First the second part, I know nothing about "Bowmalloy bolts" sounds
like some kind of brand name instead of something that meets a standard...
Now the First part:
The "preference" for Grade 8 bolts is the premise that "Bigger is Better",
and "Stronger is Better". Unfortunately, it isn't always true, as there are
always trade offs.
In simplified terms:
Grade 8 bolts have higher tensile strength, but tend to be more brittle than
Grade 5 bolts.
Grade 5s tend to yield (stretch) before failure which relieves some of the
load before breaking (things tend to get loose).
Grade 8s tend to break without yielding when over loaded (although it
takes more to break them, when they do fail, things tend to fall off).
Under dynamic loads (think suspension), Grade 8 will have more of a
tendency to break than Grade 5 due to fatigue caused by repeated cyclic
loads.
Most OEM bolts used on cars are high quality Grade 5 bolts. And
they don't just use such bolts just because they are cheaper.
If you really want stronger bolts, use aircraft bolts (AN series) as they
are stronger, and more ductile. A desired failure mode on aircraft is to
have bolts stretch, and not fail (think wing twisting instead of falling off).
The only downside to aircraft bolts is the cost.
On a side note, I once (25+ years ago) had car that kept breaking the
Grade 5 bolts on the air conditioner mount. After the third time, I installed
a Grade 8 bolt. Instead of the head of the bolt breaking off, the mounting
boss on the block snapped off. I had to buy a new short block...
Tim Mullen
>>> <LBC286@aol.com> 9/25/2001 9:41:44 AM >>>
Grade 5 vs. 8
OK, here goes the bolt strength thread (no pun intended) again.
On another list there is a heated discussion about using all grade 8 bolts on
restoring a car. Can anyone give me a simple explanation as to why it may or
may not be a good idea to replace grade 5 with grade 8?
Part deux: What about Bowmalloy bolts, from Bowman? They are higher
strength than grade 8, according to Bowman. Anybody know about these?
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