Contents Major Engine Failure Analysis

© 1996 BRIGGS & STRATTON CORPORATION Form CE8034 10/96 2

Introduction Page 13

Chapter 1 Abrasive Ingestion Page 16

Chapter 2 Insufficient Lubrication Page 16

Chapter 3 Overheating Page 25

Chapter 4 Overspeeding Page 29

Chapter 5 Breakage 2

Chapter 6 Combination Failures Page 35

Chapter 7 Cause / Effect Flow Charts Page 37

Glossary of Terms Page 43

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Introduction Major Engine Failure Analysis


FAILURE ANALYSIS

What do we mean by the term Failure Analysis? Let’s look at some of the descriptions given to us by the dictionary:

FAILURE: “A state of inability to perform a normal function, neglect or non-performance”.

ANALYSIS: “Separation of a whole into its component parts, an examination of a complex item,its elements, and their relations”.

Introduction

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The term “Failure Analysis” is often used bytechnicians when discussing the results of inspectinga failed engine that has been brought to them. Manyof these technicians have spent a great deal of timetrying to analyze the failed engine without a firmunderstanding of the dynamics of why certaincomponents can fail. Even more often, sometechnicians will just plain fail to analyze.

Without having a strong understanding of thecause and effect relationships of many of thecomponents, some of the clues the engine will havewill be completely missed or mistaken for somethingcompletely different.

As an example, when abrasives are allowed toenter the intake system at the filter element, evidencewill be found on all contact surfaces from the filterelement to the crankshaft. However, if the problemwas a bad gasket at the intake manifold, the evidencewill start at the gasket and travel towards thecrankshaft. No evidence will be found in thecarburetor. An injustice could have been dealt theoperator by telling him the engine failed because ofdirt ingestion due to lack of maintenance--when in factthe problem was a defect in the gasket.

True, the failure was abrasive ingestion, but theproblem was not the operator’s fault.The abrasive andresulting wear were nothing more than an effect; thecause was the bad gasket.

No two engines that have failed under the samecircumstances will normally exhibit the exact samedegree of damage. There are too many variables inthe manufacturing process that make every enginejust a little different than the next. Knowing thepatterns of the component failure and howcombinations of these events occur will be the besttechnique for understanding how to investigate majorfailures.

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Perhaps the most challenging task the servicetechnician will undertake is that of Major EngineFailure Analysis. An accurate, cost-effectivediagnosis is not possible by attempting to memorizevisual evidence and applying it to future situations. Tohelp in this process, Briggs & Stratton has developedthis comprehensive Failure Analysis Workbook.Whenused with the companion video tape #CE3019, mosttechnicians will develop a comprehensive under-standing of the dynamics of failure as it pertains toindividual components and their relationship to theengine as a complete unit.

Engines can fail for a variety of reasons. FIvecategories cover 99% of all failures. The mostpredominant category is abrasive ingestion followedclosely by insufficient lubrication. The final three areoverheating, overspeeding and breakage. In thisworkbook, we will cover the five most common areasof major failure and how they can be compoundedtogether. For good measure some unrelated exam-ples of component failure will be added.

FACTOID: For every gallon of gasoline consumed, ablock of air approximately 100 ft. x 100 ft.and 10 ft. high will be consumed.

If an engine was to run for 1,000 hours at3600 rpm the engine would complete thefollowing:

• The piston will complete 432,000,000strokes.• The crankshaft will rotate 216,000,000revolutions.• Each valve will contact its seat108,000,000 times.• At 40 hours per week, it will take 25weeks to complete 1,000 hours.

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Chapter 1 Abrasive Ingestion Major Engine Failure Analysis


The ingestion of abrasives is all too common in the small engine industry. An abrasive is aparticle that is commonly described as a piece of undesirable foreign material that exhibits anexceptional hardness. The most abrasive particle in the small engine industry is silica.

Silica is a compound of the elements silicon (Si) and oxygen (O2) and is commonly foundin sand, and to varying degrees, in dirt. Silica (the main component in quartz) exhibits a hardnessof 7 on the Moh’s scale of mineral hardness. Only the minerals topaz, sapphire and diamond arerated harder. The degree of hardness of the abrasive particles is chief in understanding thedynamics of an abrasive ingestion failure of a Briggs & Stratton engine.

When discussing abrasive particles, it is important to have a good understanding of thesize and type of particle we are dealing with. The silica particles we are concerned about are assmall as 1 micron, and are of a crystalline structure, with very sharp edges. Most of the particlesthat lead to excessive wear are on the average of 25 microns and larger.To give you a perspective,25 microns are roughly equivalent to .001″ (.024mm). This is about 1/20th the inner diameter of apilot jet orifice.

Chapter 1Abrasive Ingestion

FIG. 1-1

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When using the term “dirt ingestion”, an imagecomes to mind of particles the size of beach sand.Nothing could be farther from the truth. Except for rareoccasions, the abrasives encountered are very small.Most air cleaner assemblies use some type ofcyclonic system that removes the larger particles.What is left are very small powdery abrasive particlesthat can pass through nearly any openingencountered. The sample silica in the picture variesfrom about 3 microns to 80 microns, averaging about20 to 50 microns. SEE FIG. 1-1

AIR CLEANERS

The function of the air cleaner is to filter asmuch abrasive material out of the incoming air aspossible. As a filter element begins to becomeobstructed, less and less air can penetrate. When theelement works as designed, air will stop flowing atsome point in the process. Before air stops flowing,however, the engine will no longer be running properly.No matter how bad the outside components look, thecarburetor side will be clean if the air cleaner isfunctioning properly. SEE FIG. 1-2

When an air filter is not serviced properly,abrasives are allowed to enter the air intake stream. Atear in either a foam or paper element will allow theair to follow the path of least resistance. Evidence willbe found when looking on the carburetor side of theelement and inside of the air cleaner. Any dirt in theseareas is a sure sign of a damaged air cleaner elementor sealing problem. SEE FIG. 1-3

Considering the environment most air cooledequipment functions within, it is not hard to imaginethe amount of abrasives the engine could potentiallyingest without proper filtration.

FIG. 1-1 Laboratory sample of silica abrasiveused for testing air cleaner designs andengine performance.

FIG. 1-2 No matter how dirty the air cleanergets, there should not be any sign ofabrasives on the carburetor side.

FIG. 1-3 When the air filter is damaged orinstalled improperly, abrasives will pass intothe carburetor.

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The outside of a carburetor will in all probabilitybe very dirty and encrusted with debris.This conditionis considered normal and does not pose any greatproblem. SEE FIG. 1-4 The problem occurs when theair cleaner element becomes clogged. This conditioncreates a restriction for the incoming air. Remember,air always attempts to follow the path of leastresistance. The next place the air will enter is thethrottle and/or choke shafts. The air will locate anyweakness relating to gaskets or the air cleanerelement.

CARBURETORS

If the abrasives have gotten past the air filter,they will continue to travel through the carburetor.Theair stream will be traveling as much as thirty five milesper hour. At this speed, abrasives will begin to impacton any surface they come in contact with. SEEFIG.1-5 Evidence will be seen in and around thechoke shaft, choke plate, any air bleeds and theventuri.This area on the carburetor should never showany signs of foreign material.

Before we leave the carburetor, lets takeanother look at this process. Remember the air wantsto follow the path of least resistance. If the air filterelement is clogged, air will start affecting the throttleand choke shafts. SEE FIG.1-6 Since these items aremoving and wet with gasoline droplets, any abrasivesmoving through the carburetor will migrate to theseshafts and stick.

