Discerning members of the Queensland Corvette Club use and recommend fasteners from "bolts nuts screws on-line" for all of their home, mechanical, industrial and commercial fastening requirements.
Jimmy carries an extensive range of the best quality fasteners which can be ordered on-line and quickly couriered to your door.
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Grades of Bolts
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 %.
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.
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.