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MG MGB Technical - Replacing Transmission gaskets

just pulled the engine and transmission from my 1970 mgb roadster. I have separated the two to replace the clutch. It is a four-synchro transmission without OD.

One of the "might as wells" that I am doing while all this is out of the car is to replace all the gaskets and oil seals on the transmission.

I have cleaned the exterior of the transmission and removed the clutch fork (in preparation for a new throwout bearing). I am getting ready to start replacing the gaskets/seals. The front cover gasket and oil seal look pretty straightforward, as does the side cover gasket.

I bought the Moss gasket kit, which includes the gasket between the main gearbox and the rear extension. This one has me a little worried. If I simply unbolt this and pull of the extension, am I going to find myself with a handful of gears and bearings that are going to be difficult to reassemble. I guess I'm just asking what I should look out for as I dissassemble the transmission.

What, if any, gasket sealant should I use? Or are the paper gaskets good enough?

Thanks,
Paul Noble
Paul Noble

Paul,
If that gasket isn't leaking, leave it be. The rear bearing is pressed on the shaft and doesn't slide off easily. You will need to remove the shifter interlock so the rear shift linkage can disengage the shift rods.
Leland Bradley

Paul. I agree with Leland. When I reseal a tranny, I replace the front cover gasket (and often find that new shims are needed), the front oil seal, the side plate gasket and the rear oil seal. All is pretty much straight forward. You will find the rear flange nut can be loosened with an impact wrench fairly easily. The front cover has shims between the front cover and the bearing. Be aware that the Moss supplied .004" shims are actually .005". (Chris Betson mentioned in an e-mail that the shims he gets are .005" and I measured the five I had on hand. All were .005". This is significant when your tolerance is zero to plus .001") The .002" measure .002". So pay attention. Most of the boxes I have resealed over the last year have needed the shims replaced or adjusted. When you have the side cover off, clean out all of the oil you can see and look for any metal pieces or other swarf. I used HiTack on all the paper gaskets, both sides. I would also suggest that you consider rebushing the throw out bearing arm while you have it apart. You can check the hardened bolt it pivots on, but the last three I checked were all good with less than .001" of wear over the many years of use. Insert the bolt into the bushing and try to move it from side to side. If you feel play, install a new bushing. I made up a special piloted driver to remove the old bushing and press in the new one. Start with a short length of 3/4" round stock and turn it down to .003" under the outer diameter of the new bushing, then turn one end down to the diameter of the pivot bolt. Easy to use by hand or in a press. Les
Les Bengtson

Les,

I did rebush the throwout lever fulcrum. I don't have a micrometer, so I can't say exactly how much wear there was, if any. However, when I put the new bushing in and re-inserted the fulcrum bolt, it is a rather tight fit. I was able to get it in all the way, but it is difficult to turn it by hand. Using a socket wrench, it turns, but is that too tight a fit, or will the bronze bushing wear a little bit and loosen up?

Last night, I removed the front cover plate to get to the gasket and oil seal. There were shards of what must have been the shims you refer to. I'm glad I asked the guestion, because I wasn't even aware that there are supposed to be shims. Guess I'd better go study the shop manual again.

A cursory examination of the inside of the transmission via the side cover, showed no signs of metal shards or anything else that might cause damage. I'm no expert, but the gears had no knicks or broken teeth. The synchronizers did not appear to be excessively worn. This fits with my experience with the transmission. I have had no trouble shifting or with it popping out of gear, which is usually a sign of bad synchros, isn't it? Anyway, I am only doing this because I have the transmission out of the car and I wanted to reduce oil leaks.

The inside of the bell housing was caked with grease and dried oil. The flywheel appears pretty clean, so I suspect that it was coming from the front oil seal of the first motion shaft. BTW, the new oil seal for this seems to not be a very snug fit. There is no noticeable side-to-side play, but I expected a little tighter fit. When I looked at the shaft, it appears that there is a sort of rounded shoulder on the shaft, just before it enters the gearbox, as if this should presses against the back of the seal, rather than the inner circumference of the seal riding on the shaft itself.

Thanks for the help.
Paul Noble

Another question on the shims. I just looked at the Moss online catalog (don't have shop manual here). Are the shims you refer to numbers 462-020 (0.002") and 462-025 (0.004")?

They appear to go between the large nut and the bearing race that the nut secures. Is that right?

Since my shims have torn up and disintegrated, I almost certainly need new ones. How do I know which thickness or combination of thicknesses to use? Should I order a couple of each size and experiment? Can I even do this without having a micrometer to measure end-play? I wasn't expecting to have to go this far into the transmission, but if they're bad, I'd like to replace them now, rather than having to pull the engine again.
Paul Noble

A timely thread for me as I'm doing the same job to reduce oil leaks except that my transmission is an overdrive LH. Any other things to watch for with it ? The breather is missing as well...would this cause leaks ? Thanks !
Al
Al McMillan

Paul,

Determine the number of shims thus:

Put the front plate back on without shims or gasket - hold it firmly against the bearing out race using one hand. With the other hand measure the gap between the plate and the gearbox casing throught the hole for the release arm using feeler guages.

If there is no gap, add a 5 thou shim or more till there is a gap.

Take the gap measurement and take off 12 thou - the result is the thickness of shims required ( plus any already there).

Go up to one thou under but not over with the combination of shms used.

Example: measured gap 18 thou - minus 12 thou equals 6 thou - fit one 5 thou shim or 3 x 2 thou shims.

Use some grease or gasket sealant to hold the shims in place on the cover when fitting.

Chris Betson

Chris,

I think I understand, except for one small detail. Exactly where do the shims go? When I removed the front cover, several broken pieces of one of more shims fell out. It was not immediately obvious where they belong.

The diagram in the Moss catalog seems to show them between the nut and the bearing race of the first motion shaft. Or do they go outside the nut, between the nut and the oil seal in the cover plate?
Paul Noble

Paul. Yes, those are the shims. The Moss part number 462-025 (Shim .004") actually measures .005". Moss is aware of this and has been for some months now. Turn your front cover over so the inside is facing upwards. The area where the cut out for the forward extension of the bearing is where the shims go. While I find Chris' method to be workable, I find a dial caliper works better for my measuring use. There is a Chinese model, available from Harbor Freight and most tool stores for about $20-25. Comparing its accuracy to my $125 Starret, there is no difference. Both read exactly the same. The least thick shim pack I have seen measured .007" and the thickest measured .017", so there is significant variation between trannies. I keep five of each shim on hand, but I am currently maintaining/setting up five cars. All of the trannies I have pulled apart either had broken up shims in the front cover or required a different combination of shims than were in them when the front cover was removed. All of the trannies were working before and after the reseal. Hence, I do not know exactly how important all of this is, but prefer to do it right as long as I am doing it. Les
Les Bengtson

This thread was discussed between 25/06/2002 and 26/06/2002

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