My 73 AIM says to torque all body mount bolts to 40-50 Ft Lbs.
I also read through some older forum threads where they discuss that the first factory torque setting is 40-50 then after the car is driven the body mount bolts are all loosened and then torqued to 25 Ft Lbs.
It is referenced in the AIM Notes and called “the roll out torque”.

I’m getting ready to re torque all my new body mounts and I’m concerned about torquing the bolts to 40 Ft Lbs, especially #2, #3 and #4 bolts which as you know are going into 50+ year old captive nuts.​​​​​​

​I feel really comfortable torquing them to 25 Ft Lbs, but not anything over that.

What are you guys really torquing the body mount bolts to, or are you torquing them at all?

Thanks in advance!
Greg

 

I think you need to consider the effects of compression on the cushion. I just looked up "compression of an elastomer". Lots of science! I think it shouldn't be so loose you don't get some pushback.

Plus the mass of the body vs. the frame produces some kind of dynamic going over bumps and things. Up and down, up and down.

Again lots of science if you like it, but that's just me. A random sample below - I would find out what sort of material you are dealing with and go from there. Myself I elected to use the aluminum spacers, there are no physical properties worth knowing.

https://rogerscorp.com/blog/2024/eve...foam-materials

 

Well when I did all my body mounts. My thought was these 8 bolts hold the body onto the frame. Not wanting the frame to go around a corner and the body to keep going straight.
I torqued those baby's up! All the way up. 50 plus foot pounds. No messing around. I used new bolts of course and had cleaned all threads with a fresh tap.
Undertorque a body mount??? Not this guy.

 

It would seem that over-tightening the body mounts would lead to more "cracking" in the fiberglass body and potentially a very uncomfortable ride. I am not sure if this holds true in the real world, but with a steel frame and a fiberglass body, I would want some play (give) in-between the body and frame to allow for some cushioning action. A compressed body mount would not do much to make the ride better or dampen the hard jolts that could damage the body itself.

The early C3's like my 1968 C3 had metal spacers used in place of the rubber body mounts and from what I have read, that caused more spider web cracking than the later years with their rubber (cushions) body mount bushings. My car was switched over to the rubber body mounts by a former owner. He owned a Body shop in Maine and fixed the damage to the body caused by the previous owners before painting the car Daytona Yellow instead of the Safari yellow that my car came in.

Tightening it up to 50 ft. pounds torque and then backing it off to 25 ft. pounds might help getting the parts "centered better" using the higher torque. Then by reducing the torque on the mounting bolts to 25 ft. pounds the body is able to have more cushioning which will help the body last longer. The looser torque number would relate to a better ride in the Corvette.

Personally I would likely tighten them up to the 25 ft. pounds as I am getting older and no longer desire the ability to determine whether the coin I ran over was a nickel or a dime, let alone the heads or tails of the dime.

 

Tighten all to 2 grunts. Then do again one more time.

 

I watched all the body mounts and how much they squished as I tightened them.
The bolts are all tight but not torqued and the rubbers is all squished pretty compacted and the difference in torquing the bolts to 40 ft lbs over 25 ft lbs won’t achieve much.
I’ll torque the bolts to 25 ft lbs and see how that feels.

 

The idea of undertorquing for a better ride is a mute point. The body mount bushes have steel sleeves that limit the amount of rubber compression. So, once the upper and lower washers have contacted the sleeve. That's it. No matter how much torque you put on that bolt you are not compressing the rubber anymore.
However, a 7/16" Diameter bolt is DEFIANTLY under torqued at 25 pounds. Under torquing a bolt is not good practice as it can lead to bolts coming out. Thus my statement about the frame going around a corner and the body going straight.
I agree with above, at least 2 grunts and then go over them. This is a really, really bad place to leave bolts loose.

 

I can feel that the washers and the sleeves are in contact with each other and fully understand that the torque on the bolt has nothing to do with the ride or squish of the rubber.
I also agree that torquing the bolts to at least 40 ft lbs would be best in the perfect world.
However my concern is the age, structural integrity, condition of the captive nuts and how many times over the years these nuts and nut plates have been previously torqued.
The rubber body mounts I removed from my car were not the originals and we all know how some shops operate.
Nuts and bolts should only be torqued once and once only.
Torquing any bolt or nut twice is beyond the engineered design of both fasteners and their mating threads.
The last thing I want is to take the torque over what the nut’s 52 year old captive cage will handle.
If the cage containing the nuts fail and any of the nuts spin, I’ll be pulling the body back off and starting over.
I’ve been torquing fasteners all my life and can feel when it feels right and know when to stop if something doesn’t feel right.
If I feel 25 ft lbs feels right and is as far as I’m willing to torque the fasteners, I’ll stop there.
If I feel they could get snugged a little more I’ll go up to 30 ft lbs and check the feel.