When an abrasive is present in the carburetor,it will begin to affect the throttle shaft. The more theshaft rotates within its bearing surface, the greater thewear that will take place. This is why the throttle shaftgenerally wears more than the choke shaft. SEEFIG.1-7 As the wear increases, more air passesthrough bringing more abrasives with it. With theincrease in air flow through the throttle shaft theair/fuel mixture becomes leaner and causesperformance problems.

FIG. 1-4 The outside of the carburetor willusually be covered with dirt, but will not affectnormal performance.

FIG. 1-5 Abrasive particles will becomeembedded on any part they come in contactwith.

FIG. 1-6 Abrasives will continue totravel through the carburetor to thethrottle shaft.

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MANIFOLDS

As the high velocity air and abrasive mixturecontinues to flow into the engine, more evidence willbe noticeable. As the air bends to follow the path of theintake manifold, the mass of the abrasive will forcesome of the particles to impact the inside of themanifold. This will cause an etching effect to occur.

VALVES & VALVE GUIDES

Next, the particles encounter the intake valveand seat. As the particles travel toward the cylinder,they will be grinding away at the surface of the valveseat. Any particles on the seat as the valve closes willbe further ground and crushed. Since this actiontakes place mostly in the path of the flow, the evidencewill be in line between the valve guide and thecylinder. The appearance will be a valve seat with awider portion towards the cylinder and a narrowerportion in the opposite direction. The valve face willshow a noticeable impression or “dishing”appearance. SEE FIG. 1-8 This wear will only occurwhen abrasives are present. The “dishing” willgenerally be uniform around the face of the valve asthe valve rotates randomly during engine operation.Loss of valve tappet clearance can also occur as facewear increases.

When the abrasives are affecting the valve faceand seat, they are also affecting the valve guide. SEEFIG. 1-9 Any object in the line of travel of theabrasives will cause some particles to come out ofsuspension in the air flow and stick. As the valve stemmoves up and down, the abrasives will migrate intothe valve guide and begin wearing the guide and valvestem. The appearance of the valve stem will bepolished and most likely have vertical scratches. Theguide, whether it is machined into the cylinder,sintered iron or brass material, will show the effects ofthe abrasive wear. It will be difficult to identify wearunless we use the valve guide plug guage todetermine if service is required.

FIG. 1-7 When abrasives are present at amoving part such as the throttle shaft, wearwill take place.

FIG. 1-8 Dishing occurs on the face of theintake valve when abrasives are present.

FIG. 1-9 Abrasives migrate down the valvestem and work into the valve guide.

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CYLINDERS & BORES

Following the path of the air flow, the abrasiveswill travel from the intake port, across the cylinder boreand impact on the cylinder wall opposite the intakevalve. Because of the “eddy currents” of air, some ofthe particles will flow back onto the cylinder wall belowthe intake valve. Since there is a film of oil on thecylinder wall, abrasives will stick. Some of theseparticles will become embedded in the cylinder wall,while others will begin wearing the cylinder wall asthe rings and piston move up and down in the cylinderbore. SEE FIG. 1-10

When the abrasive particles that are rubbingbetween the piston, rings and cylinder wall are largerthan the oil film separating the two surfaces, wear willtake place. As wear takes place, loss of thecrosshatching on the cylinder bore will be the firstevidence present. The exception to this will be acylinder with a DIAMOND BORE™, which has nocrosshatch. SEE FIG. 1-11 Under normal runningconditions, little or no loss of crosshatch will takeplace.When a deep ridge has formed at the top of ringtravel in any bore type, it is a good bet a large quantityof abrasive material has passed through this area.

To better understand what happens whenanalyzing this kind of wear, one must understand therelationship of the materials we are dealing with. On aKOOL BORE™ engine there are three basicmaterials. The cylinder is an aluminum alloy, the ringsare cast iron or steel, and the abrasive ispredominately silica. You can readily see the softestmaterial is the cylinder wall with the silica being thehardest. Pressure is exerted on the silica particles asthey are squeezed between the rings and the cylinderwall. Since the rings are harder than the cylinder wall,the silica particles tend to be forced into the aluminumwhere it is held much like grit on sand paper. SEE FIG.1-12

FIG. 1-10 Loss of crosshatch will be one ofthe first signs of abrasive ingestion.

FIG. 1-11 When properly maintained, thecrosshatching will remain on the cylinder wall.

FIG. 1-12 Abrasive particles will embed inthe cylinder wall of a KOOL BORE™ engine.When the rings move, wear will result.


ABRASIVES

CYLINDERWALL

PISTON

PISTONMOVEMENT

RING

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PISTON & RINGS

Because abrasives tend to get embedded in thecylinder wall, small scratches will start to form from thelarger particles while the smaller ones tend to polishthe surface of the piston. This is evidenced by theappearance of the two pistons shown. SEE FIG. 1-13Some very light scratching is normal and occurs fromthe break-in process. If your fingernail does not catchwhen rubbing them, there is no problem. As we lookcloser at the piston, you will notice the light scratchingevident on the piston skirt.

Let’s look at a piston with a greater amount ofwear. The most striking appearance is the coloring ofthe piston skirt. What has happened is the abrasivequalities of the material embedded in the cylinder wallhave worn some of the chrome plating off the piston.SEE FIG. 1-14 If this condition persists, the piston willgradually begin to weld to the cylinder bore. If you lookclosely, you can see the beginning of this process.

FIG. 1-13 Light scratching can be normal.Most will “heal” over time.

FIG. 1-14 Wear will take place given timewhen abrasives are present.

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The oil ring exerts greater unit pressure thanthe other rings. Because of this, the oil ring will wearfaster than the compression rings. As we look at theseoil control rings, we can see different levels of wearwith the ring on the right being new. SEE FIG. 1-15 Asthe face of the oil control ring begins to wear, the facebecomes wider. The wider the face becomes, themore it tends to ride up on the oil film covering thecylinder wall. Once this occurs, more oil is left for thecompression rings to overcome. Since these rings arenot designed to control oil, oil consumption begins toincrease.

When we look at compression rings for wear, itis not as noticeable when looking at the face of therings. Even though the same relative amount of weartakes place. However, if you look at the ring from thetop, you will notice that the ring will generally vary inwidth. SEE FIG. 1-16

So where does the abrasive and material thathas worn off of the rings and cylinder wall go? Thelower end! As the material enters the crankcase, itmixes with the oil. Once the abrasive has entered theoil, it then will travel to all of the bearing surfaces in thelower end. Most noticeable will be the connecting rod.SEE FIG. 1-17 The bearing surface will have a dullgray polished appearance.

FIG. 1-15 Oil control rings will show wearvery quickly when subjected the abrasives.

FIG. 1-16 Compression rings will wear, withthe evidence being scratches in line withpiston travel.

FIG. 1-17 Connecting rod wear will appearas a dull gray look on the bearing surface.

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FIG. 1-19 Main bearing wear will look verymuch like the connecting rod. The wear canbe very smooth.

In this view, you can notice the wear on the PTObearing journal. SEE FIG. 1-18 The damage is notfrom metal transfer like a lubrication failure and showsno signs of heat.

Everything discussed about the cylinder wall,piston and rings will be true for the main bearings.Thebearing surface will have the same scratchedappearance as the connecting rod.

