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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, thenits 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 slightlyfrom these three grades. The lowest grade is 4.6, commonly known as commercial grade. Next comes grade 8.8, knownas structural grade, and finally, the highest grade is 12.9, known as high tensile bolts. The first number represents the

ultimate tensile strength of the bolt, 400 MPa, 800 MPa or 1200 MPa respectively. The second number represents thepoint at which the bolt will permanently stretch. (Officially, the 0.2% proof load stress). A 4.6 bolt permanently stretches a60% of its ultimate, an 8.8 at 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 scwith hexagon socket (Allen Key) drives. Corvettes use a lot of Torx drives, and thereother types of patented and tamper proof drives such as Safe-T drive. Grade 12.9 boltalso be obtained with hexagon heads. Popular brands of grade 12.9 bolts are UnbrakoHolo-chrome. It is possible to obtain zinc or chrome plated versions, but not galvanGalvanising destroys the heat treatment of the steel. Beware! it is not possible to so12.9 grade stainless steel bolts. 316 and 304 stainless steel will only be equivalent to g4.6. They may look cute in an engine bay, but they do not meet the grade. Grade 12.9

are not mGrade 8.8 bolts are usually supplied as hexagon head bolts, often galvanised, sometzinc plated, but usually black. They are distinguished by the three radial lines on the hor these day, by the numerals 8.8 stamped on the head. Grade 8.8 nuts are taller grade 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 andlight objects. Grade If you need to replace a bolt, and the grade is doubtful, use grade 8.8 and be safe. The head of agrade 4.6 bolts 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,with minimum cyclical fluctuation of stresses in the bolt under working conditions. In order to achieve this aim, high tensilebolts are pre-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 averageCorvette 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 totighten a grade 12.9 bolt. Automotive manuals will give the tightening torque required. Grade 12.9 bolts do not workeffectively with spring washers or shakeproof washers. These don't bite into the hardened surface of the bolt. There areonly 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 buy pre-drilled cap screws. Mating surfaces of a 12.9 bolted joint are not painted. For serious work, neverre-use a fully tensioned 12.9 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 thesize, length etc. The torque wrench method is used commonly, often in the form of an air operated rattle gun. Special loadindicating washers, such as Coronet washers can be used. These have dimples which indent the mating surface. A feelergauge is used to determine the pre-load. Grade 8.8 Bolts are used in High Strength Friction Grip applications. If this is thecase, the mating surfaces must not be painted or galvanised. For added security of the joint , use any one of the plethoraof systems such as spring washers, shakeproof washers, Loctite, wired heads, split pins and castle nuts, locking tabs,lock nut or patented nut systems (Nyloc is common). For serious work, use castle nuts, either with split pins or wiredtogether in groups. Also for serious work, do not re-use fully tensioned grade 8.8 bolts, Nylocs, split pins etc.

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