I posted the question about which is better, grade 5 or grade 8, in
automotive terms for a restoration, to the Shop-Talk list on team.net. Here
are some of the well informed responses. I hope you read thru it, it is
worth it.
Allen Hefner
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There are many people that would never use a grade 8 bolt on a race car.
The thinking is that the grade 5 would stretch, bend, etc and provide
a long slow failure, rather than the sudden failure of a grade 8,
under identical conditions.
The grade 8 being much harder, would snap.
Made sense to me, and do not use grade 8's.
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Cost, and the embarrassment of proving one doesn't know what they
are doing are 2 reasons which immediately come to mind as why one
shouldn't use grade 8 anywhere in a car except maybe the driveshaft.
Using grade 8 in any stressed engine part is foolish, they are not strong
enough. Use ARP or SPS bolts instead.
Using a grade 8 bolt to hold a fender on is pretty ridiculous too. It's the
equivalent of telling people that one uses racing slicks, on the street,
because you want to go faster. It may impress 15 year olds, but those
who know, just laugh.
Fasteners work by stretching, do you really need a yield strength of
120,000 psi to hold that plastic MG fuse box in place or is that going
to seem just a little foolish?
Check out the ARP website for more info on bolts and how they work.
Also see Carroll Smiths books.
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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...
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Hoo Boy Allen,
You really opened a can of worms. I bet they argue you should use Grade
8 nuts and lock nuts too.
Bolts and bolting are often very misunderstood, even by the engineers
who are charged with selecting them. I've learned everything I know
about bolting from the Germans. They don't allow conventional wisdom to
color their judgment, they actually go prove the science.
As simply as I can put it. Grade 8 bolts, while extremely strong, and
not very ductile. They do not tolerate bending and shear loading very
well. Grade 8 should be used in through bolt situations where you can
insure proper preload (torque,) flat mating surfaces and little or no
shear load.
If you have less than ideal bolting situations: tapped holes, flexible
components to be bolted, high shear loads then grade 5 is probably a
better choice. This is because they are far more tolerant to abuse due
to a phenomena called localized yielding.
This is the subject of many volumes of writing, and I've greatly
simplified the situation. No bolt will withstand high numbers of cyclic
loads, over torque or corrosion for very long.
All that being said, I'm amazed at how some high strength suspension
bolts are mounted. My guess is that their saving grace is that they are
significantly oversized thus lowering overall stress levels.
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Well, cost is one factor (and a big one!). One good reason not to use grade
8 where grade 5 or 2 is spec'd is tightening torgue. A bolt is tightest
when it stressed nearly to its elastic limit -- the point at which the
threads stretch permenatly. For grade 8, that's very much higher than it is
for lower strength grades. If all you're doing is clamping something
together, that's not much of an issue. If your threading the bolt into a
threaded hole in a cast piece, you risk ripping the threads out of the hole,
or bolts that are more likely to come loose. A loose bolt is more likely to
brake than a properly fastened one.
Most automotive applications, grade 5 is suitable.
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