SEE FIG. 1-19 Because the wear can be sofine and be mistaken for machining it is necessary tocheck the size of the main bearing using a mainbearing plug gauge. If the bearing is not cleanedproperly, damage will continue to occur because of theabrasives embedded in the bearing material. Ballbearings are also commonly overlooked when itcomes to ingestion problems. The microstructure ofthe races and balls will be damaged just as badly.

FIG. 1-18 Under normal conditions, nomarks or scratches will appear on the mainbearing journals.

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FIG. 1-20 Any buildup in the crankcase willbe evidence of improper maintenance.

FIG. 1-21 Not cleaning the oil fill will result inabrasives entering the engine.

FIG. 1-22 If abrasives enter the engine fromthe oil fill, they will generally be much larger.The damage will be more severe.


As contaminants build up, they start to settle outof the oil when the engine is not running. If the oil is notchanged on a routine basis, this buildup will continueuntil a sludge begins to form. SEE FIG. 1-20Operators tend to form patterns in the level ofmaintenance they perform. A operator that does notchange the oil on a regular basis probably does notservice the air filter very well either. When sludgeappears in the crankcase, there is a good chanceother routine maintenance procedures have also notbeen followed.

As an example, the engine you are looking atexhibits massive abrasive ingestion. All the signsare there, but when you look into the crankcase, itis relatively clean. This would most likely haveoccurred because during the last maintenance, theair cleaner assembly was not installed correctly.Thiscould have been an oversight, but the damage canstill be very severe. Abrasive damage can occurvery quickly when you consider the piston willcomplete as many as 432,000 strokes per hour.

Another common source of abrasive ingestionis the oil fill. If the area is not cleaned before openingthe cap, debris can fall into the crankcase. Externalevidence will be debris in the threads of the cap and inthe threads of the fill. SEE FIG. 1-21

Lower end ingestion will be very noticeablewhen looking at the piston skirt. Looking at the pistonshown, notice the deep scratches in the piston skirt.SEE FIG. 1-22 The scratches follow the path ofmovement and stop at the lower oil control ring. If thedebris had come from the upper end, there would bescratches in the ring land area also.

To illustrate how well a KOOL BORE™ cylindercan “hold” abrasives, notice the deep cuts on the face ofthese rings. In fact, you can line up the cuts indicatingthat all ring rotation has ceased. SEE FIG. 1-23 Thisclearly shows how hard the abrasives can be.

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Since all engines have a certain level ofcrankcase vacuum when running, any path into thecrankcase can be a potential source of ingestion.

As an example, let’s look at the crankcase oilseal. SEE FIG. 1-24 Some technicians think thefunction of the crankcase oil seal is to keep oil in anengine.This is only true of any seal below the oil leveland when the engine is not running. The purpose ofthe seal is to keep air out of the crankcase. If the seal“wears out”, air is allowed to enter through the seal. Ifair is entering, so are abrasives. Similar to the throttleshaft, abrasives will wear the bearing closest to theseal. This bearing will exhibit the greatest amount ofwear when compared to other bearing surfaces. Asthe abrasives mix in the oil, the failure will look likeother lower end ingestion examples.

Whether abrasives enter from the air cleaner,oil fill, or any point in between, the evidence will followpredictable patterns. The abrasives are harder thanthe materials the engine is made of.The best analogywould be the abrasives are like very sharp cuttingtools, and the parts are moving. If you move metalagainst a cutting tool, metal will be removed. Anyabrasives larger than the oil film that separates themetal surfaces will result in wear. SEE FIG. 1-25

Since the evidence follows these predictablepatterns, a technician, following a systematic approachto failure analysis, will be able to determine the causeof most abrasive failures. Most operators do notrealize the damage that can be caused by not payingclose attention to proper maintenance.

FIG. 1-23 Large particles embedded in thecylinder wall were cutting the rings.

FIG. 1-24 Because of crankcase vacuum, a worn oil seal will allow abrasives to enterthe engine.

FIG. 1-25 Evidence will show on every partthat comes in contact with abrasives.

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Chapter 2 Insufficient Lubrication Major Engine Failure Analysis

Engine lubricating oils are complex hydrocarbons refined from basic crude oil stocks.Finished products are blends of refined crudes, carefully tailored by the addition of additivepackages. Well defined standards developed by the petroleum industry, automotive industry andother business partners assure consistency for consumers.

Webster defines a lubricant as “...a substance capable of reducing friction, heat and wearwhen introduced as a film between solid surfaces”.

In analyzing failures due to insufficient lubrication, the technician is presented with a two-foldproblem. Not only will mechanical parts fail, but the lubricant will as well. Though the process isconsistent, visual evidence may vary dramatically.

Chapter 2Insufficient Lubrication

FIG. 2-1


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Viscosity

The main consideration when choosing alubricating oil for an internal combustion engine is theviscosity.The viscosity of a fluid is its resistance to flowcaused by internal friction. It is this property thatcauses it to resist flowing past a solid surface or otherlayers of fluid. Resistance to movement, in essence,causes oil to be incompressible.

An oil film between two surfaces adheres to thetop and bottom. If one surface is moved, thecorresponding film travels at the same velocity. Theopposite film remains stationary. The whole picturethen, is multiple layers between the two, each movingat a different speed. A higher viscosity oil has moreresistance to movement and vice versa. Twocomponents separated by an oil film are essentiallyprevented from contact as long as there is movementand an adequate supply of lubricant. SEE FIG. 2-2

Asperities

The surface of a component describes a needfor separation. Machined surfaces are characterizedby asperities, or minute peaks and valleys, left behindby the finish machining process.They serve a definitepurpose in that the valleys act as lubricant reservoirs.SEE FIG. 2-3

The peaks are sheared off during the break-inprocess, forming plateaus. The plateau becomes theactual bearing surface. SEE FIGS. 2-4 and 2-5

FIG. 2-2 A lubricant, clinging to bothsurfaces, forms into layers moving at differentspeeds.

FIG. 2-3 Asperities are microscopic peaksand valleys formed by the machining process.

FIG. 2-4 During the break-in process,asperities collide and shear off the tallerpeaks. Plateaus are formed which becomethe main support surface of the bearing.

VISCOSITY RELATES TO INTERNAL FRICTION

MOVING

STATIONARY

.00008”

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Before failures due to insufficient lubrication areexplored further, it must be noted that the lubricantitself experiences a degradation process. This is adirect result of the environment it must function in aswell as the time it is allowed to remain there.

Any plain bearing at rest displays metal to metalcontact between the plateaus of the journal and thebearing. As plateaus move toward each other in thepresence of a lubricant, there is a tendency to pushthe lubricant out of the way, much like a snowplow.Theviscosity of the fluid resists this attempt. The plateausinstead begin to lift and ride up on a film of oil. Whenrotational speed is sufficient, a complete separation ofcomponents is achieved. A lubricant’s viscosity willdirectly relate to the degree of separation attained.SEE FIG. 2-6

FIG. 2-5

FIG. 2-6


PLAIN BEARING AT REST

PLAIN BEARING WITH OIL SUPPLY

PLAIN BEARING WITH OIL SUPPLY AT START-UP

ADEQUATE AMOUNT OF OIL - IMPROPER VISCOSITY

LOAD

LOAD LOAD

LOAD

METAL TOMETAL

CONTACT

MINIMUM FILM

THICKNESS

LINE OFCONTACT

Metal To MetalContact

OIL SUPPLY

OIL SUPPLY

OIL SUPPLY

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FIG. 2-7 Combustion byproducts,contaminants and oxidized lubricant build upover time to form sludge.

