There are so many old wives tales inside of the hot rod world. It particularly makes my blood pressure rise a bit to see a semi-mainstream media publication let one of their editorial writer, who happens to own one of the most recognized hot rod name brands, continue to enumerate and lend legitimacy to those myths.
Most of these are pretty harmless, but to state that a grade five bolt is preferred over a grade 8 bolt when mounting suspension components is really dangerous. This from the September 2011 Goodguys Gazette,. written by Brent Vandervort.
In actual fact, Grade 5 bolts are often preferred over Grade 8 . A Grade 8 has a very high tensile strength, but can be brittle. In applications such as suspensions, a very high number of load/unload cycles can produce work hardening, which produces an immediate, or catastrophic failure. It’s often much safer to use a Grade 5 bolt which will bend, thus providing a warning, rather than a Grade 8 bolt breaking without warning. As long as the Grade 5 bolthas been sized large enough to take the expected load, we don’t want to trade toughness (resistance to stress cycles) for the ultimate tensile strength of the Grade 8 fastener.
While he did leave himself a caveat with the statement of properly sized hardware for the task at hand, this subject has been debated and discussed on numerous message boards. Google for yourself and read the threads if you are interested.
I have read numerous pages of information regarding this subject because logically it never made sense that a “softer and malleable bolt” GR5 would ever be superior “than a hard and brittle bolt” GR8. What in the hell is the purpose of even making bolt with higher gradings than the 5, why would anyone make a brittle bolt?This makes no logical sense.
And furthermore, the issue of toughness. Why would a bolt of a lower grade be “tougher” and therefore more resistant to stress cycles than one of a higher grade that is stronger in tensile strength? Again, this make no logical sense.
Well, when logic fails the observations of these wives tales something is obviously wrong. Using this reasoning, an even lower grade bolt than a 5 might even be preferred because its even more malleable therefore tougher, cough. I can visit websites like SAE and BoltScience and get true and factual, means tested information without the hotrodder spin.
Please, if you are even remotely interested in this, open and bookmark these websites and tables.
The Engineers Handbook
This thread on Pirate 4X4 is a very enlightening read, posts #9 and the response from Billavista #11 fit the discussion to a T. And here is the summary if you don’t care to click:
Then there’s fatigue strength. Bend a piece of metal back and forth a bunch of times; it may not break right at first, but it will break eventually. For any given metal, there is a fatigue limit, which is lower than the ultimate load. Below the fatigue limit, bend it as many times as you want and it won’t break; above that, it will eventually break. Again, in general, the higher the ultimate strength, the higher the fatigue limit.
Finally there’s impact loading. This is tested by notching a steel bar and hitting it with a hammer (very precisely, of course). This is the one case where a softer steel might do better, as harder materials tend to be more brittle. However, for most of our applications it’s not that important (unless you’re running a snowplow or trying to snatch a stuck Jeep out with a chain instead of an elastic nylon strap), as the suspension tends to damp out and spread loads below the sharp spike necessary for impact failure. Also, even grade 8 bolts aren’t all that brittle; after they’re heat treated to get the ultimate strength up they’re tempered to bring back the impact strength. Impact strength might be an issue if something’s loose, so that it shifts and bangs around before it hits whatever is stopping it (i.e. the loose bolt).
Conclusion: grade 8 bolts are stronger in virtually any application on a Jeep. Whether you need that strength, and want to pay extra for it, is another issue (though I bought some 5/16 grade 8’s yesterday for $0.60 each; grade 5’s were only ten cents cheaper). There may also be situations where you may want the bolt to fail before the expensive part it’s holding breaks, but that’s another issue. ”
As Billavista said, Grade 8 bolts are stronger. The scientific data supports it and real world testing proves it. Brent is wrong and has done damage to the hot rod community for pushing such an ignorant myth.
I have a challenge for proponents of GR5 being better suited for suspension use to find one or more makes of mass produced cars or trucks that use lower graded hardware on suspension components, take a picture and send them to me or post them here or somewhere public where we can see the evidence.
It is far more important to size and properly torque your hardware than any chemical or mechanical thread lockers can overcome.