Hey folks,

 

My mission continues to treat my ageing engine as well as I can. She's leaking ~1 litre of oil every 100h or so out of various locations. I believe this is primarily from the crankshaft rear oil seal. In fact, I stuck a little camera down there and can see that there's definitely a leak. I know this is no small project and have a shop manual to guide me, but since the tools described are impossible to find, I was wondering if anyone has gone about the process of replacing the rear oil seal and has any tips, tricks, or how-tos they can pass along.

 

As always, I really appreciate your time!

Edited December 16, 2025Dec 16 by thematt

I don't have a parts list, but I think the backplate and flywheel bolts/nuts probably use lock tabs, you may need new ones along with the new seal.

 

I think that the seal is a press fit into the engine backplate, so :- 

 

Gearbox off

 

Drive plate off

 

Put witness marks on flywheel and end of crankshaft if you can, just in case any locating dowel is missing, or between flywheel and backplate - the timing marks will do as log as you don't then rotate the engine. This is to ensure you refit it in the correct position relative to the crankshaft.

 

Remove the flywheel, You will probably need a leaver under each side to start rocking it away from the engine because I think it sits on dowels, but it may use dowel bolts and if so it should come off a bit easier.

 

Then scavenge about to see if you can find a length of pipe/large box spanner or similar that fits over the flywheel end to drive the new seal back in so it is easier to drive in square.

 

If you now insert a screwdriver between the crankshaft and seal lip, you can usually lever it out, but in doing so damage it.

 

When fitting the new seal ensure it goes in nice and square, even if you can only use a hammer/hammer and soft drift to tap in gradually on opposite sides to keep it square.

 

 

1 hour ago, Tony Brooks said:

I don't have a parts list, but I think the backplate and flywheel bolts/nuts probably use lock tabs, you may need new ones along with the new seal.

 

I think that the seal is a press fit into the engine backplate, so :- 

 

Gearbox off

 

Drive plate off

 

Put witness marks on flywheel and end of crankshaft if you can, just in case any locating dowel is missing, or between flywheel and backplate - the timing marks will do as log as you don't then rotate the engine. This is to ensure you refit it in the correct position relative to the crankshaft.

 

Remove the flywheel, You will probably need a leaver under each side to start rocking it away from the engine because I think it sits on dowels, but it may use dowel bolts and if so it should come off a bit easier.

 

Then scavenge about to see if you can find a length of pipe/large box spanner or similar that fits over the flywheel end to drive the new seal back in so it is easier to drive in square.

 

If you now insert a screwdriver between the crankshaft and seal lip, you can usually lever it out, but in doing so damage it.

 

When fitting the new seal ensure it goes in nice and square, even if you can only use a hammer/hammer and soft drift to tap in gradually on opposite sides to keep it square.

 

 

 

Pretty much spot on. 

 

Changing oil seals of that type was a regular job. You might find that rather than digging a screwdriver into the seal, make a pinprick with a small spike and screw a self tapper or two in, at 180 degrees to each other. By using a mole grip-or two-clipped to the screws, the seal can often be pulled out quite easily. In fact, I had a five inch American 'Vise Grip' with a bolt welded to the adjusting screw that fitted into my small slide hammer. Clip it on each self tapper in turn, a couple of operations of the slide hammer weight, Robert is the name of your Fathers Brother!

 

Good luck.

46 minutes ago, Mike Coombes said:

 

Pretty much spot on. 

 

Changing oil seals of that type was a regular job. You might find that rather than digging a screwdriver into the seal, make a pinprick with a small spike and screw a self tapper or two in, at 180 degrees to each other. By using a mole grip-or two-clipped to the screws, the seal can often be pulled out quite easily. In fact, I had a five inch American 'Vise Grip' with a bolt welded to the adjusting screw that fitted into my small slide hammer. Clip it on each self tapper in turn, a couple of operations of the slide hammer weight, Robert is the name of your Fathers Brother!

 

Good luck.

 

Now, that is a good idea, pity I am very unlikely to need to use it. Not so sure if it would be so easy on a seal with a metal body insert under the "rubber" because it would require drilling a hole for the self tappers.

6 hours ago, Tony Brooks said:

 

Now, that is a good idea, pity I am very unlikely to need to use it. Not so sure if it would be so easy on a seal with a metal body insert under the "rubber" because it would require drilling a hole for the self tappers.

 

When using that trick with a metal outer seal I just greased the bit so any swarf stuck to it. A Desoutter 1/4 inch chuck air drill made it a doddle-no need to centre punch-which could drive the seal in further-as the Desoutter had a REALLY slow, easily controlled rotation, always started the hole easily. I must have done hundreds when I was 'on the tools'! Last one was a Classic BMW 2002 TIi. Did plenty of motorcycles too. The outer seals on a horizontally split two stroke twin or triple crankcase often had a lip on the outer which closed into a recess as the cases were mated. Once removed, we 'shaved' the lip off of the new seal and used an adhesive jointing compound on the outer face. The inner seals on such motorcycles needed the crankshaft splitting and the bearings and seals changing-an expensive job which often exceeded the value of the bike.