FIG. 2-8 If sludge is allowed to form, oil couldbecome blocked from the oil pump.

FIG. 2-9 As the asperities break off, heat isproduced. An adequate supply of oil will carrythis heat away.

A normal deterioration due to oxidation occursin the crankcase as a result of agitation with air. Oil willalso experience thermal cracking as a result of thehigh temperatures in and near the combustionchamber.This is basically a continuation of the refiningprocess that formed it in the first place and results inheavy hydrocarbon residues which add to theformation of sludge. Contamination by unburned fuel,soot, dirt and combustion residues add many solids tothe oil. Water, resulting from the combustion process,is always present, particularly during cold enginewarmup and also adds to sludge formation. SEEFIGS. 2-7 and 2-8

Failure of the lubricating medium then, is adeterioration of the medium and its viscosity byoxidation, heat and contamination over time.

Insufficient lubrication should not be confusedwith lack of oil, particularly when explaining a failure toan operator. Insufficient lubrication is an oil film that isinadequate in preventing premature wear betweencomponents.

As oil deteriorates, it loses its viscosity. It is thenature of a viscous fluid to separate two movingsurfaces. This ability is proportional to its viscosity. Allthe oil in the world may be surrounding two movingcomponents but if the viscosity level is not sufficient,there will be metal to metal contact. Wear andeventual failure due to insufficient lubrication will bethe end result.

A lack of oil reduces the distance betweenmoving components. Surface asperities make contactand weld together. Though they normally break asquickly as they form, new asperities are formedcausing more damage as movement continues. Heatgenerated from friction rises dramatically. The oilbegins to break down from the high heat, loses itsviscosity, and more metal to metal contact occurs.Scoring and/or seizure are usually quick to follow.Though the causes (poor maintenance) and failures(discoloration, scoring, galling and seizure) areusually the same, the process differs. SEE FIG. 2-9

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To comprehend what happens to an enginewhen it fails due to insufficient lubrication, it is firstnecessary to understand what is expected of thelubricant in the first place.The function of lubricants inan internal combustion engine is to:

1. Prevent Weara. Prevent Metal to Metal Contact (Lubricate)b. Prevent Corrosion

2. Cool the Enginea. Transfer Heat from Internal Components to

the Cylinder Block b. Prevent Sludge Formation which Insulates

the Engine and Retards Heat Transfer

3. Seal the Enginea. Reduce Deposit Formations which Prevent

Rings from Free Movement b. Reduce Wear which is Detrimental to Sealing

Rings to the Wallsc. Provide a Viscous Fluid Film Between

Components

4. Clean the Enginea. Reduce Deposit Formation on Pistons and

Valve Stems b. Suspend Dirt and Debris c. Reduce Sludge Formation which Interferes

with Oil Distribution

All engines will wear over time. Premature wearis considered a major engine failure. Premature wearin an engine falls under two categories: abrasive wearand adhesive wear. Though both have differentcauses, the end results may look the same.

FIG. 2-10 Scratches are the result of metal tometal contact.

FIG. 2-11 Closeup of Fig. 2-10.

FIG. 2-12 A score is a deeper, morepronounced scratch.


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Abrasives may enter an engine and not causedamage. If the abrasive particle is smaller than thethickness of the oil film separating components, it willbe suspended in the oil causing little or no damageand can easily be removed during the nextmaintenance cycle. If the film is thinner than theparticle, a corresponding scratch, cut or gouge willoccur with movement. SEE FIGS. 2-10 and 2-11

It may be difficult to tell the difference betweenan abrasive wear failure and an adhesive wear failure.Other evidence must usually be interpreted and usedto make a decision.

Adhesive wear failures result from a lubricatingfilm that is too thin, allowing metal to metal contact.Metal welds to metal. More pronounced inaluminum/steel bearing configurations, a piece ofaluminum may be pulled from the cylinder wall, mainor connecting rod bearing surface, and draggedagainst the cylinder wall or bearing, creating a score.SEE FIGS. 2-12 and 2-13 Displaced material fromthe score is rolled out of the groove, creating a furrowhigher than the average surface height of theplateaus. The procedure repeats itself on the newfurrow, and soon larger pieces of aluminum are rippedaway. The damage now is generally referred to as agall, and can be evidenced by aluminum that appearsto be ripped or torn and/or aluminum wiping, or metaltransfer to the steel component. SEE FIGS. 2-14 and2-15

FIG. 2-13 When a component is scored,metal may be lifted above the surface of thematerial. The oil film may be penetrated andfurther damage caused.

FIG. 2-14 A gall occurs when a group ofasperities weld together at one time. Thepiece of material that is ripped loose isdragged against the mating surface...

FIG. 2-15 ...causing extreme damage andpotential seizure.

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Metal transfer will only occur from insufficientlubrication. SEE FIG. 2-16 High frictional tempera-tures are created that cause the aluminum to becomevery near a molten state. If the surfaces are drivenwith enough force, they wipe the aluminum betweenthem, much like a paint roller moves paint in front of it.

Cast iron and steel have higher melting pointsand will not transfer metal like an aluminum surface.Instead, scuffing will be present, in particular on thering faces. SEE FIG. 2-17 A corresponding scoremark will follow down the cylinder. SEE FIG. 2-18

All insufficient lubrication failures will follow thissame pattern. Unfortunately, the evidence provided forthe technician to analyze may vary in appearance.High localized heat will be present which is the causefor the typical discoloration of and around aluminumbearing surfaces. SEE FIG. 2-19 The discoloration isactually failed lubricant that has carbonized on thesurface of the component. SEE FIG. 2-20 More oftenthan not, this appears on the connecting rod at thebearing that has failed. The constant wiping of thepiston rings prevents this evidence from being overlyapparent on the cylinder walls. If the cylinder is deeplyscored, carbonized oil may be seen in the valleysaway from ring contact. A new engine, however, runwithout any lubricant, has no lubricant to carbonizeand will not exhibit discoloration.

FIG. 2-16 Metal transfer will only occur frominsufficient lubrication.

FIG. 2-17 Cast iron and steel will not melt asreadily as aluminum. Insufficient lubricationmay cause areas of “scuffing”’ on the ringfaces.

FIG. 2-18 A failed cylinder and piston withevidence of scratches, scoring and galling.


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Discoloration

Discoloration due to localized heat, particularlyat the connecting rod bearing in conjunction withscoring and/or galling, is a key factor and indicator offailures due to insufficient lubrication.

Several areas of the engine function underconditions of boundary lubrication. These are areaswhere a full oil film is not always present to separatecomponents. Of note is the top piston ring at top deadcenter. SEE FIG. 2-21 The oil supply for the top ring isthe amount of oil that has been squeegeed up thecylinder walls by the rings themselves. The supplymust be carefully controlled. Too much oil and theexcess will be burned in the combustion chambercausing high emission output and oil consumptionalong with the risk of causing deposit buildup on thering itself. Any deposit formation on the rings maycause a lack of sealing ability against the cylinderwalls which hinders engine performance. Too little oiland the ring loses the film which separates it from thewalls themselves.

