Body Shims

[David W.]

Hey Guys,

Ok, since my previous question regarding missing shims is getting zero responses I thought maybe I should ask another way.
If you change frames due to damage, how is the best method for determining body shim numbers and placement?
There has to be an easier way to do this than sitting the body on the frame and looking at gaps. I can only imagine how many times this method might take to get it correct.
Is there no way to "measure" (assuming the frame is on a dead level surface) mounting pad height and shim the lowest points up to the height of the tallest pad?

Thanks for any help?

[Harmon C.]

Re: Body Shims

The way I adjust the shims if for some reason I lost the information I should have from when I took the car apart or I bought a basket case is to put two shims on the four center body mounts (#2&#3). With a T-Top their is not much you can do to adjust the door gaps so just try to get the #1 & #4 mounts with the same body pressure on them and tighten them down. If the door gaps need adjustment you can then try to add or take out shims. A picture of the large shim jig the factory used should be easy to find on this site as it has been posted many times with more information.

[Jack H.]

Re: Body Shims

"Is there no way to "measure" (assuming the frame is on a dead level surface) mounting pad height and shim the lowest points up to the height of the tallest pad?"

Perhaps the slow response is due to the fact this question has come up several times before and been answered.

The factory used a precision jig where the frame was lowered to make contact. The tallest mounting pad (sometimes more than one pad are equally tall) touched down first, and unskilled labor simply marked the other areas with the shim count necessary to bring them to equal height.

This CAN be done using a laser based level/transit...

[David W.]

Re: Body Shims

Thanks Lyle. There were few few shims and what there were came from the #2 and #3 mounts.

Regards, David

[Steve L.]

Re: Body Shims

This is the same thing I keep wondering.

At the factory they use a jig on the frame upside down, then stick a wedge in with markings on it to tell them how many shims. HOWEVER, what does the jig use for a reference. It can't be the mounting locations since they could add shims to any of the mounts.

There is something else that the jig must be referencing onto and there must be 3 points to reference to since it requires 3 points to establish a plane.

What I have done on my frame, that I have done extensive refurbashing due to rust, is put a rotating laser inside the frame at about where the gear shift is. Level the laser, turn the motor off on the laser and just aim it by hand. Then I took #2 left and right and #3 right only, to establish a plane. I measured down from the laser to these mounts and moved the entire frame up or down using bottle jacks until these mounting points measure the same. This way I know the frame is level to the laser which is also level. Then I went and measured the other mounts, compared it to the frame drawing. This way I could calculate how many shims to use at the other mounts.

BUT, I don't think this is right. since it assumes that I don't need shims at #2 R&L nor #3 R.

My body is still not on so I still have time to play with this.

My point is, that with todays cheap spinning laser levels, there has to be a way to predict what shims is required without all of the trial and error of putting the body on and then figuring out what shims.

It also assumes that the body rocker channels nor the #4 mounts on the body have not distorted.

Anyone know what the factory jig used as a reference?

[David W.]

Re: Body Shims

Thanks Jack. I did go back in the archives and read a lot of posts. Just thought there might be a "latest-greatest" method for solving this problem.

Regards, David

[Steve L.]

Re: Body Shims

"
The factory used a precision jig where the frame was lowered to make contact. The tallest mounting pad (sometimes more than one pad are equally tall) touched down first,

So this is what I'm trying to understand, "the tallest mounting pad first" but the jig itself must be parallel to something. I could take a jig and make it contact any mounting point first just be angling the jig.

The jig has to be aligning to something first, before it makes contact with the first body mount. To me, it would make sense that it would align with the spring towers and the sombrero brackets since this is where the spring force eventually rest on which then sets the body angle once the shims are in place.

[Kenny C.]

Re: Body Shims

I may be off base here (i typically am) but I think you may be over thinking this. I believe that some assumptions were probably in play, the jig was square and level and that the body mounting were as well. The jig once in place showed a "fairly accurate" number of shims to insert so the body pulled down reasonably evenly. No disrespect intended towards the GM engineers or the hard working labor on the lines but if you look at enough cars you will see that he tolerance was not exactly like a Swiss watch. I think that the process may have been a little less exacting than you are all giving credit for. From a practical point of view for one car you can do a great job with some trial and error adding and subtracting shims to even out body gaps, its not all that hard it just takes a little patience. At the factory they probably weren't quite as exacting given the time allowed and that they weren't building what we see today as a masterpiece to them it was just another car down the line. Again if I'm off base here my apologies.

