Because this post involves is a safety related issue I think some further discussion is appropriate. A statement made by Smokey Yunick was referenced which said "never use grade 8 bolts in any stressed application! There are NO Grade 8s in aircraft, and the FAA does not allow them during its inspection program'!" Smokey is one of my all time favorites. He is intelligent, experienced, interesting, and above all, entertaining. But to put things in perspective I've also seen examples of his "experience" exceeding his knowledge. Smokey has an impressive resume, but have you ever stopped to considered how well the race cars and engines HE has built have actually done? When you eliminate the cheating and the novelty factor there�s not a lot to talk about.
The reason there are no grade 8 bolts on aircraft is because the FAA requires that AN(aircraft) spec. bolts be used. Grade 8 bolts are SAE spec fasteners. Grade 8 bolts are actually stronger than most AN bolts, but do not meet the dimensional and quality control standards of AN fasteners. The other part of this statement is not even logical. Where WOULD you use high strength bolts, in LOW stress applications? If that were true grade 8 fasteners would not be used to retain, flywheels, ring gears, main caps, bulldozer sprockets, crane booms, truck cross-members, bridge beams, etc. Manufacturers routinely use 10.9(grade 8) fasteners in the suspension of their vehicles. Modern cars and trucks go through a tremendous amount of engineering and development. Every part of a vehicle's suspension is subjected to months of field and laboratory testing. If a less costly 8.8 fastener would do the job, I assure you there would not be a 10.9 bolt in the hole. In most situations a bolt needs be tightened to the point that it's clamping force exceeds the load being retained, and at the same time, the bolt must be sufficiently elastically deformed. There are cases where a structure(suspension mount and inner sleeve) may not be strong enough to allow a grade 8 bolt to be tightened to this extent. I think this may be what Smokey was iluding to. So, there are cases where the answer is, it depends. This is not a simple subject with universal answers. There�s no doubt an expert in the field could shed a lot more light on this matter than I have done. But if I wasn't sure which bolt was correct, I'd rather have a grade 8 or 10.9 bolt that may loosen up than a grade 5 or 8.8 bolt that may not be up to the job in the first place.
When comparing a broken grade 5 bolt to a broken grade 8 bolt, the grade 5 will have the appearance of having put up a better fight. A grade 5 fastener will stretch and bend a lot more than will a grade 8 before it breaks. But the fact is, the actual load required to FAIL a grade 8 fastener is much higher than for a grade 5 of the same size. If you think not, tighten one of each until it breaks, or try cutting one of each with bolt cutters. Along with this is a perception that impact loads are somehow different than "regular" loads. And that softer weaker bolts can tolerate these �special� impact loads better than a harder stronger bolt. Well, load is load, and stronger bolts can take more load than weaker ones...Al English