Frequent contact does occur between the ringsand cylinder. This is remedied by anti-wear andextreme pressure additives that are added during theblending process. In essence, these are chemicalsthat bond to the surfaces of the materials and form aprotective chemical layer to prevent excessive wear. Itshould be noted that all additives blended in the oilpackage will either be consumed as they perform theirrespective functions or deteriorate over time. There isalso a limit to their functionality. In the case of the topring, once the extreme pressure additives are wornaway, they must be replenished by fresh oil carryingfresh additives. If this is not the case, scoring andgalling will eventually occur.

FIG. 2-19 Discoloration on or around a plainbearing is a signature mark of insufficientlubrication.

FIG. 2-20 Friction due to metal to metalcontact causes high localized heat. Anyresidual lubricant will burn and carbonize onthe components.

FIG. 2-21 Several areas of the enginefunction under conditions of boundarylubrication.

CYLINDERWALL

PISTON

RING

BoundaryLubrication

CE803 BODY 4-05 4/28/05 10:11 AM Page 23


The dynamics of failures due to insufficient lubrication are fairly clear cut. They should,then, be fairly easy to determine. Unfortunately, this is not always the case. Variables such asengine load, operating conditions, maintenance schedule and tolerance stack-ups may change theseverity of the failure. A light engine load will exert less pressure against the rings and reduce theforce against the cylinder walls potentially increasing their survivability against boundary lubricationeven though the oil level may be dangerously low. A PTO bearing with heavy belt loading and a lowoil supply in the crankcase may exhibit more defined evidence of failure than the connecting rodbearing. It cannot be predicted which part will fail first. It is this seeming randomness that tends tothrow the less disciplined technician off track. All moveable internal surfaces of the engine mustbe thoroughly inspected before an intelligent decision may be reached.

FIG. 2-22


In addition, knowledge of the environment the equipment is used in, the load conditions itfunctions under and the maintenance habits of the operator will all help to put the pieces of thepuzzle together.

CE803 BODY 4-05 4/28/05 10:11 AM Page 24

Chapter 3 Overheating Major Engine Failure Analysis


An internal combustion engine converts energy from a chemical reaction into a mechanicalrotating force. By far, the greatest amount of energy produced is in the form of waste heat.Withoutmethods in place to remove this heat, the engine’s expected life span would be measured in hoursas opposed to years.

The forced air cooling system used on practically all Briggs & Stratton engines does anexcellent job of removing waste heat. Air cooled engines must deal with extreme temperatures andpressures. There is a direct correlation between expected life and any increase or decrease ofeither of these. An engine is in a state of overheating when it lacks the ability to maintain its internaland external temperature within designed parameters. The main cooling system process is thetransfer of heat created by combustion to the cylinder block and ultimately to the moving air stream.A secondary process is the transfer of heat created by internal engine friction to the engine oilwhich also transfers to the block. SEE FIG. 3-1 Cooling fins are located around the cylinder boreand head to increase the surface area thereby increasing heat transfer to the moving air. An enginemay overheat when anything serves to retard this process.

Chapter 3Overheating

FIG. 3-1

CE803 BODY 4-05 4/28/05 10:11 AM Page 25



Overheating can be caused by a number offactors, several of which are not even engine related.Blocked cooling fins are typically the biggest factor.Anything that would impede the continuous flow of airacross the cooling fins will retard heat transfer. Chaffand debris are perhaps the most common. However,wax dust from floor buffers and airborne tar debrisfrom roof cutters or even dirt buildup from tillers willhave the same effect by forming an insulation barrier.

COOLING

As the fins become insulated by debris buildup,the temperature of the engine will increasedramatically. Nearly all metals expand when heatedand return to their original size and shape whencooled. Different materials will expand and contract atdifferent rates. Consider steel head bolts torquedagainst an aluminum cylinder head. As the aluminumexpands, it increases the clamping force of the bolt.Add the pressure created by the combustion processand each head bolt can be subjected to stressesequal to the weight of a full size pickup truck. Iftemperatures are great enough, the bolts may stretch.The same effect may occur to the threads in thealuminum block. In both cases, the aluminum materialof the block and the steel of the bolt have exceededtheir thermal yield point. This is the point at which amaterial will expand and be unable to return to itsoriginal shape and size. SEE FIG. 3-3

Blown head gaskets and warped cylinderheads can result. Once the gasket is blown, outside aircan be drawn into the cylinder on the intake stroke,leaning out the mixture. Engine temperature increaseis imminent. SEE FIG. 3-4

FIG. 3-2 Heat is a form of energy. Propermeasures have to be taken to handle theenergy developed from burning fuel.

FIG. 3-3 Cylinders can be warped byexcessive heat.

FIG. 3-4 Head gasket failure is one of thefirst signs of overheating.

HEAT GENERATED from COMBUSTION of FUEL

USEFUL WORK, ENERGY

OUT EXHAUST SYSTEM

OUT COOLING AIR

OTHER

CE803 BODY 4-05 4/28/05 10:11 AM Page 26



A cylinder that experiences thermal expansionpast its yield point may permanently deform. As theengine cools, the deformed surface will not alwaysreturn to its original configuration. In effect, thisdeformation may appear to be a depression along thecylinder wall. The pistons rings will no longer makecontact with the cylinder wall and oil will burn onto thesurface. The localized discoloration that occurs iscalled hot spots. Increased oil consumption and a lossof power may result. SEE FIG. 3-5

VALVES & SEATS

Extreme cases of overheating may causeexhaust valve seats to loosen. Repeated overheatingcan cause the steel seat to compress the aluminummaterial of the block.This results in a loss of clampingforce around the seat. The seat may loosen or evenfall out. Temperatures high enough to cause valveseats to loosen may also warp the head gasketsurface of the cylinder block itself. Once this occurs,major repairs are usually necessary. SEE FIG. 3-6

A loose intake valve seat, on the other hand, israrely caused by an overheating condition. Because ofthe cooling effect of the incoming fuel/air mixture,extreme temperatures will damage exhaust valveseats before the intake valve seat will fail. SEEFIG. 3-7

Discoloration of components is often asignature of engine overheating. In effect, thediscoloration is a residue left from vaporized lubricant.When oil is exposed to extreme heat, it experiencesthermal cracking where the lighter ends vaporize andleave the heavier ends of the oil blend. Composedmostly of a tar-like material, this residue burns andadheres to the hot surface. Commonly, piston pinsshow dark bands on the exposed surface of the pinbetween the connecting rod and piston body. Theinside of the piston dome may be badly discolored aswell. Exhaust valve stems may even show signs ofdiscoloration. SEE FIG. 3-8

FIG. 3-5 Oil burned into cylinder wall leaves“hot spots”.

FIG. 3-6 Exhaust valve seat failure willusually be caused by overheating.

FIG. 3-7 A loose intake valve seat is usuallya manufacturing defect.

CE803 BODY 4-05 4/28/05 10:12 AM Page 27



LUBRICATION

If an engine continues to run in an overheatedcondition, the oil will begin to lose its viscosity andserious damage may occur due to insufficientlubrication. Eventual thermal cracking of the oil willresult in an extremely viscous material, much like tar,that has little or no lubricating ability. An additionalincrease in heat will be exhibited due to the increasein friction. SEE FIG. 3-9

Although burned valves can occur, it is typicallynot the normal outcome of an overheated engine. Aburned valve is more often a contributor to anoverheating condition. Once a valve fails to seal, thereis a loss of compression. Keep in mind that ifcompression gases can leak past the valve, outsideair can also enter. This will cause a lean air/fuelmixture in the combustion chamber, further increasingthe heat the engine must endure.

