Posted by Alan Bowes on February 25, 1998 at 14:56:28:
In Reply to: Military nuts and bolts posted by Steve Ostroski on February 24, 1998 at 21:21:36:
Here are some general recommendations:
Whenever possible, for authenticity, I like to use the original "script" bolts. I bead blast them and take them down and have them zinc-iridite plated. It's cheap if you have a bunch of them done at one time, and the iridite surface bonds well with paint.
Judging from the TM torque figures for M37 fasteners (plus some "pseudo"-scientific tests I've done), I'd say that most of the M37 bolts are the equivalent of the more common AN or MS fasteners (aircraft/military), which would be about 125,000 psi tensile strength. This is slightly higher than a grade 5 (120,000), but well below a grade 8.
Replacement studs should always be grade 8 or better. Grade 8 is pretty common. Generally, when replacing studs, try to match the original appearance or measurements. There are more modern stud designs with relieved areas and radiused edges, etc., but they are designed to be used in a hole that is drilled and threaded specifically for those stud designs.
In general I'd just make it a rule to use hardened, grade 8 nuts for most applications. There is no reason to use anything else, except possibly the slightly higher cost for grade 8 nuts.
When in doubt, I use grade 8 bolts just as a matter of course.
If you want to save some cash, I would DEFINITELY avoid buying grade 8 bolts or nuts from any of the big "discount" chain hardware stores. You'll pay 2 to 5 times more than you will at a good fastener supply house for the SAME brand and grade! For instance, I go to a place called Bolt and Nut Supply here in Salt Lake. They've been around for years, their fasteners are traceable to the source (thus avoiding counterfeit fasteners), they carry the same brands as the big chain stores, and I've found that their prices are about 70 percent cheaper than at certain big hardware stores. Every major city should have several such suppliers.
Make sure the grip range (shoulder) of the replacement bolt/screw is the correct length. This is referring to the unthreaded part of the bolt. If you can't find the exact shoulder length, make sure that the unthreaded part does not extend beyond the surface of the parts to be joined. In other words, you should not be able to see the unthreaded part of the bolt after inserting it into the hole. The shoulder should end just below the part's surface where the washer will contact it.
When torquing bolts (even with grade 8 bolts), if you are screwing bolts into a casting, do not exceed the original torque ratings shown in the technical manual, since the casting will usually be the weak link.
Never use weaker fasteners than the original ones. By the way, stainless steel fasteners are usually MUCH weaker in terms of tensile strength and yield than the original fasteners. Generally speaking, even with higher-strength bolts and nuts, I'd suggest using the torque ratings printed in the technical manuals. This assumes, of course, that you are also using the same thread type, since torque ratings are different for fine vs. coarse thread.
Have fun,
Alan