C3/C2 Differential Crossmember Reinstall

[Howell Jaynes]

I removed the differential crossmember and replaced the rubber bushings. The new rubber bushings appear to be repro's. They look like the OEM's but were missing GM part numbers. When I went to reinstall the crossmember, I could not get it mount flush up against the frame. The bushing "sockets" jam up against the frame bell shaped "plug" about 3/8 of an inch short of of bushing/frame contact. I would imagine many C3/C2 owners have replaced their old bushings with these repro bushings and had the same problem. Any solution?

Actually I torqued the crossmember bushing to frame attaching bolts up to 85 foot pounds and still could not get the bushing to mate to the frame. (55 ft-lbs is spec value). I even tried to remount the old bushings and they also similarly jammed with about 3/8 inches let to go, even though they previously had been flush to the frame. The bushing supplier said to just mount them with the 3/8 clearance remaining. Will driving the car force them into position? I've considered reaming out the bushing "sockets" but I can imagine if I get them too big, the crossmember will be loose. Did the factory use some sort of a press to install them originally. Did torqueing the original bushings achieve a flush mount with 55 ft lbs of torque? Thanks

[Wayne M.]

Re: C3/C2 Differential Crossmember Reinstall

As I recall, the (factory) rubber bushings never contacted the frame. Remove each bolt (separately, supporting the X-member end for safety) and see if there is an air gap between the metal of the rubber cushion and the frame (threaded) sombrero./ If no gap, means that's all she wrote.

[Chuck R.]

Re: Agree with Wayne

I applied a very light coat of antiseize to the inner surfaces of the rubbers and they pulled right up.

I then did as Wayne stated and pulled the bolt to confirm that it had traveled the full distance. Sure enough, it had mated up nicely.

By the way, I called Permatex Corp. just to double check that their product wouldn't have any adverse affects on the rubber mounts prior. They confirmed that there would be no issues.

Chuck

[Howell Jaynes]

Follow-Up Bushing Observation

Thanks. I was relieved to understand from the messages that the rubber top of the crossmember frame bushing does not have to contract the frame. I went out and looked at my 1970 Corvette to compare it with the 1968 I'm now working on. Sure enough the 1970 crossmember bushing rubber top had an air gap of a little less than an 1/8 inch with the frame. On my 1968, before I removed it, the rubber was flush against the frame. With my repro bushings now on the 1968, the gap is about 3/8 of an inch. As I said, the vendor said this was OK. But now based on the messages and my follow-up observation of the 1970, I feel better. I plan to use a rotary sandpaper wheel to try to open up the metal cup portion of the repro bushings to get them to mate up a little closer to the frame. I will use something - silicon or the permatex anti-seeze to prevent metal to metal corrosion. Thanks again.

[Mike Cobine]

Re: Follow-Up Bushing Observation

I would be very careful about opening anything up. The crossmember is not to be in contact with the frame. There is between 3/8 and 1/2 inch gap between the crossmember and the support cups on my '63 and '79 with original bushings. You need some room for the deflection of the rubber bushings under extreme load.

If the gap is equal on each side, that is important. The car will settle to whatever height it will take. As long as the cups go in most of the way and you have many turns on each bolt holding them, then you are good.

Now if all you have found is a burr that is holding the cup out, then that is different and you can remove that. But cutting them to fit better may well place them too loose or in to load the wrong part. The load is carried on the sides of the cup, not so much the bottom. That is because it is important to bear the lateral stresses as much as the vertical ones.

A simple check is to clean the bushigns and the cups completely. Spread a thin coating of the silver antiseize along the top edge of the bushing and a similar coating on the lower 1/4 inch of the metal support cups and some on the bottom of the cup in a couple of spots (very, very little here).

Install the crossmember. Using a floor jack, lift the rear of the car under the diff so that the weight of the car is on the crossmember.

Set the car on jack stands again.

Remove the crossmember.

Examine where the silver antiseize is at. This will show you where the cup has positioned itself in the bushing. You can see if it bottoms out or if it had not gone in deep enough.

[Scott Marzahl]

Re: Follow-Up Bushing Observation

Doesn't the inner cup of the bushing have a rubber coating?

[Mike Cobine]

Re: Follow-Up Bushing Observation

To be truthful, it is been several years since I had one off. But I believe that there is rubber inside partially, more like it leaked in and not like they tried to coat it.

Maybe others have seen different examples.

[Howell Jaynes]

Re: Follow-Up Bushing Observation

The original bushing has a rubber coating on the inside of the "cup." Notwithstanding the force required to pop the original bushings loose and notwithstanding their 36 year age, the original bushings still have what appears to be a relatively thick rubber coating that is basically undamaged. One of the walls shows a slight abrasion-like scuffing damage. The repro bushings had only a thin rubber film that I removed with my thumbnail!! Even with the thin film removed, they are still much tighter than the orignal;i.e. don't fit as well.

I agonized quite a bit before deciding to replace the originals. One of them had a small crack on the exterior rubber rim. The original rubber is 36 years old and I would like the car rubber suspension components to last another 36 years!...so I decided to replace them. I often wonder why people that make repro parts just don't go the small extra distance and make them fit like original (I'm particularly thinking now of repro interior parts.) Maybe the bushings were made a little tighter because to account for some hypothetical wear that would exist when they would be installed later in the car's life.

Once again thanks! I'll heed the advice not to try to open up the opeing clearance to get closer rubber bushing rim to frame fit.

Hal