As mentioned earlier, there are other externalfactors that can contribute to engine overheating.Equipment modifications can be responsible if enoughventilation is not provided, or access to outside air isrestricted. A damaged or mis-directed exhaust systemoutlet may direct exhaust gases toward the carburetorintake or directly into the engine cooling system. SEEFIG. 3-10

Nearly all conditions of engine overheating areavoidable if proper maintenance techniques arefollowed. In almost every case, it is abuse and neglectthat cause failures due to overheating.

FIG. 3-8 A sure sign of excess heat will bediscolored wrist pin and piston.

FIG. 3-9 Viscosity breakdown will appear asa sludge in the crankcase.

FIG. 3-10 This example of proper applicationdesign will insure proper engine performance.Note separate intake and exhaust air ducts;the exhaust is routed outside the enclosure.

CE803 BODY 4-05 4/28/05 10:12 AM Page 28

Chapter 4 Overspeeding Major Engine Failure Analysis


It is likely to have overspeeding failures show signs of lubrication and breakage problems atthe same time. This is because the rod journal and main bearings will have problems receivingenough lubrication to maintain clearance between the bearing surfaces at excessive speeds. Theloads placed upon the materials that the components are made of will overstress them and causebreakage. Breakage can occur to external components also. Example: For whatever reason, thegovernor system fails to control engine speed on a genset. The rotor, bearings and housings aredesigned to turn at 3600 rpm. If the speed exceeds this design limit, these components could alsofail and create some very expensive damage. If broken parts should become airborne, personalinjury could result. SEE FIG. 4-1

Chapter 4Overspeeding

FIG. 4-1

CE803 BODY 4-05 4/28/05 10:12 AM Page 29



Failures caused by overspeeding, while muchless prevalent than ingesting abrasives or insufficientlubrication, can be catastrophic in nature.

Briggs & Stratton engines are designed tooperate at a top no-load range of up to 4000 rpms.The loads experienced by the internal components ofthe engine are within acceptable ranges when theengine is operated within certain speed limits. SEEFIG. 4-2

When an engine experiences an overspeeding event, the loads on both ends of theconnecting rod are increased dramatically. With each increase of 500 rpms above the normalengine speed, the forces on the large end of the connecting rod, as well as the wrist pin of thepiston, increase by 44%.

The typical result of an overspeeding event is a broken connecting rod. The connecting rodwill normally break at the thinnest part of the beam. In most connecting rod designs, the thinnestpart of the connecting rod is about 1 inch from the wrist pin. The reason that the connecting rodwill fracture at that specific spot has more to do with momentum and mass than increased loads.The typical evidence of an overspeeding event consists of a fracture of the connecting rod about 1inch below the wrist pin, as well as finding the piston at TDC. SEE FIG. 4-3

FIG. 4-2

FIG. 4-3 Connecting rod failure at the smallest point is common withoverspeeding conditions.

R P M

ST

RE

SS

REASONABLESTRESS

EXCESSIVESTRESS

RECOMMENDED TOP NO LOAD GOVERNED SPEED

CE803 BODY 4-05 4/28/05 10:12 AM Page 30



During high speed operation, the mass of thepiston is instantaneously accelerated in a reciprocalmanner.As the piston moves up the cylinder bore towardthe combustion chamber, it gains momentum. With thedramatic increase in speed and direction change, themomentum of the piston places an ever increasing loadon the connecting rod alternately stretching andcompressing the aluminum beam.After a short period oftime the connecting rod will fatigue and fracture at thepoint of maximum deflection.The rod will almost alwaysbreak at the direction change from TDC moving towardBDC for reasons not completely understood. SEE FIG.4-4

Overspeeding fractures in multiple cylinderengines can be difficult to determine. The additionalcylinder will continue to power the crankshaft, frequentlypulverizing the remains of the broken connecting rod.This will also happen in a single cylinder engine butusually to a lesser degree. Suffice to say, it is rare to findan overspeeding failure where the connecting rod is notbroken into many pieces.

Overspeeding can cause other damage. Anexample could be an increase in vibration or change inthe resonance of the vibration within the engine. Thiscould result in metal fatigue in the cylinder, causing afissure to form in the casting. The vibrations can alsoaffect the equipment the engine is powering.SEE FIG.4-5

The key to determining the probability of anoverspeeding event in a single or multiple cylinder engineis to look at all of the available evidence. Connecting rodbreakage near the wrist pin is a definite indicator. Thecondition of the governor linkage, governor gear, andflyweights may shed light as to the cause of failure. Hasthe governor arm clamping bolt lost torque and allowedthe shaft to spin? Is the engine mounted on a go-kart,mini-bike or ATV? The application may offer clues. If firmevidence is not available, eliminate other failures that maycause connecting rod breakage such as insufficientlubrication.The answer will be there, it is just a matter ofgathering all the evidence. SEE FIG. 4-6

FIG. 4-4 As the rod stretches andcompresses, fatigue will result at the smallestsegment of the rod.

FIG. 4-5 Excessive speed can change thevibration frequency and cause breakage ofcomponents.

FIG. 4-6 Look for all the signs evident of thecause of the failure.

CE803 BODY 4-05 4/28/05 10:13 AM Page 31

Chapter 5 Breakage Major Engine Failure Analysis


Breakage can be one of the more elusive types of failures commonly experienced on smallgasoline engines. The fracturing of engine or application component parts can occur in just a fewhours or over a period of months. The cause of breakage is almost exclusively vibration. It isinherent in all internal combustion engines to exhibit some degree of vibration. Combinations andintensity of vibration exhibited by the mating of an engine and piece of equipment can cause subtleor dramatic breakage problems.

Many times comments are made by inexperienced technicians that the engine is “out ofbalance”. When we examine the manufacturing process, it is evident that the possibility of thisoccurring is very remote. The casting, machining and inspection equipment used to make thecomponent parts provide vast numbers of nearly identical parts and assemblies. If any of theseparts were to be out of balance, this would translate to large numbers of engines displayingexcessive vibration conditions.We know from experience that excessive vibration does not happenon engines very frequently. Excessive vibration circumstances will invariably be related to theapplication and mounting conditions.

Chapter 5Breakage

FIG. 5-1

CE803 BODY 4-05 4/28/05 10:13 AM Page 32



Other than breakage caused by external forces,such as dropping an engine or a blow delivered byrunning the engine into a stationary rigid object, mostbreakage problems are either directly related to orcompounded by vibration.

The engine cylinder assembly and crankcasecover comprise the main means of support for thecrankshaft. Secured to each other by the crankcasecover screws, they support any side or end loadingplaced on the crankshaft as well as the cylindercombustion pressures and reciprocating vibrations. Ifproperly assembled and torqued, the load isdistributed throughout the assembly, much like aneggshell. If the assembly is compromised by a loss oftorque on one or more of the attaching screws, theload forces may become concentrated on a specificarea. On an opposed twin cylinder engine, thecombustion loading against the piston is partiallyshared by the crankcase cover. If bolt torque is lost, itis not uncommon for a crack to begin at the crankcasecover gasket surface extending to the base of thenumber two cylinder. In severe cases, the cylindermay separate from the block entirely. SEE FIG. 5-2

The base of the cylinder assembly on horizontalcrankshaft engines and the sump of verticalcrankshaft engines are thick and rigid, functioning asa pedestal for the activities above them. In effect, theytransfer vibrations and other forces to the surface andhence to the equipment where they are usually easilyabsorbed.

