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Hi all, after checking over the work by an exhaust shop on modifying my Y pipe yesterday, I notice a bolt missing which goes through a tab from the rear of the k member and into the frame. It looks like this may have been awol for some time. As you can see the bolt hole in the frame has become partially obscured as the k member has shifted slightly. As I have used up my car time credits this week, hoping one of you knowledgeable folk can tell me the bolt diameter and thread pitch and length that is needed for this so I can get something suitable ordered?
I cannot find any torque or other details for this in the Haynes manual. And some quick searches here and at parts suppliers were fruitless.
Cheers.
Last edited by Gascan; 01-14-2022 at 05:47 PM.
Reason: moved pics to bottom
The one on the other side is the same. Take it out and get something similar. The rearward bolt gets tightened to 70 Nm (52 lb-ft) torque. The other bolts get tightened to 90 Nm (66 lb-ft).
In the wise words of Elmer Fudd, "Be vewy vewy careful" when installing the bolts. They thread into captured nuts inside the frame and cross-thread super easy at the start.
You might be able to tweak the position a bit by loosening the 6 bolts of k-member and prying it around while you HAND THREAD the bolt in place. I never tighten any of the bolts until all 6 are in place for reasons like this.
Thanks for those torque values, yes it seems obvious but I was a bit worried with ending with two misaligned bolt holes completely removing the LHS one, that's kind of the way things roll with this car, something that should take 15 minutes always seems to escalate, and I get the feeling this is going to be more of pain than it ought to be ;-) I just fitted a wonder bar over xmas and it was off by a couple of mm. I do wonder if the car had some accident damage prior to my ownership....It does have a patch weld on the LHS "Rail" which I presumed to be from the steering box issue with these cars, but maybe not.
Anyway, I feel a lot more comfortable messing around with it now I know the correct torque settings to put everything back to. Much appreciated thanks!
You'll be fine, you're asking all the right questions.
When I say "loosen" the K-member bolts, I mean just a couple turns. Just enough that the K-member barely separates from the frame to release the friction so you can bump it around a bit. You won't be able to do much though because the K-member is located with pins that poke up into the subframe.
Just for safety sake, you maybe should pull out each bolt one at a time (while leaving the others tight) just to inspect how much thread engagement there is. If your car has been messed with then you never know if somebody put in a short bolt. I wouldn't want you to loosen all the bolts a couple turns and then find out the K-member drops to the ground!
Those linked threads suck and don't come to a real conclusion.
Proper tightening torque of a bolt is not a mystery in most cases, we've got standards that can be referenced when in doubt. My stock k-member bolts are sitting on a shelf so I measured them for you...
* All bolts are M12-1.75, verified with my thread checker tool.
* The four long bolts are ISO Class 10.9, and the two short bolts are ISO Class 9.8.
* Approximate dimensions are shown in picture below, measured with my caliper. I made that graphic from a picture of my bolts.
A lot of the torque values you see on the internet are maximum allowable, not what you should do. I have access to some of the best engineering standards in the world and it's not something you're going to find on the internet. And the specs I reference are generally very similar or same as what GM uses.
* 100 Nm standard torque for M12 Class 10.9 bolt when no other torque spec is given.
* 75 Nm standard torque for M12 Class 8.8 flange head bolt.
* I don't know the spec for a Class 9.8 bolt but it is probably somewhere in between.
Now the GM specs I quoted earlier are very close to those values and seem very reasonable. It passes the laugh test. And GM might have intentionally lowered the torque a bit because the abutment material (K-member) can't handle larger values. The K-member is stamped steel and maybe yields under the bolt which is maybe why people are reporting loose and missing bolts after some time.
So why do the later service manuals have much greater torque? I can't tell you without knowing more about the bolts used on those cars. Maybe it's a typo. Maybe it's not. Maybe we're not seeing the whole story on that one page. But you sure as hell don't want to tighten a normal M12 bolt to 160 Nm. DON'T DO IT!
I remember the instructions with my BMR k-member said tighten their bolts to 130 lb-ft (176 Nm). I blindly followed it and broke one of the bolts during install. Well no sh*t, that's way too much for an M12 bolt. Ever since then I changed my habits to always double check against trustworthy standards.
Thanks for the extra information - before I saw this, The boss wanted to go shopping today, so not wanting to miss the opportunity I removed the LHS bolt to measure it, and it is slightly different to yours, still a M12 x 1.75 (checked with my thread chaser set) but the threaded length is only 35mm, it is also stamped 10.9.
At the two hardware shops we visited they only had 8.8 grade, but figured it was better than no bolt, so bought a pack. Once home I slackened the other bolts off and was able to pry the k member just enough to expose the thread. Cleaned it with a chaser and bolted up no problems, tightened them all up as per your spec.
I did find the middle bolts required little force to undo compared to the fronts considering they are supposed to be the same torque.
All good now, will see if I can get a duplicate of the LHS bolt at some stage.
Those linked threads suck and don't come to a real conclusion.
Proper tightening torque of a bolt is not a mystery in most cases, we've got standards that can be referenced when in doubt. My stock k-member bolts are sitting on a shelf so I measured them for you...
* All bolts are M12-1.75, verified with my thread checker tool.
* The four long bolts are ISO Class 10.9, and the two short bolts are ISO Class 9.8.
* Approximate dimensions are shown in picture below, measured with my caliper. I made that graphic from a picture of my bolts.
A lot of the torque values you see on the internet are maximum allowable, not what you should do. I have access to some of the best engineering standards in the world and it's not something you're going to find on the internet. And the specs I reference are generally very similar or same as what GM uses.
* 100 Nm standard torque for M12 Class 10.9 bolt when no other torque spec is given.
* 75 Nm standard torque for M12 Class 8.8 flange head bolt.
* I don't know the spec for a Class 9.8 bolt but it is probably somewhere in between.
Now the GM specs I quoted earlier are very close to those values and seem very reasonable. It passes the laugh test. And GM might have intentionally lowered the torque a bit because the abutment material (K-member) can't handle larger values. The K-member is stamped steel and maybe yields under the bolt which is maybe why people are reporting loose and missing bolts after some time.
So why do the later service manuals have much greater torque? I can't tell you without knowing more about the bolts used on those cars. Maybe it's a typo. Maybe it's not. Maybe we're not seeing the whole story on that one page. But you sure as hell don't want to tighten a normal M12 bolt to 160 Nm. DON'T DO IT!
Interesting, the shorter M12x1.75 that came out of my 91 firebird was 35mm long.
this is what I'm using Fastenal Industrial Supplies, OEM Fasteners, Safety Products & More
EDIT: Another difference that I just noticed is that the shorter bolts from my 91 are class 10.9 bolt like what Gascan seen.
Last edited by 91banditt2; 01-20-2022 at 07:04 AM.
01-14-2022, 05:45 PM
Ever since then I changed my habits to always double check against trustworthy standards. 
