C3 - Body Shim Question

[Jim Markevitch]

I'm in the process of doing a frame-off on my 72 Coupe. Both rails have been replaced as well as #2 & #3 body mounts. When I put the body back on, do I still use the same number of shims at each mounting location as I found upon removal? The frame was repaired by a qualified body shop on a frame jig so it is "on the screws". I'm assume the body had the imperfections that required the use of shims in the first place. Is that correct?

Thanks in advance.

[Wayne P.]

Re: C3 - Body Shim Question

You'll have to shim to adjust the body fit. Highly doubtful the frame is now the same as the factory. That original shim count might be a good place to start and then adjust as needed from there.

[John H.]

Re: C3 - Body Shim Question

Originally, the body was considered "on the money" dimensionally, as it was assembled on a precision-machined steel truck with the underbody bolted down to the steel pedestals; the shims dealt with dimensional variations in the frame to create matching vertical datums between the two.

[Jim Markevitch]

Re: C3 - Body Shim Question

Ok, so how do I view the shims at #2 & #3 position to know if I need to add or remove shims? Am I missing something very basic?

Again, thanks in advance.

[Chuck R.]

Re: C3 - Body Shim Question

I think it's all about door gap alignment all the around especially if the nose is still attached to the body........I think

Then it's a matter of lift and either place or remove as needed.

I'm going to be watching very closely for additional posts as I'm going to be going through the same process in the fall (I hope).

Regards,

Chuck 32205

[Mike McKown]

I've never shimmed a Corvette body, but

If I had, I would start with three shims each at the "A" and "B" pillar points. If the body sat level to the frame at this point, as indicated by the other mounts, I would go to the front and shim to take all the strain off the front end by shimming the gap out plus 2 mils . Next, I would go to the rear and shim the gap out of the door/Bpost margin and make it parallel, top to bottom and also get the right height of the pillar to the door, and then add maybe two mils or whatever it took to take the strain off things. In other words, start in the center and work your way out to the ends, just like you torque an intake manifold down. You may, at some point, have to remove/add shims at the "A"/"B" pillar

As John says, the body was built, netted off the underbody. This means the underbody is theoretically zero heightwise minus any reefing done over the years due to an improperly shimmed frame and/or distortion due to fatigue of mating parts.

I'd like to add, if this was my project, I would have the engine transmission in the chassis and full body trim loaded to the body when I did the shimming. Some body frame combinations I am familiar with (not Corvette) will really do some wierd things when you load the weight to the chassis and cause it to flex all over the place which would render the shimming useless if done with a bare frame and body. A Corvette may not react this way, I don't know.

[Dave McDufford]

Re: C3 - Body Shim Question

My observations from just putting a body with a new front clip on a newly rebuilt frame.

The shims are 1/16th of an inch. Tightening the bolts and/or the weight of the body will take up the slack of a couple of shims (+/-) without any problems at any of the body points. So looking to see how much space exists after the body is on is of limited use.

I used the shims to set the door gaps to between .12" to .24" (if I remember correctly) measured with a dial caliper. I started with my best guess by measuring the height of each body mount above the floor (with side rails level) compared to the frame dimensions in the shop manual. From this point after a few tries (adding body and door shims and moving the door on the hinge) I added two shims to the left #2 body mount (convertible) and one to the right #2 body mount. This widened the door opening so both doors fit well and the gaps were even and within spec on both doors and the doors were flush with the body (door shims). The right door is a couple of hundredth closer but not enough to add another shim. I was surprised at how easy it worked. Each shim at the #2 mount seemed to add about .05 to .10 to the total door gap.

As part of this process I also refit the bumpers (they mount through the body) to make certain they fit and were level within the AIM spec.

It sounds harder than it is, but it takes a while. Hopefully it will fit the same for the final assembly.

Dave

[Jim Markevitch]

Re: C3 - Body Shim Question

Thank you all for the help! What confused me is that on pictures of frame restorations with all the markings you always see the number of shims required at each mount location marked in crayon before the body ever was installed. The corvette plant must have had some kind of system to determine the number of shims. I figured if they could determine the number of shims before the body ever reached the frame so could I. Not that lucky I guess.

Again thanks.

[Jack H.]

Re: C3 - Body Shim Question

They had what was refered to as the 'geo' machine (frame geometry). This was a high precision fixture that obviated the need for traditional alignment tools and high skilled labor.

Load the frame in the geo machine and appropriate 'spaces' appeared.... Non skilled labor simply 'filled' the exposed holes or used crayon to indicate the shims necessary. Look through pictures of the final assy line and you'll note the ABSENSE of ROWS of Bear alignment machines....

The geo machine also mimiced the weight of body/power train to compress the springs in the suspension and 'open' the front A-frame gaps so unskilled labor could simply stuff the appropriate number of shims in to set camber/caster....