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Old 01-12-2012, 04:23 PM   #1
bmwhacker OP
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BMW Rear Main Seal Nightmare

I just completed (so I thought) a rear main seal replacement / oil pump seal replacement on my R100S engine.
The new seal began leaking within a 100 miles. Pulled the flywheel back off yesterday and carefully cleaned / dressed the mating surface on the flywheel. The surface had a little discoloration prior to cleaning it up with crocus cloth. The flywheel surface looks good, no wear or grooves visible to the eye.
Put it back together this AM and again am beginning to lose oil from behind the transmission.
The old original seal was loose in the bore and I was able to pull it out with my fingers.
The new seal went in nice and snug. ....I have an aluminum disc that fit the seal outside diameter and was able to "tap" the new seal in without any trouble. The seal looked straight and was not messed up on installation.
One thing I noticed, the seal lip on the "new" seal is a very hard material and not soft like most seals I've seen.
Is that normal?....or did I get a 40 year old "new" seal?
I'm running out of ideas on what in the world is wrong...or what I'm doing wrong....
Any inmate insight and opinions would be appreciated.

I just ordered another new seal and I'll be able to remove the transmission blindfolded with all this practice.
Bike is hooked to a sidecar so that makes it even more fun to remove.
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Old 01-12-2012, 04:57 PM   #2
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There's another hidden o-ring that could be causing this. And I don't know why it is - makes no sense to me, but it's on the guide ring - that piece on the end of the crankshaft the flywheel bolts to. Part # 11221337099.

I've gone through similar with an R65.

If it's not that, then it's time to check for cracks in the block.
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Old 01-12-2012, 05:05 PM   #3
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Originally Posted by Wirespokes View Post
There's another hidden o-ring that could be causing this. And I don't know why it is - makes no sense to me, but it's on the guide ring - that piece on the end of the crankshaft the flywheel bolts to. Part # 11221337099.

I've gone through similar with an R65.

If it's not that, then it's time to check for cracks in the block.
Ya, I did replace that o-ring too. Found a little "crud" in the groove but cleaned it well before installing the new o-ring.
I'll also remove the new oil pump o-ring and inspect it to be sure I didn't mess it up. I ordered another pump cover o-ring too.
I'm still questioning the "brittleness" of the "new" main seal material. I've never handled one before but the rubber was really hard and stiff....
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Old 01-12-2012, 05:19 PM   #4
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The new seals are teflon - kind of a brownish material. And they're flat when you get them, no spring like the old ones.

I remember ten years ago guys having difficulty with these things - and there were three or four different ideas how to install them - some even contradictory. I've installed a couple with good luck and they haven't leaked, but always kept my fingers crossed.
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Old 01-12-2012, 05:30 PM   #5
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Those seals have been out now for 30 years? 25 for sure. They are ten times better than the rubber ones. Install them just like a normal seal.

Pump O-ring? Somebody was sealing the wrong part recently. It wasn't BMW.

The O-ring is on the guide plate to keep oil from leaking out the flywheel bolts. Later flywheels have the same thing for the same reason. It works.
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Old 01-12-2012, 05:40 PM   #6
pommie john
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I replaced my seal a few times and it kept leaking.
It turned out to be a crack in the block.
See here

http://www.advrider.com/forums/showt...=527956&page=5
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Old 01-12-2012, 05:54 PM   #7
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Originally Posted by pommie john View Post
I replaced my seal a few times and it kept leaking.
It turned out to be a crack in the block.
See here

http://www.advrider.com/forums/showt...=527956&page=5

I'll look the block over real good when I go in for the 3rd. time.
The 1977 engine rear main seal seats directly on the flywheel. I see in the thread above photos that the configuration is different.
Is there any way oil could come through the flywheel mounting bolt threads?
The flywheel internal o-ring wouldn't stop oil from getting through the threads. I did replace the flywheel bolts.
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Old 01-12-2012, 05:55 PM   #8
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Did you make sure to put the seal in and then the guide ring? The teflon 'ribs' won't ride on the guide ring if you put in the guide ring first. If the guide ring is already on the crank when you install the seal then the ribs will get pushed out when you press it in.
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Old 01-12-2012, 05:57 PM   #9
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Originally Posted by bgoodsoil View Post
Did you make sure to put the seal in and then the guide ring? The teflon 'ribs' won't ride on the guide ring if you put in the guide ring first. If the guide ring is already on the crank when you install the seal then the ribs will get pushed out when you press it in.
Nothing resembling a "guide ring" under the flywheel. The flywheel bolts directly to the crank end. It appears that the flywheel end sits directly on a thrust bearing surface.


mine doesn't look like this....this must be a later model.