Edited December 16, 2025Dec 16 by Mike Coombes
Add Text

You folks are all legends. I really appreciate the help!

 

Woo! underway. This is a large task. I hope it works.

 

Question: The manual says to remove the gearbox adapter plate once the fly wheel is removed. sounds like this step can be skipped with the self drilling screw and pipe idea. Can you confirm?

 

 

Hi,

When I had a BMC1.5 years ago, it used to leak oil from the rear oil deal if filled to the MAX mark on the dipstick, when it reached the mid point twix max and min it stopped, It had reached it's 'happy' point..........seemed a lot easier to fill it to this level than remove the flywheel.

51 minutes ago, LEO said:

Hi,

When I had a BMC1.5 years ago, it used to leak oil from the rear oil deal if filled to the MAX mark on the dipstick, when it reached the mid point twix max and min it stopped, It had reached it's 'happy' point..........seemed a lot easier to fill it to this level than remove the flywheel.

 

At least some, if not all, 1.5s used a scroll and oil thrower for the rear crankshaft seal and if the engine was installed at an excessive angle, or it was overfilled there was nothing to stop it leaking when stationary. The 1.8 uses a lip seal.

 

4 hours ago, thematt said:

 

Question: The manual says to remove the gearbox adapter plate once the fly wheel is removed. sounds like this step can be skipped with the self drilling screw and pipe idea. Can you confirm?

 

 

As long as you can see the outer circumference of the seal than the answer is yes, it can be skipped. 

8 minutes ago, Tony Brooks said:

As long as you can see the outer circumference of the seal than the answer is yes, it can be skipped. 

Thanks Tony. As always your quick replies are life savers!

1 minute ago, thematt said:

Thanks Tony. As always your quick replies are life savers!

and even if you find it can't be skipped, it is not too difficult to take the backplate off. Westerbeke seem to have done their own thing with the marinsiation, so one can never be 100% sure, but the manual image looks like a standard BMC backplate.

Edited February 5Feb 5 by Tony Brooks

Thanks, I may just go for it. I have a gasket ready. might as well given the ammount of effort it took to get here.

 I don't think those 'service tools' will be much of a problem. From memory it is just a straightforward nuts and bolts job, no delicate bits or springs suddenly whizzing out or getting lost. Basically big Meccano. Good Luck.

I think that the OP needs to assure himself that it is the oil seal leaking and not a leak between block and backplate. That can be hard to spot because the sump gets in the way. The photo seems to show staining plus a lack of rust from a leaking oil seal.

Well folks, I have progress, but sad news. I used a 3inch PVC pipe cap to hammer in the new seal but I believe I broke my two I ordered. Are there any tricks to make these damn things go in easier? Oil the outside? Silicone lube? Still couldn't get it flush even after hammering away.

 

Edit: measured the depth of the new seal. It's ~1-1.5mm deeper. Not sure if due to fresh rubber or just different spec. My original seal says 'made in the UK'. The manual says install until flush. is 1mm proud too much?

 

Edited February 6Feb 6 by thematt
more info

I would get a new seal. Take the old one to a bearing supply specialist and they will probably understand the sizing. It looks to me as if what looks like an aerosol can lid is too small diameter wise and with a wall that is too thin. The pipe wall needs to be thick enough to spread the load of the blows and be all but the same diameter as the seal. Typically, inside the rubber sheathing of the seal here is a ring of L section steel ring. The horizontal led across the seal face and the vertical one going down next to the engine block. It seems to me that the undersize "cap" has either pushed the horizontal arm down and has cut through the seal like a knife.

 

This time, forget about pipes unless you can find a steel one with a thick wall and very much closer to the seal OD. Now the engine backplate is off carefully start the seal by using a smaller hammer by tapping at 12 o'clock, 6, 3, 9 and the halfway between each of those so it starts to go in square (I suspect you may have got it stick at an angle). Then switch to a soft metal or hard wood drift and larger hammer (not a lump hammer) moving it around the seal in the same way. The drift face should span the whole of the seal face. It can hang over the outer edge. keep an eye on the way the seal sits and if it starts to angle or one part drives lower drive in a way that corrects it.  Use blows that are only heavy enough to move the seal a little with each blow. Once it starts square you can work round in a circle but keep it square. 

 

The instructions say there is a seal retaining plate, so I doubt that will fit properly as the seal is now, and fear it might leak from the split.

Thanks Tony,

 

That makes sense. I've ordered a few more replacements as they're relatively cheap. I just have to wait for them to arrive.

 

I'll attempt your method as well as look for a more suitable aid for install as you described while I wait.

