Modifying The Moss Rear Main Oil Seal Conversion.


I recently renovated an 1949 MG TC. Part of the renovation was the engine. I fitted new crank bearing (main and big end), new pistons and rings and cleaned the head and ground the valves in. The head had been overhauled about 6000 miles earlier.

Whilst I had the engine stripped I fitted a Moss rear main conversion kit. I did not know then that many have problems with this kit and some change back to the scroll as they cannot get the seal to be leak free.

I was in no rush as the bodyshop was behind with the body renovation. I took 3 days to fit the kit and followed each step of the instructions religiously,

When I stared the engine, some months later there were no leaks. The engine probably ran for two hours before being driven. On 28th May the bodyshop finished and I drove the car home, about 24 kilometres. When I got home I noticed oil under the car. Lifting the bonnet, I saw oil coming from the starter aperture. It was running out, not dripping. Engine was quickly taken out. Turned out that when I painted the engine I had “sealed” the drain hole at the bottom of the bell housing which was now half full of the oil running out of the seal.

I bought a new seal and this time a “Speedy Sleeve” as well. I hadn’t fitted one first time as the crank was clean and I polished it so it should not have required a sleeve. There is no tool supplied to get the lip of the seal over the razor sharp edge of the sleeve. Taping the edge of the sleeve with masking tape (as suggested in the instructions) didn’t help and the Teflon seal was cut. So was the next one. The third went on undamaged and I ran the engine on the shop floor for 2 hours and it was leak free, I refitted the engine, started it up and 20 minutes later the oil starts running out again.

Engine out again but this time I give it a very close inspection. The lip of the seal is on the vey edge of the Speedy Sleeve. I check all the measurements and everything is within the tolerances set in the instructions. However, I measure where the lip sits in the relation to the housing and position of the sleeve and find the seal needs to come rearwards 0.045” to be in the centre of the crank boss/sleeve.

A decision has to be made. Do I remove the crank and pack the housing backwards 0.045” or do I make up seal faced sized gaskets to pack the housing so the seal is moved back the required amount? More measuring and I realise the inside face of the flywheel will need cutting back to clear the housing when it’s moved back. On this basis I decide not to remove the crank and move the housing back but to pack the seal housing and move the seal back leaving the housing where it is. This means that I have to cut the flywheel back by the diameter of the seal, not the larger diameter of the housing.

So, I make 5 rings from a Weetabix packet (what else are cereal packs for?) and cut a groove in the flywheel 0.060” deep.

I then soak the new seal in boiling water and, with the edge of the Speedy Sleeve covered in PTFE tape I slide the seal on backwards. This makes the fitting of the seal very easy and risk free from cuts. The sleeve is then fitted to the crankshaft using a flat piece of wood on the flange of the seal to tap it on.

As the seal is already on the sleeve you cannot cut the flange off the sleeve. I left a 0.060” gap between the seal and the flange so they didn’t touch. I then refitted the lower half of the housing and the flywheel. Sump back on the engine goes back in and has now done nearly 300 miles, some of it 30+ c temperatures (I live in France) and there is no leak from the seal whatsoever.

My only query is how can Moss get the measurements so wrong. The instructions call for 0.020” to 0.025” gap and mine was 0.022” But it needed another 0.045” to get the seal in the centre of the sleeve.

My theory is that the crankshaft bosses vary over the production cycle of the XPAG engine. The only purpose the crank boss serves is to provide the 4 threads and two dowels for the flywheel to be affixed to the crank. If some cranks have a deeper boss, then the seal will fit correctly with the Moss measurements. NOTE. I have since found an old instruction set and it cites that they have found crankshafts with shorter bosses. They suggest to fit the Speedy Sleeve further on, which may be easier but it may touch the housing. This advice seems to be missing on the latest set of instructions.

To do your seal as above. Measure where the seal lip will be when fitted and decide if you need to move the seal back and by how much. The lip of the seal from the inner face of the seal is listed below. Using a pair of digital callipers, it is easy to see if the seal is fitted without gaskets in the housing, where the lip will sit. Just as easy to work out, using the measurements below, how much packing you need by marking the centre of the boss with a felt pen and then measure the gap between the inner face of the top housing and the mark in the centre of the boss.

Have the flywheel machined at least 0.015” more than the packing used to move the seal back. 




LIP OF SEAL IS .300 IN FROM OUTER FACE OF SEAL WHICH IS THE CLOSED SIDE. THEREFORE, IT IS 0.060” FROM THE OPEN, INNER, SIDE. (This measurement is approx. It’s difficult to measure the edge of the lip as it’s flexible and may not run 100% true)


The seal fitted in the housing.

A close up showing how much of the boss is exposed. The white powder is talcum powder, used to determine where the leak may be from.

The seal being “boiled” in hot water. The open side, the side that faces the engine is uppermost.

The Speedy Sleeve wrapped in PTFE tape. Do not wrap too far down as it will be difficult to remove when the seal is on.The flange is on the table.

The seal on the sleeve. It goes on very easily, just gently open the lip with your fingers as you slide it over the edge of the sleeve. Remove the PTFE tape.

One of the sleeves cut by the razor edge of the sleeve. For those who have worked on the BMC A Series engine as fitted to the original Mini you will know of the tool used to fit the clutch idler gear seal. This was a tube that fitted over the idler gear and was tapered at the other end. The seal was slid over the tapered end, along the length of the tool and into the housing thereby protecting the lip of the seal from the sharp edges of the idler gear. To fit the Moss seal after the Speedy Sleeve has been fitted and trimmed needs a similar tool.

The seal fitted in the top housing (the housing is a spare I have). This is before the new gaskets have been fitted.

The location of the seal before packing. Not a good picture but it is right on the inner edge of the crank boss.

5 Gaskets made from a Weetabix box. Total compressed thickness is 0.046”.

The groove being cut in the flywheel. It is, in this case, 0.060” deep and slightly wider than the seal.

The engine running on the shop-floor for testing. I ran it for two and a half hours before refitting it. It has now been driven 288 miles and is leak free. On two days I’ve been out in it, the outside temperature was 32c and 38c. A good test.