Mine looks more like this:
(Stole this from "pbeckm's" /6 thread)



Photo from the first flywheel removal / cleanup.....I was surprised to find a drilled flywheel living there. It doesn't weigh much.

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bmwhacker screwed with this post 01-12-2012 at 06:30 PM
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Old 01-12-2012, 06:27 PM   #10
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The flywheel O-ring doesn't keep oil from going through the bolt holes. It keeps the oil from getting to the bolt holes.
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Old 01-12-2012, 06:32 PM   #11
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Originally Posted by supershaft View Post
The flywheel O-ring doesn't keep oil from going through the bolt holes. It keeps the oil from getting to the bolt holes.

Gotcha.....The flywheel bolt holes in the crank must be "dead end" holes.(?)
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Old 01-12-2012, 07:19 PM   #12
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Gotcha.....The flywheel bolt holes in the crank must be "dead end" holes.(?)
Yes, they are blind holes. Occasionally oil will get past the o-ring and trickle past the bolts in a very small quantity. It is obvious when this has happened-a bit of oil will be found on the inside of the wheel, and some will run out when a bolt is removed. If this is the case, put a light ring of sealant (I use Hylomar) under the bolt heads where they engage the bolt hole only. It is most likely that your seal preparation and installation weren't correct; the teflon lip must be gently shaped to accept the flywheel. You can do that with a clean fingertip. Use a small amount of WD-40 to ease the seal into the case. The outer face of the seal should be just inside of the bevel in the seal bore. Do not lube the seal surface where it contacts the flywheel with grease; it is intended to be run in dry. Grease won't hurt the seal material, but can prevent it from "seating".

Don't lose heart. I think all of us were stunned when this seal was introduced. I remember calling BMWNA and telling them they'd got a batch of unfinished seals. History has proven this seal to be an excellent design.

This seal was introduced about 1988, so SS is on the mark. It pleases me that BMW continued to refine these bikes so late in the series run.
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Old 01-12-2012, 07:46 PM   #13
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Originally Posted by bmwrench View Post
Yes, they are blind holes. Occasionally oil will get past the o-ring and trickle past the bolts in a very small quantity. It is obvious when this has happened-a bit of oil will be found on the inside of the wheel, and some will run out when a bolt is removed. If this is the case, put a light ring of sealant (I use Hylomar) under the bolt heads where they engage the bolt hole only. It is most likely that your seal preparation and installation weren't correct; the teflon lip must be gently shaped to accept the flywheel. You can do that with a clean fingertip. Use a small amount of WD-40 to ease the seal into the case. The outer face of the seal should be just inside of the bevel in the seal bore. Do not lube the seal surface where it contacts the flywheel with grease; it is intended to be run in dry. Grease won't hurt the seal material, but can prevent it from "seating".

Don't lose heart. I think all of us were stunned when this seal was introduced. I remember calling BMWNA and telling them they'd got a batch of unfinished seals. History has proven this seal to be an excellent design.

This seal was introduced about 1988, so SS is on the mark. It pleases me that BMW continued to refine these bikes so late in the series run.

On the 2nd. go-round I did use a small amount of "BMW #10" lube on the seal..... Oops...Used no lube the first time around. .Thinking about running it a while as is and see if the leakage subsides...wishful thinking.... I only drove it about 10 miles and see a small bit of oil coming from the "shelf". I'll run it a few more miles and see what happens. Just don't want to oil "foul" the new clutch plate.
If the leak persists I'll replace the two o-rings and the seal again and maybe go back to a stock flywheel.
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Old 01-12-2012, 08:52 PM   #14
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My nitty gritty advise is slightly different. I never preshape the seal with my fingers. I don't touch it per BMW's slightly later advise. The seal boss in the case often needs chamfered with a Exato razor blade or the sharp edge will peel the ribs on the seal's OD. I lube the OD with a bit of motor oil. Knock on wood I have never had a problem that I know of.

What about your pump cover O-ring? There are some wrong parts for that out there.

Don't worry about reusing the 11mm bolts per BMW but I always replace the 10mm bolts!
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Old 01-12-2012, 08:52 PM   #15
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Quote:
Originally Posted by bmwhacker View Post
On the 2nd. go-round I did use a small amount of "BMW #10" lube on the seal..... Oops...Used no lube the first time around. .Thinking about running it a while as is and see if the leakage subsides...wishful thinking.... I only drove it about 10 miles and see a small bit of oil coming from the "shelf". I'll run it a few more miles and see what happens. Just don't want to oil "foul" the new clutch plate.
If the leak persists I'll replace the two o-rings and the seal again and maybe go back to a stock flywheel.
make sure it's actually motor oil on the shelf. Trans oil smells totally different.
Also, sometimes the oil pump cover is actually loose!
I have seen that more than once.
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