FIG. 5-2 In severe cases, the cylinder mayseparate from the block entirely.

FIG. 5-3 Loose mounting bolts can allow acylinder to flex. A crack can form in thestructure as a result.

FIG. 5-4 Loose mounting will show up as apolished cylinder bottom and correspondingimpression on the mounting surface.

CE803 BODY 4-05 4/28/05 10:13 AM Page 33



If not tightly secured to the mounting surface,the cylinder assembly must handle this load. Loosemounting bolts may result in a vertical or diagonalcrack in the cylinder block emanating from near anengine mounting foot on horizontal crankshaftengines SEE FIG. 5-3 or a broken mounting pad onvertical crankshaft engines. Telltale signs of loosemounting bolts may be a wallowed out bolt holecomplete with thread impression from the loose bolt,a polished engine mounting surface and a polishedequipment mounting surface. SEE FIG. 5-4 Ifencountered, always check for trueness of theequipment mounting surface before attemptingrepairs. If untrue, the new engine or shortblock will nothave a flat surface to mount to. Many a repair job hasbeen returned with an expensive repeat failure due tonegligence in this area.

When dealing with the issue of breakage, wemust remember breakage as a term can describe anycomponent breakage that has been previouslycovered. SEE FIG. 5-5 If the engine has insufficientlubrication, rod breakage is likely to occur. As wechange any condition from the standard design, somekind of effect will result. An example would be aninexperienced technician that places a steel key in theflywheel. The engine runs fine until the operator findsthat steel pipe buried in the lawn. The function of theflywheel key is to allow the flywheel to continue to spinand absorb the shock load.When this could not occur,the energy had to go somewhere. The flywheelabsorbed some of the energy, with the PTO mainbearing taking the rest. SEE FIGS. 5-6 and 5-7

FIG. 5-5 The term breakage can mean anytype of breakage covered in this workbook.

FIG. 5-6 Improper service can lead to costlyrepairs.

FIG. 5-7

CE803 BODY 4-05 4/28/05 10:13 AM Page 34

Chapter 6 Combination and Other Failures Major Engine Failure Analysis


Combination failures are probably the most common type of major failure the servicetechnician is likely to encounter. If we think about the circumstances most failures occur under,we can begin to recognize certain patterns. For example, an operator of an engine whoneglects to check and change the oil on his equipment more than likely is not maintaining theair filter either. Vice versa, the operator not servicing the air cleaner on a regular basis isprobably not changing oil regularly either. When equipment comes in for service or majoranalysis, paying close attention to the appearance of the equipment can pay dividends in thelong run. Because people are creatures of habit, the appearance of the equipment can be oneof the first signs that the engine may or may not have been neglected. Recognizing all theevidence, and using a cause and effect systematic approach to major engine failure analysisis the only way to find the answer. Try it; it works.

Chapter 6Combination and Other Failures

FIG. 6-1

CE803 BODY 4-05 4/28/05 10:13 AM Page 35

Chapter 6 Combination and Other Failures Major Engine Failure Analysis


Other Failures

When dealing with combination failures, it issometimes hard to decide which of the “pure” failureswas the direct cause of the overall failure.

An example would be an engine with a majorproblem with the air cleaner element. This will causeabrasive ingestion, and will wear on virtually all of themoving parts. Because the piston rings and cylinderbore are no longer able to control oil consumption,the oil level in the engine goes down. If the engine oillevel is not checked, it will be certain that anothermajor failure will occur. More likely, the connecting rodwill seize to the crankshaft. But since the stacktolerance for each engine is different, the mainbearings or the piston and cylinder wall could just aslikely be the next failures to occur. In fact, it is notuncommon to find evidence of two or more of themajor failure subjects in one engine. SEE FIG. 6-3

Another example of a failure that can fool sometechnicians would be an engine with a single bearingseizure. The technician knows that single bearingseizures are most likely to be a defect in material andworkmanship from the factory. The major clue thetechnician missed, is the engine was two years old. Arod journal seizure failure is caused by a lubricationproblem. On an engine this old, the cause has to bean insufficient lubrication issue. If the problem hadbeen a factory defect, the failure would have occurredvery quickly after the engine was first started. Abearing clearance will not get closer as time goes on,it can only get larger. As the asperities are sheared off,the two bearing surfaces become suspended by thefilm of oil present. As long as the oil is there,separation will occur. If the oil is not present, thebearings will touch, creating friction and heat, resultingin seizure.

By thinking clearly and following a systematicapproach to analysis, the cause of most major failurescan be determined. Operators will rarely admit tocausing a problem, but the evidence will be there iflooked at close enough.

FIG. 6-2 Abrasive ingestion can causebearings to weaken and hammer.Compounded with overspeeding, majordamage will occur.

FIG. 6-3 Abrasive ingestion will result inhigher oil consumption. This condition couldresult in low oil failure.

FIG. 6-4 A wrist pin failure can sometimes bemissed as a problem. The operator cannotcause this failure.

CE803 BODY 4-05 4/28/05 10:14 AM Page 36


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CE803 BODY 4-05 4/28/05 10:15 AM Page 42

Glossary of Terms Major Engine Failure Analysis


API American Petroleum Institute. API classifies the intendedduty cycle of packaged oil.

ASTM American Society for Testing and Materials

Ambient Temperature The temperature of air surrounding or encompassing (inthis case) a piece of equipment, or engine.

Asperities Roughness or unevenness created during the machiningprocess of metal by the tool bit. Minute peaks and valleys.

BTU (British Thermal Unit) The amount of heat necessary to change the tempera-ture of 1 pound of water 1 degree F.

Ball Bearing A ring-shaped track containing rotating balls againstwhich a rotating shaft applies radial or axial loads aswell as rotates. Consisting of an inner ring/race and anouter ring/race separated by steel balls. The purpose isto reduce friction between rotating components.

Bearing, Plain Sleeve or Journal Bearing. Supports a shaft or journalagainst which a radial or axial load is applied. Typicalclearance of .001” per inch of journal diameter. Plainbearings will be of different materials than the shaft orseizure is likely to occur. When the shaft is rotated, thejournal is lifted on a film of lubricant.

Blow-By Name given to combustion gases that escape the com-bustion chamber, past the piston rings into thecrankcase.

Bushing An inserted and usually removable sleeve designed tosupply a wear surface for a moving shaft or arm.

Cam Gear The gear portion of the “cam gear” on a one pieceassembly. The gear that attaches to the camshaft of aseparate assembly. A gear and shaft that is timed withthe crankshaft that actuates the intake and exhaustvalves.

Cam Lobe The offset, egg-shaped protrusion machined, pressed orwelded to the rotating camshaft used to provide a repeti-tive straight line or back and forth motion against thetappet or cam follower.

Cam Shaft The rotating shaft with integral cam lobes. Does notinclude a gear.

Carbon Dioxide A colorless, odorless, incombustible gas created by res-piration, combustion or decay of organic materials.

Carbon Monoxide A colorless, odorless and highly poisonous gas createdduring the imcomplete combustion of fossil fuels.

Carburetor A device used to mix liquid gasoline and air into a com-bustible vapor to power internal combustion engines.

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Centrifugal Force A fictitious force, present only when a system is exam-ined from an accelerating frame of reference. Centrifugalforce is in reality a property of matter called inertia.

