8/3/2019 Grades of Bolts

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Grades of Bolts

An Engineer RamblesIs it possible to write an essay about bolts? If you are a Engineer, and your livelihood depends on machine design, then its

possible to write a book about bolts.

There are three grades of bolt in common use in Australia. Most bolts which you will come across will only deviate slightly fromthese three grades. The lowest grade is 4.6, commonly known as commercial grade. Next comes grade 8.8, known as structuralgrade, and f inally, the highest grade is 12.9, known as high tensile bolts. The first number represents the ultimate tensilestrength of the bolt, 400 MPa, 800 MPa or 1200 MPa respectively. The second number represents the point at which the boltwill permanently stretch. (Officially, the 0.2% proof load stress). A 4.6 bolt permanently stretches at 60% of its ultimate, an 8.8at 80% and a 12.9 at 90 %.

Grade 12.9 bolts

Grade 12.9 bolts are most often supplied as socket head bolts or socket head cap screwswith hexagon socket (Allen Key) drives. Corvettes use a lot of Torx drives, and there areother types of patented and tamper proof drives such as Safe-T drive. Grade 12.9 boltscan also be obtained with hexagon heads. Popular brands of grade 12.9 bolts are Unbrakoand Holo-chrome. It is possible to obtain zinc or chrome plated versions, but not galvanised.Galvanising destroys the heat treatment of the steel. Beware! it is not possible to source12.9 grade stainless steel bolts. 316 and 304 stainless steel will only be equivalent to grade

4.6. They may look cute in an engine bay, but they do not meet the grade. Grade 12.9 nutsare not made.Grade 8.8 bolts are usually supplied as hexagon head bolts, often galvanised, sometimeszinc plated, but usually black. They are distinguished by the three radial lines on the head,or these day, by the numerals 8.8 stamped on the head. Grade 8.8 nuts are taller thangrade 4.6, but do not have any distinguishing marks

Grade 4.6 bolts come in a vast array of configurations. Hexagon head, countersunk slot drive, galvanised, zinc plated,oxy-sealed (ie gold zinc plating), chrome plated. On a motor vehicle, grade 4.6 bolts are used to hold on trim parts and lightobjects. Grade If you need to replace a bolt, and the grade is doubtful, use grade 8.8 and be safe. The head of a grade 4.6bolts is easily scratched with a file, not so a 8.8 bolt.

Correct design of a bolted joint is quite involved. Serious joints are designed on the basis of maximum stretch in the bolt, withminimum cyclical fluctuation of stresses in the bolt under working conditions. In order to achieve this aim, high tensile bolts arepre-tensioned, bolts are made as long as possible, and the minimum size bolt is used. Often the shank of a bolt (ie a cylinder head stud) is reduced in area to assist. Bolted joint design too complex to delve into here. The average Corvette enthusiast will

be replacing bolts in an existing designed joint and need not worry to much about the design of joints.

How do you tighten a bolt?Grade 12.9 bolts are tightened with a torque wrench, right up to their 90% proof load. There is no other effective way to tightena grade 12.9 bolt. Automotive manuals will give the tightening torque required. Grade 12.9 bolts do not work effectively withspring washers or shakeproof washers. These don't bite into the hardened surface of the bolt. There are only two ways of adding extra security to grade 12.9 bolts. Use Loctite on the threads or drill the heads and wire the bolts. It is possible to buypre-drilled cap screws. Mating surfaces of a 12.9 bolted joint are not painted. For serious work, never re-use a fully tensioned12.9 bolt.

Grade 8.8 bolts are tightened by the part turn method, torque wrench, or by using load indicating washers. In the part turnmethod, the bolt is done up to snug tight, and then advanced one, two or three flats of the hexagon, depending on the size,length etc. The torque wrench method is used commonly, often in the form of an air operated rattle gun. Special load indicatingwashers, such as Coronet washers can be used. These have dimples which indent the mating surface. A feeler gauge is used todetermine the pre-load. Grade 8.8 Bolts are used in High Strength Friction Grip applications. If this is the case, the matingsurfaces must not be painted or galvanised. For added security of the joint , use any one of the plethora of systems such asspring washers, shakeproof washers, Loctite, wired heads, split pins and castle nuts, locking tabs, lock nut or patented nutsystems (Nyloc is common). For serious work, use castle nuts, either with split pins or wired together in groups. Also for seriouswork, do not re-use fully tensioned grade 8.8 bolts, Nylocs, split pins etc.

Grade 4.6 bolts are not pre-tensioned in the joint. They are tightened to snug tight only. For all intents, this means a reasonablehand effort on a spanner. Use good springy chrome-moly spanners, and never put an extension bar on the spanner. All of thesecurity systems available for 8.8 bolts are available on 4.6 bolts. If you need much more than a spring washer, questionwhether the joint really requires an 8.8 bolt. Mating surfaces of a grade 4.6 bolted joint are usually painted prior to assembly.

Grades of Bolts http://www.qldcorvetteclub.com.au/tech/tech_talk/bolts.ht

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