Differential Mounting Repair & Modifications - TR6

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It is not uncommon to have a broken differential mount on a TR-6. For most owners, a clunking sound is usually the first sign of a cracked differential mount. The front of the differential unit is mounted on a bracket usint two part rubber mountings. The front right (passenger side - U.S.) bracket is the most likely to crack due to the torque applied by the driveshaft. It is possible to fix the bracket on the car by welding the crack and welding a reinforcing plate onto the bracket. Care must be taken as the fuel tank and hoses are in close proximity.

You may or may not have a problem finding a welder who will do this for you. Before I started the conversion project I tried to find a welder who would repair the crack. Many welders wouldn't do the job - they didn't say exactly why but I had a feeling that they didn't want the potential liability associated with frame repair. I wound up welding the crack with the assistance of a neighbor. This was before I took a welding course. I recommend taking a welding course first and then attempting the repair.

The rest of this article will deal with replacing the bracket and reinforcing it with plates to "box it in".

The diagram below shows the individual pieces of the brackets and where they are located on the frame.


The picture below shows the earlier attempts I made to repair and reinforce the brackets. This was done by putting the car on jack stands, crawling under the car, and welding the plates onto the brackets. The yellow line shows where the crack occurred on this bracket.


The top of the crossmember is also an area which should be carefully inspected because, you can't see it in this photo, there is some cracking near the stud.


After ordering new brackets, I removed the old one by using an angle grinder with a 4 1/2" cut off wheel. Using a regular grinder wheel, I cleaned up all of the old weld remnants so the new bracket would fit correctly.


Obviously I don't have the factory jigs so I had to improvise to set the brack height. It may not be critical but I didn't want the two brackets to be a different heights with relation to each other. Since I had only cut one of the brackets out I could use the other as a reference point. Rather than measure the height of the existing and hope that I could hold the new one in the correct position while I welded, I decided to use C-clamps to mount two steel bars to the existing bracket. I measured the distance from the bars to the bottom of the crossmember at both bracket locations and found the distance to be the same. I then clamped the new bracket to the bars and proceeded to tack weld the new bracket in place. By using a bar in front of the pin and one behind, I was able to get the bracket oriented correctly side to side and front to back.


I believe the factory welded the stud to the bracket before the assembly was attached to the crossmember. Since I hadn't removed the stud and had mounted the bracket, I faced a dilemma because I couldn't reach the inside to the bracket with a welder - should I weld the stud to the bracket from the "outside" and run the risk that the weld would interfere with the rubber mount or should I do nothing? I knew that I had to weld the stud to the bracket or else the stud would eventually break away from the crossmember. I worked on something else while I thought about it and then the answer came to me. A stick welder does have one advantage - you can bend the welding rod (see picture below). I bent the welding rod into a "U" shape so it would fit inside the bracket. Since the frame was on a rotisserie, I could turn the frame until I had a comfortable position for welding. It was rather unusual position for welding but it worked.


I used a portable sandblaster to clean the immediate area where I would be welding. I sprayed the area with weld-through coating to provide some corrosion resistance.


I didn't buy the reinforcing plates since I wanted to make my own. I used light cardboard to make a template for the reinforcing plates and made the reinforcing plates out of some 3/32" galvanized sheet steel that I had in the scrap pile. Most diagrams show the reinforcing plates with three and a half holes in them. The half hole is located on the bottom edge and allows any water to drain out of the bracket assembly. I made the three holes with a 1/2" drill bit and used the angle grinder to notch the reinforcing plate for the water drain. The main reason for making my own plates was to make a modification for ease of installation. It looks as if all of the reinforcing plates are flat. You fit the plates up to the bracket and weld them into place. Since I was using a stick welder, this required some odd angles to get the welding rod into the correct position. It is a real pain in the @&% and I wasn't confident that my welds would be strong. I modified the reinforcing plates by making them larger on three sides by about 3/16". I bent the edges of the reinforcing plates so that they were the correct size to fit into the crossmember. The picture shows two reinforcing plates - the one on the left has been cut as a flat reinforcing plate and the one on the right has been cut oversize and the edges bent.

The edge allowed me to insert my welding rod straight down into the crossmember and make a strong, neat weld. It requires a bit more work to make the reinforcing plate but it made the welding much easier. The picture below shows the reinforcing plate fitted next to the bracket. I welded the top of plate to the bracket and used a hammer and brass punch to tap the edges until they firmly touched the crossmember. This creates tight seams and promotes a good weld joint. Because welding heat caused the seams to separate, I would weld a small portion of the seam, stop, tap the seam closed again, and repeat until the entire seam had been welded.


Once the brackets were finished, I welded any cracks I found in the crossmember and made reinforcement plates for the top of the crossmember. I cut some of the 3/32" sheet about 6" wide by about 10" long. I drilled a hole in the middle of the plate so that the stud (and weld) would just pass through. I think the hole was about 3/4" - 1" in diameter. I placed the plate over the stud on top of the crossmember so that the plate hung over the front and back of the crossmember. I marked the plate so that 1" of the plate would extend beyond the crossmember front and back. I trimmed the plate, repositioned it on the crossmember and tack welded it into position. Using a BFH, I worked the reinforcing plate overhang until it conformed to the crossmember and proceeded to weld the plate around its entire periphery. When that was finished, I used a hammer and brass punch to ensure the plate was touching the crossmember directly around the stud weld. Once I was satisfied that I had tight seams around the stud, I welded the plate to the stud and crossmember. This isn't a very good picture but the yellow arrows point to the reinforcement plates on top of the crossmember.


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