I picked up a friends 96 ZJ for a song. In the process of fixing DW I replaced the trac bar. The OEM frame end bolt is a 12mm 10.9 bolt. Specs call for 72 ftlbs of tightening torque. That bolt can be replaced after a slight redrilling with a 3/8 G8 fine thread bolt which is said to have like strength, yet typical torque values from every mfg'er I looked up for a bolt of that spec are only 50 lbs or there abouts. What am I missing here?
Bolt Torque Question
- Thread starter Gilaguy23
- Start date
Are you sure you have your numbers correct? 3/8 is smaller than 12mm by a good bit.
You are missing that 12mm is just slightly under 1/2" and would easily take 75ish foot pounds of torque for an appropriate grade fastener. The 3/8" being slightly smaller than 10mm will not do that. There is no redrilling required to use a 3/8" bolt in place of a 10mm since .375 is a smaller diameter than the 10mm at .393.
12mm is .472, 7/16" is .437 so 12 is larger than 7/16 and slightly smaller than 1/2" at .500.
My guess is since he is drilling out the 12mm (about 1/2”), the OP is replacing it with a 9/16 or 5/8 g8 bolt.
My guess is that we are all guessing.
You guys are correct as I missed the mark on the OEM bolt diameter. A 3/8 is a few thousands larger than the OEM bolt on this thing, my bad on that one. I see several kits that offer a 9/16 G8 bolt, however on this early GC if the bushing was drilled to accept a 9/16 there wouldent be much left of the inner steel bushing. I should have used my micrometer on both for accurate diameters, but the 3/8 will not go thru the new Trac bar.
You've got something else going on unless they are using a very oddball 9mm bolt and I have never seen one.
I have the old one I pulled off a few days ago. I'll take a pic and and show the OEM bolt as well. Scary small IMO especially when it DW's. I yanked the stabilizer and the DW cycling was so fast and hard I could not turn the steering wheel and had two wheels off the pavement in a corner. Nothing like the TJ. This poor thing has seen 30 miles of wash board roads 3-4 days a week for the last 5 years on the PO's work commute. Kinda shocked its in as good of shape as it is underside.
Geez...I think sometimes I'm a in line for early Alzheimer's, no joke as my dad seemed to fad away a bit early and I past his age of slipping a bit. Regardless, yes I screwed up. A 3/8 is smaller then the OEM bolt. A 7/16 fine thread is what I will upsizing to which calls for @ 78 ftlbs torque. Looks like I answered my own question. 
Geez...I think sometimes I'm a in line for early Alzheimer's, no joke as my dad seemed to fad away a bit early and I past his age of slipping a bit. Regardless, yes I screwed up. A 3/8 is smaller then the OEM bolt. A 7/16 fine thread is what I will upsizing to which calls for @ 78 ftlbs torque. Looks like I answered my own question.
If the OEM bolt was 12mm, than the 7/16” bolt is slightly smaller. How is that upsizing?
Without getting into the whole deformation before failure discussion Grade 8 is still stronger than Grade 5 by a fair margin.
Never. Mainly because it is not a shear application. Even then, a harder bolt is stronger than a softer bolt. If your argument is that the softer will bend before it breaks, what you missed is it will bend long before the harder bolt even begins to get into force levels where it can be broken. If it is a bolted connection with a nut, then the higher grade bolt will develop far more clamp load and prevent the connection from slipping to bring the shank of the fastener into shear.
To add to Mr. Blaine's and Blackjack's great responses, the grades for USA fasteners are specified in Society of Automotive Engineers (SAE) Specification J429 and the grades for metric fasteners are specified in SAE J1199. Those specifications are copyrighted, and SAE is protective of that copyright. As such, my price as a long-time member is about $87 for each one (I get a whopping 10% discount for having ponied up dues, which are over $100 per year currently, for almost 40 years!)
I left the automotive engineering world in 2007, so I no longer have access through an employer to those standards. I'm going off very old memories, but I don't believe that the mechanical properties in those standards covered shear strength (in psi). They only cover tensile properties (proof strength, yield strength, tensile strength, elongation). That said, for ferrous steels, most shear strengths are in the 50-60% of tensile strength range. In other words, shear and tensile strengths are roughly proportional. The J429 spec for the tensile strength of Grade 5 and Grade 8 bolts is 120,000 psi and 150,000 psi, respectively. So, a Grade 8 bolt is nominally 25% stronger than a Grade 5 bolt in both shear and tensile.
Additionally, the way to analyze the "deformation before failure" aspect is to look at what percent of tensile strength the yield strength is. The closer to 100%, the more brittle it is (glass is effectively 100%). For Grade 5, that percentage is about 77% and for Grade 8 it's about 87%. That's a significant difference, but not so much that Grade 5 should always be chosen. We don't inspect our rigs often enough for deformation to be a factor. Deformation is usually a factor when tensile strength isn't and the joint is highly visible. In this case, tensile strength is more important.
I left the automotive engineering world in 2007, so I no longer have access through an employer to those standards. I'm going off very old memories, but I don't believe that the mechanical properties in those standards covered shear strength (in psi). They only cover tensile properties (proof strength, yield strength, tensile strength, elongation). That said, for ferrous steels, most shear strengths are in the 50-60% of tensile strength range. In other words, shear and tensile strengths are roughly proportional. The J429 spec for the tensile strength of Grade 5 and Grade 8 bolts is 120,000 psi and 150,000 psi, respectively. So, a Grade 8 bolt is nominally 25% stronger than a Grade 5 bolt in both shear and tensile.
Additionally, the way to analyze the "deformation before failure" aspect is to look at what percent of tensile strength the yield strength is. The closer to 100%, the more brittle it is (glass is effectively 100%). For Grade 5, that percentage is about 77% and for Grade 8 it's about 87%. That's a significant difference, but not so much that Grade 5 should always be chosen. We don't inspect our rigs often enough for deformation to be a factor. Deformation is usually a factor when tensile strength isn't and the joint is highly visible. In this case, tensile strength is more important.