Body Shims

[Mike K.]

When changing frames, how do I know the number of shims to use at each new body mount?
Are the shims used because the body is uneven, or is the frame uneven?

[Jack H.]

Re: Body Shims

Shims are used because there are minor differences (frame to frame) in the EXACT height of each body mounting pad. The shim count is what the factory originally determined to be the number of shims required to bring each of the pads up to the height of the TALLEST mounting pad in order to form a level platform for the body to rest on.

If your donor frame had its factory original shims intact, use those. If the donor frame was purchased TOTALLY bare, then you're going to have to 'determine' the correct number of shims to use at each mounting pad.

The factory had a special machine to automate this process. Essentially, it was a controlled plane that was lowered until it made contact with the tallest pad. That left air gaps at the remaining pads for unskilled assy techs to go measure and chaulk the adjacent frame rail with the shim count needed to fill the gap(s)...

There are prior threads in the archives about how you can approximate this process. But, they boil down to one of two methods:

(1) Devise a precision measuring system using the equivalent of a laser transit and re-create the shim count(s).

(2) Go 'seat of the pants' and lower the body onto the frame, do a walk around and visually determine (based on body line + door gaps) where you need to shim and what number of shims to use.

[Tracy C.]

Re: Body Shims

Mike,

Here is how I solved my shim count mystery....with the frame supported by the cars suspension on the ground, I put some modeling clay bars sandwiched between tinfoil at each body mount location. It is important to start with equal thickness of clay at each location. (I used 1/2" square bars)

I then placed the body on the frame and let it sit overnight with just the weight of the body pressing down on the clay bars (no bolts were used). The next day I lifted the body and removed the clay/foil sandwichs from each body mount and recorded the location they came from.

I then used a 0-1" micrometer to measure thickness of each sandwich. When I found the thinnest sandwich I extablished that location to be "ground zero" and I subtracted it's thickness from the thickness of each sandwich from the remaining mount locations. I then calculated how many shims it took at each location to reach a comparible delta thickness. (the ground zero location recieved a zero shim count)

This method is fundamentally similar to using plastigage to establish bearing clearances during an engine rebuild. It will result in an easy way to get a close count on body shims at each location. I expect that there may be some final adjustments made to obtain proper door gaps, but it certainly beats a byguess and bygolly approach when you have no idea.

good luck,
tc