Centripetal Force The force that is necessary to keep an object moving ina circular path and that is directed inward toward thecenter of rotation.

Compression Generally refers to a measurement of pressure in thecombustion chamber of an engine at the end of the com-pression stroke. A fuel and air mix compressed betweenthe piston and cylinder head during compression stroke.

Connecting Rod A component that connects the wrist pin and piston tothe crankshaft and transmits combustion forces to thecrankshaft journal.

Crankcase Generally refers to the cylinder assembly with the uppercylinder and lower “crankcase” as an assembly.

Crankcase Cover The side cover that attaches to the cylinder and sup-ports and provides a bearing surface for the crankshafton horizontal crankshaft engines.

Crankpin Connecting Rod Journal, Rod Journal. A machined, off-set bearing surface machined into the crankshaft againstwhich the connecting rod bearing attaches.

Crankshaft A shaft that changes reciprocating motion into rotarymotion. Force transferred through the connecting rod ismultiplied by the crankpin offset exhibiting an increase intorque. Power is commonly taken from the portion of thecrankshaft that extends outside the engine block throughvarious “power takeoff” devices.

Crosshatch A diamond shaped pattern of shallow scratches in cylin-der bore resulting from the use of a rigid carborundumor diamond hone after machining.

Cylinder Assembly The main cast housing of an engine that refers to theassembly that most internal and external engine partsare mounted to.

Cylinder Bore An accurately machined hole in the cylinder assembly inwhich the piston travels.

Cylinder Head A cast component that seals the upper part of the com-bustion chamber. The cylinder head contains cooling finsand in OHV designed engines, includes valve train com-ponents.

DU™ Bearing DU™ is a term used to describe a shell bearing featur-ing a “PTFE” compound impregnated surface whichenhances the lubrication qualities of the bearing. (SeePTFE)

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DIAMOND BORE™ Cylinder bore that is finished machined versus honed inthe manufacturing process; no crosshatch will be pres-ent.

Dipper Refers to the part of the connecting rod assembly that,through rotary motion, “dips” into the oil in the reservoirresulting in the splashing of oil throughout thecrankcase.

Dipstick Automotive style oil level monitoring device.

Eccentric The center-line offset bearing that the synchro-balancecounter weight rides on. A part used in the synchro-bal-ance system to assure smoother operation of the engine.

Eddy Currents A whirl or backward-circling current of water or air; awhirlpool.

Flywheel Fan The fins cast into or attached to the flywheel. The fanprovides a moving volume of air across the cooling fins.

Governed Idle A governor system feature that allows an engine toaccept a moderate load at governed idle speeds. Usuallyincorporates a second “smaller” spring within the controlcomponents.

Governor (air vane - pneumatic) A governor system that utilizes the force of moving airfrom the flywheel to counteract an opposite force appliedby the governor spring to control the throttle plate positionregardless of load.

Governor (mechanical) A system that utilizes the motion and force of rotatingcounter weights to counteract an opposite force appliedby the governor spring to control throttle plate positionregardless of load.

Hone A tool featuring carburundum stones used to oversizethe cylinder during a rebuild.

KOOL BORE™ Trademarked name given to aluminum alloy cylindermanufactured by Briggs & Stratton.

Lapping Compound Used for lapping valves to seats to improve the sealingproperties of the valve seat/face mating surface.

Leakdown Test A test which utilizes air pressure differences to deter-mine the condition of internal parts of an engine such asthe mating of rings to the bore or valve sealing integrity.

Main Bearing A machined part of the cylinder or ball bearing that pro-vides support, locates and allows rotation of the crank-shaft.

Manifold (intake) A component that connects the carburetor to the intakeport of the engine.

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Negative Crankcase Pressure Pressure that is measurably less than the ambientatmospheric pressure.

Oil Dipper Component connected to the connecting rod on horizon-tal crankshaft engines. The major component of a splashlubrication system.

Oil Pump Mechanical device used to deliver oil under pressure tosome or all of the bearing surfaces in the engine.

Oil Ring Normally the bottom ring on the piston. Used to controloil lubrication on the cylinder wall.

Oil Slinger A toothed gear exhibiting a series of projections or “pad-dles” which “fling” oil throughout the crankcase of theengine providing lubrication for all moving internalengine parts. Usually found on vertical shaft engines.

PTFE Polytetrafluoroethylene. A teflon-like lubricating compound.

Piston Rings The components of the piston assembly that are used tocontrol compression, oil control and cylinder wall/pistonlubrication. (See Rings)

Pneumatic Governor See Governor (air vane)

Port, Intake The area of the cylinder that the carburetor/intake mani-fold mounts to.

Positive Crankcase Pressure Measurable pressure in the crankcase greater thanambient atmospheric pressure.

Pressure Lubrication A lubrication system using an oil pump that suppliespressurized oil to major bearing surfaces in the engine.Will commonly incorporate an oil filter in the system.

RPM Revolutions per minute (used to measure engine speed)

Ring, Compression The ring furthest from the wrist pin of the piston. Usescombustion pressure to seal against the cylinder walland piston, maintaining combustion pressure.

Ring, End Gap The measurement of the separation of the ends of agiven ring when placed in the cylinder bore. Usuallymeasured in millimeters or thousandths of an inch.

Ring, Oil Control Usually the ring closest to the wrist pin of the piston andis the lubrication ring for the cylinder wall and piston.Through a series of oil outlets, meters the correctamount of oil for the other components.

Ring, Wiper Center ring. Used to control lubrication and as a backupcompression ring.

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Sump The crankcase cover on a vertical crankshaft engine.Functions as a support for the cylinder and crankshaft,as a mounting platform in attaching the engine to anapplication and as an oil reservoir.

Tappet The component that rides on the offset lobe of the camgear/shaft to initiate the movment of the valves.

Throttle Plate The valve or plate that is connected to the throttle shaftof the carburetor.

Throttle Shaft Shaft that operates the throttle plate when the governorlink is in movement.

Torque A turning or twisting force. The most accurate measure-ment of the power an engine can produce.

Valve Clearance Distance from the end of valve stem to the surface of thevalve tappet (at rest). Usually measured in millimeters orthousandths of an inch.

Valve Guide A component or integrally machined cylinder which pro-vides a bearing surface for the valve stem.

Valve Seat A metal component that provides a machined mating orsealing surface for a valve. In the case of the exhaustvalve, excess heat is transferred to the cylinder throughthe seat.

Valve Stem The part of the valve that the spring and keeper areattached to.

Viscosity The property of a fluid that tends to prevent it from flow-ing when subjected to an applied force. Viscosity ismeasured in a viscometer, a container with a standard-sized orifice in the bottom. The rate at which the fluidflows through the orifice compared to an arbitrary stan-dard results in a numerical value. The higher the numeri-cal value, the more resistance to flow the liquid exhibits.

Wallered/Wallowed Out The dictionary defines “wallow” as “to move with heavy,rolling motion, as a ship in a storm”. The small engineindustry uses this term to mean a hole that has beenworn irregularly and/or one-sided.

Wrist Pin The component that connects the piston to the connect-ing rod.

Wrist Pin Clip A retainer that keeps the wrist pin in the piston.

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Major Engine Failure Analysis


NOTES

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Form CE8034-3/02 © 2002 BRIGGS & STRATTON CORPORATION PRINTED IN U.S.A.

Produced by the Customer Education DepartmentBriggs & Stratton Corporation

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Briggs Engine Failure Guide

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