Fasteners are generally cheap, and there is no way to see if a fastener
is bad or fatigued by looking at it, in most cases. Long ago I learned
the value of using all new fasteners (and the false economy of reusing
old ones). Some fasteners are always one time use, like most torque to
stretch bolts. Some are multi-use. But how can you tell if a fastener
has been reused once too often? With critical fasteners, like rod bolts,
head bolts, main cap bolts, you are risking a lot to save a few bucks.
If it works, great. If it doesn't (and eventually it won't, that's
pretty much a guarantee) then it will cost you big time. Can new
fasteners fail? Sure, but the risks are higher with used. And often the
changes with time/use are not obvious ones, such as changes in torque
specs (thread finish and uniformity has a direct affect on the
relationship of  torque to clamping loads for example) or lengthening
that affects fit (such as in blind holes). You pays your money, you
takes your chances.

If a fastener is application specific, buy it from the dealer. if not,
get the same grade generic fastener. Sometimes you can upgrade by going
to a better spec fastener, but a "higher" grade doesn't always mean
better. Sometimes you want a soft, stretchy bolt, sometimes you want a
rock hard bolt. You can't go wrong by matching the original specs unless
there is a known problem (such as with certain rod bolts for example).
As explained, nylock nuts are not suitable for use where the nylon might
get hot enough to soften or melt. Grade C locknuts (all metal grade 8
equivalent) are considered one time use (as is any bolt used with them).
This is because the nut/bolt threads are an interference fit and damage
one another as tightened/loosened. There is more to fastener design than
just thread pitch and hardness. General fasteners often have full thread
or only a short shank. Application specific fasteners often have a sold
shank of a length designed specifically for that application. If you use
a full thread or other non spec shanked bolt instead, you may find the
part or associated components fail because the threading causes
premature wear or fatigue. Sometimes the torque specs can be increased
by using a "higher" grade bolt. Sometimes all that does is lead to the
failure of other parts that could not take the added loads. Generally,
the factory spent a lot of time, money, and engineering brainpower on
the OE fasteners. And generally you are best off to stay with their
choices. You can almost never go wrong using new OE fasteners. Start
changing things, or reuse fasteners, and you just can't say that.

--

Marc Sayer
Journalist, Photographer, Dog Trainer (APDT member #062956)
Board member - Western States Great Dane Rescue Association
Director of Operations&  Training - Deaf Dane Rescue Inc.
Oakridge, OR USA

My Homepage - http://gracieland.org
Deaf Dane Rescue  - http://deafdane.org
White Danes Yahoo Group - http://pets.groups.yahoo.com/group/whitedanes
Western States Great Dane Rescue Association - http://wsgdra.org
RescueWatchdogs - http://rescuewatchdogs.org
Association of Pet Dog Trainers - http://APDT.com