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 Grades of Bolts

An Engineer Rambles
Is 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 from these three grades. The lowest grade is 4.6, commonly known as commercial grade. Next comes grade 8.8, known as 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 the point at which the bolt will permanently stretch. (Officially, the 0.2% proof load stress).  A 4.6 bolt permanently stretches at 60% 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 screws with hexagon socket (Allen Key) drives. Corvettes use a lot of Torx drives, and there are other types of patented and tamper proof drives such as Safe-T drive. Grade 12.9 bolts can also be obtained with hexagon heads. Popular brands of grade 12.9 bolts are Unbrako and 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 source 12.9 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 nuts are not made. Grade 8.8 bolts are usually supplied as hexagon head bolts, often galvanised, sometimes zinc 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 than 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 and light objects. 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 4.6 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 tensile bolts 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 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 tighten a grade 12.9 bolt. Automotive manuals will give the tightening torque required. Grade 12.9 bolts do not work effectively with spring 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 buy pre-drilled cap screws. Mating surfaces of a 12.9 bolted joint are not painted. For serious work, never re-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 turn method, 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 indicating washers, such as Coronet washers can be used. These have dimples which indent the mating surface. A feeler gauge is used to determine the pre-load. Grade 8.8 Bolts are used in High Strength Friction Grip applications. If this is the case, the mating surfaces must not be painted or galvanised. For added security of the joint , use any one of the plethora of 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 wired together in groups. Also for serious work, 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 reasonable hand effort on a spanner. Use good springy chrome-moly spanners, and never put an extension bar on the spanner. All of the security systems available for 8.8 bolts are available on 4.6 bolts. If you need much more than a spring washer, question whether the joint really requires an 8.8 bolt. Mating surfaces of a grade 4.6 bolted joint are usually painted prior to assembly.