[Randy R.]

Re: Body Shims

Is the main purpose of shiming to adjust door gaps? I have observed numerous posts discussing various methods of determining the number of shims to use where but I don't recall a discussion of the purpose or is it just assumed that it is to adjust gaps? The arangement of shims that result in the best gaps on my 67 convertible is not what was used originally.

Randy

[Timothy B.]

Re: Body Shims

In the archives there is a much discussion and pictures about this process.

In short, the body is assembled on a flat jig and assumed to be flat/level. The frame body mounts are checked with a large tool that indexed to the frame. The frame is then marked to identify the low mounts with the # of shims to bring it level to the jig.

[Terry M.]

Re: Body Shims

Hey Guys,

Ok, since my previous question regarding missing shims is getting zero responses I thought maybe I should ask another way.
If you change frames due to damage, how is the best method for determining body shim numbers and placement?
There has to be an easier way to do this than sitting the body on the frame and looking at gaps. I can only imagine how many times this method might take to get it correct.
Is there no way to "measure" (assuming the frame is on a dead level surface) mounting pad height and shim the lowest points up to the height of the tallest pad?

Thanks for any help?

Four of the five "free" searches at the bottom of this thread relate to shims and shimming. They might be worth looking at. Just page all the way down.

I seem to remember someone (either John H or Mike H) posting a picture of the fixture that began use in the C3s along with a description of the factory process. Someone also posted a photo of the gauge shim that was used to generate the number that was marked on the frame.

[Steve L.]

Re: Body Shims

The frame body mounts are checked with a large tool that indexed to the frame. .

Thanks, but where do they exactly "index to the frame".

I remember the jig but can't find the pictures. Can anyone point me to it.

[Randy R.]

Re: Body Shims

Thank you for the info. I was very useful.

Randy

[Jeff B.]

Re: Body Shims

The problem that your are trying to get answered is that in order to use a laser transit to do the job, you first have to have both the laser and the chassis level. The laser is easy, but the problem is what to reference on the frame to level it. The factory had the advantage of jigs that accomplished this. I believe that any measurement excercise is useless unless you can get this done. Here is what you can do if you want to spend the time.

I'm not sure about the C3 but with the C2 if you look in the frame section of your corvette shop manual, you will see a detailed drawing of the frame. This drawing references an imaginary datum plane under the frame which is assumed to be level. There is a guage hole in the front section of the frame that is the primary point that establishes this datum plane. On the C2 the measurement from the hole center to the datum plane is 16.05". But you will reference this as Zero as you set the car up to be level. Put the frame on jackstands at four corners. With your laser level measure the height of the guage hole center and zero out the measure stick. This is your initial reference. Now you have to do some math using the other points of reference on the frame. For example, the center of the guage hole (for a C2) is 16.05" from the datum plane and the top of cross member at the rear of the frame is 21.69" from the datum plane. 21.69 - 16.05 = 5.64 inches. So the with your laser you will need to shim the frame so the top of the rear frame cross member is 5.64" higher than the center of the guage hole (+5.64"). There are several points referenced so you cross check these points to get the frame level. Now that the frame is level and your laser in the same position, you can start your measurements of the body mounts. Remember that you should use a minimum shim count of two (2) even at the highest point to allow for adjustment if necessary. Also remember that it's important that the body mounts at the front of the frame dont contact "hard" and hold the body away from the #1 mount. Or you could just set the body on there and shim it up by to fit

[Timothy B.]

Re: Body Shims

Jeff, your method is a good starting point if one wants to spend the time..

All that assumes the body is flat as when built but after it's first life who knows.

The most important thing is to shim the box flat, the rear mount is easy on a coupe and adjustable on a convertible but it's important to have the weight of suspension and car for a proper rear shim count.