Grades refer to the tensile strength of the fastener.
Bolts are generally marked with hashes on the head in a radial
pattern, higher grades of fasteners having more marks, and more
tensile strength.
When you torque a fastener, what you are really doing is stretching
the unit to its proper tensioned length, with the torque a convenient
method of determination. This ensures that the fastener will not
loosen inappropriately or fail due to over stress.
Different grades of fasteners are made of different alloy steels, with
varying amounts of carbon, moly, etc. which are quenched and tempered.
Very high tensile fasteners are also specially plated with various
flavors of zinc chromates for corrosion resistance and to facilitate
proper torque levels without having the head twist off (lubricated in
other words).
Some super duty fasteners are matched sets of components (bolt,
washers and nut) and exceed 180,000 PSI tensile strength.
Stainless steel is not always the correct option here, although using
it for exhaust applications is cool.
When I build a motorcycle (one of my other afflictions) I use aircraft
grade SS nuts and bolts, mainly for strength and because SS polishes
up like chrome. Boy does that bill hurt!
For the complete skinny, get a machinists handbook such as is sold by
Griot's Garage, or get your hands on a good vendor's catalog such as
McMaster-Carr (908-329-3200, the ultimate toy store for tools and
such).
Never use less than a grade 5 bolt (usually 3 hashes) on any
automotive application and you should be safe. Internal engine,
suspension and other strength-critical applications warrant more
attention to detail. I used grade 8s when replacing the spring-pan HW
on my front end and the brake backing plate HW on the rear axel, for
example.
I know there are a couple of MEs out there who will probably provide
more info...
Hal
______________________________ Reply Separator _________________________________
Subject: Re: Fasteners
Author: <jphender@soli.inav.net > at ccout
Date: 1/13/98 8:42 AM
Thanks for the advice Shawn,
Can someone give a quick tutorial about "grades" of fasteners? What is
unacceptable and why? Are they marked with a code? I don't think asking
my local hardware guy would necessarily yeild a correct answer. I am a
RELATIVELY bright guy, and I didn't even know I had a choice!
The only thing I've used stainless on was the manifold/block and
manifold/pipe junctions. Are there any others I should change before they
freeze up?
The "stock-up" list that Shawn started (may it grow ever more complete!)
is great. There is a visible path from my house to the Coast-to Coast
store. (My '87 horizon drips a bit. Who wants to work on one of those
anyway!)
Jim Henderson
'69 Sprite MKIV HAN9U78817G
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