Introduction #
Rear Differential Gasket Replacement was another new job for me on an MGB. I took a few pictures and thought I’d document this for those that follow me in doing this bit of maintenance. The drips and leak from the rear differential on my 72 MGB Roadster had been bothering and annoying me for some time. Recently seemed to be getting a bit worse. Having parked it in the part of the garage where the regular cars reside, I noticed a fairly large spotting of gear oil. This was enough to prompt me to take action. I had bought the necessary gasket for this job quite a while ago. So, time to Replace Rear Differential Cover Plate Gasket!
It occurred to me that the job would be much, much easier if the petrol tank was not installed. However, it could be done without removing it. After the fact, I discussed my approach with John Mangles. He suggested placing the jack stands behind the rear mount points of the leaf springs. This would enable the rear differential to hang lower. Unfortunately I didn’t have this bit of wisdom before doing the job. So I probably made this a bit harder than it needed to be. It was quite hard to get at the top bolts and not permanently wedge a socket and ratchet up there against the petrol tank – live and learn!
Digging into the details #
First, I’d suggest making sure that you can remove the fill plug. I’ve done this a few times and I find that I can remove the fill plug with just a 3/8 inch ratchet. This fits inside the fill plug. For the drain plug it seems a 1/2 inch ratchet does the job. I’ve read online that using a socket adapter and grinding it down a bit will help with getting a good fitting for the fill and drain plugs. But it hasn’t been necessary for me.
Step 1 Drain the Differential #
Of course the first step would be to drain the rear differential.
Step 2 Removing the Cover Plate #
Having drained the rear diff I left it to drip while I started work on the bolts securing the plate. There are 2 studs used at approximately the 11 and 1 O’clock positions on the plate. On my 72 these are used as mounting/securing points for a section of the rear brake system hard line. I removed the 2 nuts that were holding the brake line clamps in place. Then carefully took the clamps off the studs. The brake line does have some flexibility to it but don’t get too heavy handed with it or you’ll have more stuff to fix! With these clamps removed I was able to carefully push the brake line up and over the top of the plate. It rested out of the way of the cover plate coming cleanly off the rear diff.
That left me to remove the bolts/lock washers and the 2 longer bolts that were securing the hand (emergency) brake cable compensating lever. The only tricky ones were the top most bolts. I didn’t have much room to work and the deep socket was a bit too long and a normal socket on a short extension was also too long. In the end I used a socket with a 1/4 inch to 3/8 inch adapter which worked pretty well. With everything removed I was able to carefully pry the cover plate off without digging into the diff casing or the plate. A few taps with a rubber mallet might also help. Depends on what has been used previously to seal the cover.
Step 3 – Cleaning Time #
Once you have the cover plate off its pretty much cleaning time. You want to get both the mating surfaces really clean. I don’t have a wire wheel or the like. So for me it was a gasket scraper and what felt like hard labor on one of our very hot weekends. Fortunately for me 90% of the gasket was stuck to the cover plate. The actual main differential was fairly clean. I carefully cleaned around the back of the diff. For protection I covered the gears and the area in general with some shop towel to keep the crud out. This didn’t take much time and I finished up with some 320/400 grit sand paper to get the diff’s mating surface as clean as I could.
The cover plate was not nearly so easy! It took some manual labor and patience. I just kept working at it to remove the old gasket material which had attached itself pretty firmly to the cover plate. If you have a wire wheel or other power tools this will be much easier. With most of the gasket removed I also used the sand paper on the mating surface of the cover plate. Eventually it was as clean and shiny as possible. I was also checking the plate for any warping or other problems. I didn’t find any so I was ready to proceed with the new gasket install.
Step 4 – New Gasket #
The gasket seemed to be quite thick. I tested it for good fitment and alignment with the cover plate holes. As expected, the gasket appeared to be a bit small. As I have discovered in the past, you often need to soak gaskets in water for a short while. This gets them to expand a bit. So out came the bowl of water and in went the gasket for 5 to 10 minutes. After the soaking I carefully dabbed off the excess water with a shop towel. The test fit was perfect.
Step 5 – Gasket Sealer, or Not? #
My final debate was whether to put the gasket on dry or use some sealer. In the end I went with some Permatex 2B non hardening sealer. I put it on the plate cover itself and then laid the gasket onto this. No sealer was put onto the actual diff mating surface. I then moved fairly quickly. To ensure the rear cover plate is on and in place with several bolts before the gasket dried out and shrank again. Getting the first one in was a little tricky while trying not to have the gasket move around. I got things started and had several bolts loosely installed to hold everything together.
Step 6 – Final Bolts and Add Gear Oil #
I worked my way around the plate getting either the bolts, studs or longer bolts back in place. The 2 trickier ones at the top were done last. With everything in place I proceeded to tighten them all in a crisscross pattern. Everything was nicely snugged up and evenly tight. I then got the brake line back up over the top of the diff and in front of the cover plate. This enabled me to reattach the line brackets and secure them on the studs with the 2 nuts.
That just left me to refill the diff itself. I decided to wait a while to do that to give the gasket time to seat and for the sealer to do its thing. After adding about a quart of 80/90W gear oil nothing came pouring out. This was good and also as I had remembered to install the drain plug before starting the refill. My car was back flat on the ground at this point. So I followed the instructions to fill the diff until you start to get the gear oil coming out of the fill hole. I’ve driven the car around since doing this and everything seems to be fine.Most importantly the leak does seem to be solved. I haven’t crawled back under for a close up inspection but the checks I have made seem to show no more drips. So, hopefully, mission accomplished
I hope this is helpful if you need to Replace Rear Differential Cover Plate Gasket.
Other Materials #
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