 

The plate is back on (photos out of order). I was worried about damaging the seal without the crank shaft there to support. Would you recommend pulling the plate back off? (I have another gasket on the way as well)

 

 

Edited February 6Feb 6 by thematt
spelling

2 minutes ago, thematt said:

Thanks Tony,

 

That makes sense. I've ordered a few more replacements as they're relatively cheap. I just have to wait for them to arrive.

 

I'll attempt your method as well as look for a more suitable aid for install as you described while I wait.

 

The plate is back on (photos out of order). I was worried about damaging the seal without the crank shaft there to support. Would you recommend pulling the plate back off? (I have another gasket on the way as well)

 

 

 

I don't understand the bit about needing the crankshaft to support.

 

It looks as if you have put the seal in the block, but the manual says it goes onto the backplate. The recess in the block may well not be deep enough to allow the seal to go in flush.

 

The backplate fits on two? dowels that ensured the hole in the backplate and thus the seal are concentric with the crankshaft.

 

The end of the crankshaft at least has rounded edges, but I suspect there is a small taper that helps the seal lip slide onto the crankshaft. Oil the shaft before trying to refit the backplate and seal so it helps the seal slide onto the shaft.

 

Yes, take the backplate off. Lay it flat on a sheet of "wood". Then fit the seal with it horizontal. that is normally easier than trying to do it in situ. You can then use a drift or hammer with a face that is wider than the seal because there is no danger of hitting the crankshaft end.

 

When refitting the backplate with seal you may be able to slightly rotate the plate back and forth a little to help the seal slide over the crankshaft before you have to locate the plate onto the dowels, but even if you can't the seal should still slip over the oiled crankshaft.

Understood. Thanks Tony. Your recommendation is my new plan when the replacements arrive.

 

RE. fitting it into the block: I did not. I erroniously reinstalled reinstalled the back plate thinking it'd be easier to align the seal with the crankshaft. I was wrong. Hope my malleting did not damage anything (I avoided the cranksaft)!

Edited February 6Feb 6 by thematt
clarity

I mistook the now rust free backplate for the engine block, a good bit of cleaning there.

 

If you want to, once you have started the seal into the plate you could put a stout piece of wood either side, with nut and bolt through the middle and use the bolt to pull the seal into the plate, that way it should go in square.

6 hours ago, Tony Brooks said:

If you want to, once you have started the seal into the plate you could put a stout piece of wood either side, with nut and bolt through the middle and use the bolt to pull the seal into the plate, that way it should go in square.

 

Now that's sounds fool proof! It might just work for me

Update: Tony's plan worked for getting the seal set evenly. However, the big issue is the seal is indeed 1/16 inch (~1.5mm) too wide - My callipers indicate 5/16" for the original and 3/8" for the new one. I managed to get the seal flush, but it then protrudes out the back of the gearbox adapter plate. I then reinstalled and that pushed the seal proud of the the plate again. Less so then that of the original photo above. 

 

Now, my question: Would it be foolish to think this will work? Installing the oil seal retainer shows it to sit proud, but not enough to interfere with the flywheel, I think.

 

Thanks for any help, this project is driving me crazy.

 

 

 

 

Looking at the photo showing the retainer in place. I think the outside of the large part of the crankshaft may fit into a recess in the flywheel, so the flywheel pushes over it a bit.

 

The retainer seems to have bowed between the bolts, and it LOOKS as if it will be very close to, or even touching, the back of the flywheel where the flywheel recess fits over the end of the crankshaft. Look at the bright ring around the end of the crankshaft.

 

Now, all crankshafts have a degree of end float, and the older the engine the greater this is likely to be so I fear the flywheel may rub on the retainer when running. 

 

I would take the retainer off and see if it is bent, if so try straightening it and refit to see if it is then clear of that bright ring.

 

Have you taken the old seal to a bearing supplier to see if they can supply one 5/16" thick?

7 minutes ago, Tony Brooks said:

Looking at the photo showing the retainer in place. I think the outside of the large part of the crankshaft may fit into a recess in the flywheel, so the flywheel pushes over it a bit.

 

This is not the case. The flywheel mating surface is completely flat. The callipers slow ~4mm of crank shaft beyond the retention plate.

 

14 minutes ago, Tony Brooks said:

I would take the retainer off and see if it is bent, if so try straightening it and refit to see if it is then clear of that bright ring.

Sadly there's no unbending it with the oil seal being proud. 

 

14 minutes ago, Tony Brooks said:

Have you taken the old seal to a bearing supplier to see if they can supply one 5/16" thick?

That's my next step. I thought I was surely doing something wrong with getting the thing fully seated.

 

I've called Calcutt and they seem willing to help with a measurement.

I've finally tracked down a source that has a seal of the correct dimensions. The only challenge is the seal is only rated for 8 psi. Is the rear main seal holding back the 80psi that the guage is showing or is the rear main exposed to less pressure?