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When I first got the car I noticed all of the six crossmember bolts were either loose or not as tight as they should be (118 lb-ft). I torqued them back then but recall that one or two seemed to be stripping (maybe I was crushing some grease and dirt) and I did not torque them all the way down as I was afraid of fully stripping them. I will be checking them again when I go under the car next time but I was also thinking why not have a shop weld it up to the chassis. I can't think of a bad reason as I have no plans to remove it ever.
When I first got the car I noticed all of the six crossmember bolts were either loose or not as tight as they should be (118 lb-ft). I torqued them back then but recall that one or two seemed to be stripping
I don't know where you got that torque value from but it's horribly high. Front and middle bolts are tightened to 90 Nm (66 lb-ft), and the rear bolt is tightened to 70 Nm (52 lb-ft).
All your bolts are now ruined. Throw them all away and buy new. Honestly, I'm surprised the rear bolts didn't flat out snap off. Hope to heck that you didn't ruin the captured nuts inside the frame rails.
That graphic doesn't pass the laugh test. Standard torque for an M12 bolt is 100 Nm, High torque is 105 Nm, and Low torque is 50 Nm. 160 Nm is waaaaaay out of bounds.
Actually I think your engine is about to fall out.
160 Nm is correct. My 92 service manual tells the same torque and I torqued my (new) bolts all to 160 Nm without any problems when I reassembled my engine bay two years before.
105 Nm in the '88 Firebird service manual, Page 3C-10. Same number as the High torque value I mentioned earlier, which is typical of M12 bolts used in structural joints.
Well, back to the original question, I've built several cars for myself as well as others using M2 kits, always boxed the frame and welded the kits in, the longest running one I still have is my '36 PU with Heidts SR2, I have it torn down now, maybe 80K miles on it, other than the paint faded on the chassis, looks just like it did when I installed it.
So, can you weld it in? Depends on the frame, how the frame is structurally and if the pieces are made correctly to work with being welded in. Rivets and bolts allow some flex, welding doesn't. When I weld in a front X-member, I fully box the frame and usually use a tri-bar rear set up. I let the front and rear suspension do the work.
When I put in a X-member and I cant/don't want to, get to everything, I bolt it in, I'm not relying on the bolts allowing movement, but as the frame is working under twisting/etc forces, bolts just never crack like a weld can...well, not the weld normally, but the metal along side of it.
All these different documentaion examples are odd.
Personally, I just can't buy into that larger 160 Nm (118 lb-ft) number. For one thing I can't make sense of it -- and we should be able to make sense of it.
And I can also tell you my '89 rear bolts (that supposedly uses that huge number) literally snap off before reaching 130 lb-ft. I found that out when blindly following some bad BMR k-member instructions. Made total sense once I started thinking and not blindly following instructions. There is only 12 difference between 118 and 130, and I never even made it to 130... Seriously, think about what that means.
This is beside the obvious that Tootie is stripping the captured nuts on his car.
Bottom line is a GM service manual doesn't actually decide the correct torque spec. The bolt does. You don't even need the service manual to know the correct torque spec for 99.9% of hardware on your car.
Proper torque to be applied in structural joints depends on many things. You are ultimately actually aiming for the proper bolt stretch so that the bolted joint in the end is secure. Some of the items that influence the actual torque to be applied are the bolt material grade, mating thread properties, applied lubricant, etc. An example of how torque value would change for a given joint would be if the nut factor was designed with a "dry" bolt application in mind (ie only the bolt as received finish and you apply anti seize to it. It is easy to imagine how the coefficient of friction has now dramatically changed and the torque required to get the proper bolt stretch in the same joint will be much less in the anti-seize applied example versus the dry application. This is one reason why many critical engine fasteners now utilize a torque angle method or in the case of ARP they dictate their nut factor by supplying the lubricant to be utilized in fastening the joint with their bolt or stud/nuts
In smaller fasteners and low grade applications, typically the torques are low enough that everyone gets away with some amount of over torquing. An M12 isn't exactly a large bolt, but you can see by even material properties below for a dry joint has a wide range of typical torques depending on joint design based on fastener material properties. You can see if the bolt is an 8.8 the recommendation is around 100 Nm vs a grade 12.9 at 160 Nm.... again depends on the bolt, lubricant, mating fastener properties, etc what is the proper design. Did GM change bolt grades during the years???? Also on these bolts would be very careful on lubricant application to avoid breakage
07-31-2020, 10:00 AM
