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Originally Posted by 68LSS1
I have heard the same thing about grade 5/8 for many years but have yet to read anything proven this true which is a totally other subject. Bolts are rated by the yield strength and tensile strength. Shear falls into the tensile category. Can someone support their statements about 1) stainless being unable to carry equivalent grade 5 loads and 2) just for giggles that grade 5 are better for shear forces and 8 for clamping?
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The reason you can't find that info is that the shear strength of a bolt is dependent on the joint it is used in, so the industry doesn't publish the data, as a general rule. A simple shear (two equal thickness plates) versus complex (multiple plates of different thickness) have completely different values. You don't run into that problem with tensile strength.
What I try to go by is the grade of the fastener being replaced. If you follow this advice (which is what the SAE tells the technicians) you can't go wrong. If a bolt has failed, you need to determine why rather than replacing with a stronger bolt. It is also the most economical method of replacement.
Also, when replacing a steel rivet with a bolt, I drill to the next larger bolt diameter and use a grade 5. The strength of a rivet is in shear, and will always be weaker than a grade 5 bolt, possibly more than a grade 2, depending on the application.
I am not a huge fan of stainless steel fasteners. Two major issues, thread galling (where the threads meld together) and galvanic corrosion with non-ferrous metals. I've had to resolve too many corrosion issues in airframes, and am skeptical on the use of stainless. My best advice is if using stainless steel bolts is to use alloy steel nuts. That stops the galling issue. And avoid the use of stainless in aluminum. If you do use stainless bolts and nuts or stainless bolts in aluminum, always use either a thread-locker compound or anti-sieze.
Also, when buying stainless steel fasteners, buy only from a known, reputable source, preferably U.S. made. Otherwise, you are dealing with material of unknown heat treat. I used some stainless steel exhaust studs that came with an engine dress-up kit once. Had major problems with them. Went to a metallurgist, here at work, and he determined that the material had not been heat-treated, it was still in the 0 condition.