Modifications needed for 73 frame from a 69 frame

[Calvin C.]

The fact page on corvettefaq.com, indicates that the only change from 69 to 73 was frame a lowering of the body mounts for the softer body mount hardware.

1. Anyone know how much lower I would have to modify the mount area?
2. There is no mention if there is any modification to allow for rubber front bumpers?
3. And finally, are there any additional bracing that has to be done for a convertble? The 69 frame is a coupe, my 73 frame is a convertible.

thanks
cal

[Joe L.]

Re: Modifications needed for 73 frame from a 69 fr

cal-----

1) I don't know, specifically, the amount of lowering of the brackets that occurred with the change from aluminum puck, solid mounts to the rubber mounts. However, it would be pretty easy to calculate if one had examples of both the aluminum and later rubber mounts. The difference in thickness of the mounts represents the exact amount the brackets were lowered. I have both of these mounts, but it's just too much effort to get at them right now. Perhaps, someone else has some handier;

2) There was no significant modification to the frame to accomodate the 73-74 ennergy absorbing front bumpers and underlying structure. These were, basically, a "bolt-on" type of arrangement. There may have been some holes added, but these could easily be added to a 1969 frame when used for a 1973 application;

3) There is no difference in the frame between coupe and convertible for any given year. There is, however, a difference with respect to underdash bracing and components of the "birdcage". However, none of these are related to the frame. Some cars do have differences in the frame between coupes and convertibles. 1963-75 Corvettes are not among them.

The reintroduction of the Corvette convertible for the 1986 model year (after an 11 year hiatus) did result in a frame difference between coupes and convertibles. For convertibles, and "X" member, bolt-on bracing was added.

For C5 and C6, I believe it was back to the "frame" commonality between coupe and convertible.

[mike cobine]

Re: Modifications needed for 73 frame from a 69 fr

Are you putting a '69 frame under a '73 or a '73 frame under a '69?

The rubber body mounts in 1973 were approximately 1/4 inch thicker than the regular mounts from '68-'72. This is indicated in the August 1973 issue of Hot Rod Magazine in an article on using the soft mounts in the '68-'72 Corvettes to get a better ride. One modification needed for the earlier C3 was to loosen the steering column prior to lifting the body (had to) and then reattaching it after, noting that there were be a new position approximately 1/4 inch lower due to the added thickness.

Since most will notice that the rubber bumper cars sit higher than the chrome bumper C3s, I really wonder if the frame mounts were moved at all. An issue at the time was the required bumper height and raising the body 1/4 inch on the new mounts would have helped with that issue.

Realistically, would Chevrolet make rubber mounts too thick and have to fix it by redesigning the frame? I doubt it.

If you have both frames, a ruler and a straight edge will answer the issue quickly.

Joe answered #2 and #3.

[Joe L.]

Re: Modifications needed for 73 frame from a 69 fr

Mike------

As a matter of fact, SERVICE frames, right up to the last available, were the same for 69-74 Corvettes. This strongly IMPLIES that the body mounting brackets were in the same location for all years. Of course, it's possible that the later frames did have a lowered bracket location and were supplied with some sort of "spacer-adapter" for earlier applications. In this manner, the later frames could be made "backward compatible" for earlier models. However, that does not mean that the earlier frames would be "forward compatible" for later models. The part numbers for the frames were changed several times over the years.

[Jim T.]

Re: Modifications needed for 73 frame from a 69 fr

Have the boxes of rubber mounts. Part #334756 is the only rubber mount used for coupes and out of the box measures 2 1/16 inches. Part #334755 is 2 3/16 inches measured from right out of the box. These are the uncompressed heights. The #334755 rubber mount is used in only four locations on the convertible.