Something im not sure on, but was told once. Grade 5's supposedly have a higher shear point than grade 8s. Apparently the 8s have more tensile strength but are more brittle thus the lessened shear capacity. Came from the guy at Tacoma screw company, he suggested 5s over 8s when i fishplated my frame.
Grade 5 and Grade 8 bolts are both medium carbon steels that have been hardened, and have the same practical ductile (non-brittle) range. The Grade 8 bolt is alloyed with other materials to provide more strength in proof, shear and tensile ratings while not needing to be excessively hardened or "carboned up" to a state where steel would become brittle.
Rockcrawler.com wrote a short article that covers this misconception in the offroad crowd too. This generality has even been published in automotive magazines, and generally results in a weaker bolt being used without sound reasoning by hobbyists.
how about some shoulder bolts to avoid the whole thing Al was talking about with his awesome pic
http://www.mcmaster.com/#shoulder-screws/=hrdhtn
these things have a machined shank to fit in the hole much more precisely then you run of the mill bolt.
I saw this and thought heck yeah, even the chart I put in showed that the shoulder has
much higher ratings in shear that the threaded portion of a bolt, even though either can be used in shear applications as long as the ratings are respected. I clicked on that link though, and immediately saw that the "shoulder screws" linked to are NOT rated for shear loads, at all (it states in the text)!
I went digging and on McMaster and found that the bolt you need if you want to use a 'shouldered' area for higher shear loads are called
"High Strength Steel Grade 8 Cap Screws, partially threaded" - these bolts (cap screws) are rated for shear and tensile loading. The way all this jargon is worded with bolts and such, you have to be really, really careful about what you chose for your applications.
Nut is facing engine. I will check when I disassemble to see if I can tell if the bolts came loose before they sheared, but by the looks of the nuts that came out of the inspection cover the bolts did not come loose. The ends of the bolts were still in the nuts. Holes were drilled with a 3/8" bit on a drill press. The bolts didn't just fall into place so the hole isn't oversized I think..... Loctite use was wurth red high strength nuts torqued to 40 ft-lbs, 3/8-16 UNC grade 8 bolts.
I was re-reading this thread after the last couple posts, and I noticed the part I highlighted the second read through. The -16 thread per inch bolts will shear sooner than the -24 fine threads. The thread root is cut deeper, and your cross section is smaller on the coarse thread bolt, try to spec the fine threads for shear applications if you can. The best bet as stated above, if you can find the partially threaded bolts to use and put the non-threaded smooth part in the shear interface where the parts would slide against each other. It's a case of good (coarse), better (fine), best (smooth shoulder). Also note that torque specs for proper clamping will generally be lower for the coarse threads, although 40 ft lbs. falls within the safe range for both coarse and fine grade 8 3/8 bolts when assembled dry.
Liquid loctite does also act as a thread lubricant during torquing as well, although not to the same degree as moly or assembly lube. You might consider a value between the dry and lubricated specs for fasteners while using loctite. Just one more thing to add to the never ending compilation of Lucky Mod & Fasteners 101, lol.
McmasterCarr has quality bolts.
+1 - I use McMaster for all sorts of stuff I can't find local, the fact that an industry quality supply house will even deal with individuals and no minimum orders is a HUGE advantage for us all! Their shipping is usually very good as well.
9 times out of 10 the bolts on your truck probably don't need anywhere near this level of **** retentive attention, but since you've had a failure once, might as well cover as many bases as we can think of. Flywheels, tow hooks, winches - these areas usually deserve a little extra legwork I guess. I have a PDF of torques that I want to archive here for future reference too.
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This site is the bomb for all things related to steel fastener stamps and strength ratings, for bolts AND nuts!