tragopan86

Suppose I broke off the caliper mounting bolts while replacing the calipers, and want to replace them with a medium grade bolt, instead of the super hard ones. I don't see why they need to be #8 bolts. Isn't that a bit of overkill anyway?

RussB

I'd guess that in this application, tensile, compression, or shear strength (depending upon the brake design) is more important than hardness. If I remember correctly, grade 8 bolts are specified in terms of shear strength. Not sure about lower grade bolts.

BTW, for high shear situations, AN ("aircraft") bolts are superior to grade 8. Most of their shanks are unthreaded to take the shear load; only their ends are threaded. The long threads on other bolts degrade their shear strength.

Bottom line: Where brakes are concerned, I wouldn't downgrade components. For racing or heavy duty, in fact, I'd upgrade them.

cuatrokoop

If you are reading 8.8 on the bolt, that is a "metric" rating and is NOT equivalent to SAE Grade 8.

Here are the approximations for y'all:

Grade 12.9 > Grade 8
Grade 10.9 = Grade 8
Grade 9.8 = Grade 7
Grade 8.8 > Grade 5 (if crest diam > than 1")
Grade 8.8 = Grade 5 (if crest diam less than 1")
Grade 5.8 = Grade 2
Grade 4.8 = Grade 1 or 2 (crest diam. dependant)
Grade 4.6 = Grade 1 or 2 (crest diam. dependant)

RussB

In practice, designs sometimes place shear stress on bolts. In well designed systems, tensile stress is the rating to which you wanna pay attention.

RussB

Nice cross-reference. Thanks. Don't recall seeing this before.

cuatrokoop

Its from my Machine Design book ;-)