HOW TO - REPAIR A REAR DIFFERENTIAL MOUNT
Submitted by - Steve Pritchard
** This article has pictures from the ST205 but the procedure is the same for all the GT4s **
When working on a car always use the appropriate safety equipment, chock the wheels to prevent the vehicle from moving. When working underneath a raised vehicle always use axle stands.
Always read and understand the directions on any chemical products you use. Pay particular attention to any handling and/or PPE directions
Why would I need to repair a differential mount
The chief reason for repairing a differential mount is the failure of the original one. This is characterised by a heavy clunk from the rear of the car when applying power or lifting off the throttle.
Some people may wish to upgrade it as part of a significant power increase program in order to prevent the standard mount overflexing and causing symptoms like a failed mounting
Polyester resin and release agent. I used Polytek EasyFlo 65 shore. This is perhaps a little on the hard side for a simple replacement and something with a slightly lower shore might prove better
Paint/preparation materials of your choice
|Remove old mount from car (see how to coming soon)|
|Using a marker of some description (I used Tippex) make some index marks from the rubber to the inner and outer metalwork. You will need these later to partially re-assemble the mount|
|Using a prybar (I found a 3/8" socket extension bar to be about right) lever the central metal section from the outer frame. I did this by sticking the prybar through one of the two differential bolt holes and twisting the centre while holding the outer frame rigid with an adjustable. NB the photo here shows the completed mount as I forgot to take one of the original|
|After grunting and groaning you should hopefully be left with 3 pieces, the rubber and inner and outer frames. Note that you need to take some care in the disassembly process. You need the rubber in one piece for the next stage|
|You will notice that on both edges the rubber has a tongue which engages in a groove in both the inner and outer frames. Using a craft knife CAREFULLY remove this tongue from both sides of the rubber. You should now find that you can re-assemble the mount relatively easily|
|>Re-assemble the mount and line up your match marks to get the original spacing back. When it's all lined up screw it down. Carefully remove the rubber and draw round the inner and outer frames so that you can re-position them later.
You can see my outlines in the picture. You can now unscrew everything and prep and paint the metalwork.
You can see here that I used a flat sheet of melamine board as the base. I think this could be improved on (detailed later) but as I know this works this is what I have detailed.
Personally I wire brushed off the worst of the rust from the metalwork, hit it with a coat of KuRust then a coat of Hammerite. The result could be better though and is best described as a triumph of function over form
|Coat the base with a suitable release agent. The product I used has a specific release agent or suggests the use of
Vaseline. Since I had neither I actually used a thin layer of teflon silicon grease. This worked fine for me but is not exactly following manufacturers instructions so may not work for other polyurethane resins
Once you have applied release agent re-assemble the diff mount using the outlines you drew earlier. You can see my assembled setup here, with the shiny grease evident in the bottom
Here is where I made a bit of a mistake. I used a little extra grease round the outline of both metal bits hoping that this would form a seal between the metal and the board when everything was screwed down. This proved not to be the case and I suffered some leakage in the next step. I think this is where my improved method would work but, again, as I have not tried it I have not described it.
|Mix the polyurethane resin following the manufacturer's instructions and carefully pour it into the void where the rubber was on the original mounting. I guessed the amount by eye but would estimate that 225 mL would be plenty.
You should end up with something that looks like the picture, hopefully without the leakage that's evident in this picture. Leave the mount to cure following manufacturer's instructions
|The finished article after a little trimming of the leaked resin|
Possibly improved method
You can see from the final pictures that the "pour side" has a couple of mm gap between the resin and edge of the metal framework while the other side is flush with the metalwork. This is an obvious side effect of the method I used
I also had some leakage issues between the metal and the melamine board I used. I was fortunate that the cured resin released quite easily from the shiny melamine otherwise I would have had quite a mess
I think this could be improved significantly. If I were to do this again I would try the following :-
Instead of using hard board for the base I would try using a thinnish sheet of stiff foam covered by a shiny fablon type plastic coating. This needs to be put down before you draw the outline of the disassembled mount
The rest of the process is as per the description. I believe that this method would achieve two things.
You should end up with the backside having a little gap between metal and resin as per the top side.
You should get a good seal between the metalwork and the plastic since the rubber will take up any irregularities. This ought to stop the leakage issue I had
The method is essentially the same for renovating/upgrading other polyurethane mounts. Real world tests have shown that using this 65 shore compound for engine mounts, results in a great deal of engine vibration through the bodywork. While engine mounts can be upgraded using this method lower shore polyurethane is an absolute must!