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MG MGB GT V8 Factory Originals Technical - Rear Main Seal

I have had a lot of trouble with my Rover 3.5 and oil leaking out of the rear main seal.
When I rebuilt the engine I used the rope type seal, this seal leaked and at this time it might have been because I hadnt installed a PCV valve which is now installed on top of one of the rocker covers. The other rocker cover has a K&N filter to allow air to enter the engine.
Well it leaked again the next driving season and during the winter I installed the 2 piece neoprene seal. I did this by taking off the rear main bearing cap, coating the neoprene seal (the side which is in contact with the block) with silicone and installing. After letting it cure overnight I torqued the rear main bearing cap back up. Well it leaked again. I am now going to try it for a third time. This time I have the rope seal ready to go, but before I do this am I missing something, does any one have any advise to offer?
On the bright side, the underside of the MGB is pretty well protected with the oil which is coating it now.



Bruce - so far as I am aware, the only rope seal is on the timing cover - the rear seal is a onepiece neoprene ring which ideally should be drifted into position with Rover service tool RO 1014.

First make sure the caps are all fully in position - retaining bolts finger tight - tighten them equally one quarter turn to settle the caps then back off the bolts by one full turn.

Make sure the guide and journal are scrupulously clean and operative surfaces coated with clean engine oil - don't touch the seal lip at any time.

With the lip of the seal pointing towards the engine push it into the recess formed in the main bearing cap until it abuts the machined step. Now tighten the caps to the recommended torque.

The manual then recommend you go on to measure the crankshaft end float - which with your story so far - might be a wise precaution.

Hope it helps.

The early Buick 215 blocks used a rope seal in the rear too. I also opted to go the 2 piece neoprene way, but have not had enough opportunity to run the engine to see if it leaks.

I have a 1969 3.5 Rover, which explains the old rope seal. The 2 piece neoprene seal I purchased from D & D for this application.
You mentioned it might be a good idea to measure the crankshaft end float. When I re-built the engine I didnít do this because I donít know how and I didnít have a dial gauge. (I recently installed new front wheel bearings/shims) so I now have an idea what you are referring to. I do remember when I installed the crank, trying to push or pull it with no noticeable movement. How would this contribute to the oil getting past the rear seal? You know as I look at the drawing of the crankshaft it is hard to believe that the oil is getting by the flange on the end of the crank itself, let alone getting past the seal as well.
Is there any way to check the seal besides installing the engine and running it? I know when I pulled the engine I adjusted the engine tilt so that oil was easily observed getting past the seal.
I am going to pull the oil pan this weekend and take a good look at it.


I agree with you - it is difficult to imagine how the oil gets past the seal. It worries me that there should be no pressure to expel oil here and that it should simply fall into the sump below but the clue may be the very careful instructions of the manual to get the seal positioning 100% correct.

Let us know what you find when you pull the 'oil pan/sump' - will you pull the bearing cap and have a look at the shell as well ?

On the subject of fore-and-aft movement; too much and imperfections in the crank will add to the wear of the bearing shells and the seal in question. However, if you can not detect movement and/or the thrust bearings are new - then you should not have a problem.

Do let us know how you get on


I just happened to run across this website.

Don't know if it is applicable to your engine or not, but may explain some leakage if you don't pay attention to the "anti-rotation" holes.

Wayne Pearson

I had this trouble,I think a lot of it is due to lack of use during the winter,PCV valve helped me too.
Dave Lowe

I pulled the rear main cap tonight, and the seal is missing a piece. Why, how, when, I do not know, but it explains the leak. I also checked the end float and it is .008, acceptable according to the book.
Wayne. Great installation instruction.
I have pulled the rear main bearing and there are no anti-rotation holes in the Rover.

The Rover sealing diameter - crankshaft is 3.25. Would the Buick Special rear main seal and the Rover 3.5 rear main seal be interchangeable?

Bruce Mills


When I pulled the rear main seal from my Rover 3.5 (bought from a V8 specialist) there where some numbers on it and what looked like the Ford emblem.
I checked with Ford and the seal is actually a Ford seal from a 170 or 200 cid engine Part # C9AZ 6701 B.

Also the side seal are available from the 390cid. I didnít replace these so canít attest to accuracy of the fit.

Bruce Mills

Bruce - just back from a few days away - sounds as if you are home and dry ?

Over the weekend I replaced the rear main seal. I put in 4 lt of 10-30 oil and tilted the engine to about 60 degreeís.
And the oil still leaks out. It seems to be leaking at a steady 1 drop every 16 seconds. With the old seal it was just constantly dripping out.
I am not sure what to do now.
Do I pull it all apart to see if I some how nicked the seal?
Do I replace it with the old rope type seal?
Maybe it really isnít such a big deal?
After all it is hanging at about 60 degrees with the weight of 4 lts of oil against it. Maybe when the engine is level it will be okay?
How much pressure is in the lower crankcase?

Any ideas appreciated


Bruce Mills

This thread was discussed between 04/03/2004 and 26/03/2004

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