Frame Shims

[Michael H.]

Re: Frame Shim Checking Fixture

Kevin,

This was the St. Louis Corvette assembly plant in the late 70's or early 80's. All Corvette engines were blue at that time. (I'm told) I have no idea exactly when they went to blue engines from orange. It was called "corporate Blue" as I remember.

Michael

[Tom R.]

1977

Some early 77 engines had orange but most were the corporate blue...in 82 sometime went to black engines.

[Mike McKown]

One logical reason I can give for not checking

80% of the frames coming down the line is that this "shim predictor' process was probably not all that accurate. It didn't matter whether the frame was right side up or upside down. Frames, on a static check fixture will check one way. When they are loaded with body and engine weight, plus gas, water, etc. and then allowed to do their thing with the flexibility of the springs they can go anywhere. Maybe, predictably in some cases but not what a static check will illustrate. With the frame upside down, this check would be totally laughable, right side up, could be close if suspension was compressed to design height (taking into account the flexibility of the body and frame under load) but this doesn't appear to be the case.

I don't know how Corvette bodies were built but many bodies are built using four of the body mount surfaces to zero body elevation. Frames on the other hand are pinned in the side of the frame rails and from that, the rail is formed. From that point, the body mounts are welded to the rails. Well, you can see if your master locators on the frame are in the side of the rails, it can have little relevance to where the body mounts actually wind up. Maybe +/- 1/4 inch in some cases. Also, do not forget to throw in other variations such as spring hanger loctation, spring pocket depth in the front cross member. Lower control arm variations, etc.

It only takes a fraction of an inch on some component to cause your pride and joy to sit cock eyed like many complain of.

The new process of hydro-forming the frame rail on the C-5 eliminates a lot of this variation.

By the way, when I started to work in the automotive world, they walked me from the employment office to a job on the frame line that had a fixture just like Michael's picture, just prior to body deck. We called it a shim guage. For some reason, four or five times a shift, the boss would come back and tell me to ignore what the gauge said, just "do it this way" Hmmmm. Wonder what he meant? Also, the frame shimming process with the check fixtue usually only lasted on pre-production models through the first month of regular production. Discontinued until the following year with a new model launch. For the eleven months in between, it was trend shim for door fits. The fender fits were controlled by fender fitter who shimmed under the radiator support to get the fenders to fit the door margins.

If I were to re-shim a body off restoration, I wouldn't even think about counting the shim pack that was in there originally. I believe I would start with a shim pack of about 1/4" on the mounts at the A and B pillar and then shim the rest of the body to fit, leaving no gaps and no excess pressure on any body surface.

Michael, nice pictures.

[Michael H.]

Re: One logical reason I can give for not checking

Mike,

You are exactly right about the difference in results in shim thickness when measuring a "loose' frame upside down compared to one that is loaded and in design position. I posted a few bits about this earlier and was patiently waiting for someone to ask about it. I thought the changes that were made were interesting.

The original location for frame shim check was at the very first position on the chassis conveyor and was done while the frame was still upside down. I can try to find a pic of the frames moving sideways, right to left, while still upside down an all have the shim markings on them. It was checked while no load was present as obviously the engine/transmission was not installed. It was discovered that there was some frame flex occuring as the engine/transmission assy was installed and this was causing problems with door and hood fit. Gm was receiving many complaints from the field in the 60's and a fix was ordered.

The biggest change was to try to simulate the loaded chassis as closely as possible. This was done by first moving the check station to the other end of the chassis line, just before body drop, and making this check with the chassis as close as possible to being complete. The assy was now supported at the front lower control arm and the rear at the rear control arm. It was determined that adding other items such as spare tire and typical curb load of fuel had no effect on the dimension.

The Corvette body had a unique problem that made it much different than the average steel passenger car as far as flex, especially a conv. If you've ever inspected a Corvette body that was on a four point body dolly, you would see that the front and rear are drooping badly. This dim can be as great as 1/2" in the front. The entire nose will drop down and any form of body fitting is impossible. That's why it's really not possible to accurately reshim a body while just supporting it from these four center mount locations.

I've been involved assisting with many frame changes and about three or four out of ten come even close to being shimmed properly if the original shims are not available. Some are so bad that the doors won't even close no matter how they are adjusted. I agree, the fit of new 63-67 cars was not always great but at least they had a basically correct platform to start from.

As far as the method that was used at the mill to produce the body, it was on a very rigid steel body truck that locked all of the body mount points precisely to design height. The body truck exactly reproduced the dimensions of the corrected (shimmed) frame. That means that whatever came off of that body truck would (hopefully) fit exactly the same when installed on it's corrected new chassis. If it didn't, it wasn't likely that the problem was caused by incorrect shim stack height. There were a lot of other reasons that the doors and hoods at times didn't even come close to fitting properly. If body induced, most were caused by bond that "kicked' a little sooner than it was supposed to and some panels weren't fitted to their exact location. (I have one such example)

One last problem that I remember was the little pile of stray shims that was always sprinkled around the body drop area. Seems there were frequently some that were knocked off the body mount as the body was installed. This condition was not immediately correct on the line but was sent to "repair" for shim replacement. I'm certain the correct amount was not always added there.

I obviously didn't watch all of the frames go through that place but I've sure watched a bunch of them. I've never seen one skip the frame dimension check.

Thanks Mike,

Michael