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Thread: 75 R90/6 New Rear Main Seal Leaks

  1. #1
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    75 R90/6 New Rear Main Seal Leaks

    Just finished a complete engine rebuild of my 75 R90 with 120k miles. Runs great, and I've put about 50 miles on the new engine. However, I noticed quite a bit of oil coming from flywheel area.. enough that I needed to pull the tranny to see if it was rear main or oil pump cover (both renewed when I rebuilt the engine). .. I DID block the crank, by the way :-)

    Well, the flywheel came off without a hitch. Oil pump cover did not appear to be leaking, but I replaced the o-ring anyway. Back of flywheel was wet with oil. Oil seal looked fine. I looked at the flywheel sealing surface and it's ok.. nothing to snag a fingernail. no ridges.

    I reinstalled the flywheel today, and just to be sure, left the tranny off and installed carbs and tank so I could start the engine while watching the flywheel.

    Sure enough, leak is still there. Nothing coming from the oil pump cover. Oil is being slung around behind the flywheel.. It must be the seal.

    I checked the crankcase vaccuum by loosening the dip stick .. it's fine. The check valve is doing its job.

    This is the new teflon seal, installed after soaking overnight in 10w30, by the book, with the installation tool. It's square to the case, about 1mm proud.

    So.. I'm a bit frustrated. I hate the thought of replacing the seal only to have it leak again.

    Is it possible that this new seal will begin to work after a time? It's currently leaking about a pint every 100 miles.. enough to leave a large puddle wherever I go, and it's coating the rear wheel. Not a good thing. Could it be leaking somewhere else?

    I'm also thinking of going back to the old double-lip seal.

    Any suggestions?

  2. #2
    Administrator 20774's Avatar
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    Sorry to hear you're having troubles. Check out Snowbum's pages on the rear main seal...maybe there's something there that will help.

    http://bmwmotorcycletech.info/flywhe...valwarning.htm

    Also might want to bookmark his main tech page:

    http://bmwmotorcycletech.info/techindex.htm

    Enough there to while a way a bunch of rainy days!!!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
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    The teflon seal is supposed to be installed dry. I'd suggest you get another one. Wet the O.D. only with WD-40.

  4. #4
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    Thumbs up dry lip seal

    The teflon seal has been around since the 70's, I was a diesel mechanic at the time and could not believe you would install it dry. It did leak but the customer needed to haul some freight so I told him to bring it back for a replacement. The seal was 6" in diameter and that may have had an effect in breaking through the oil film as it sealed up.
    Maybe you will get lucky too?

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    Rear Main Seal Leak

    Thanks for the tips. Seems the consensus is to install a new one, dry, with a light lube on the flywheel sealing surface. I've ordered another one from Bob's and will give it a shot... and I'll leave the tranny off till I'm convinced it's oil-tight.

  6. #6
    Registered User 88bmwjeff's Avatar
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    FWIW, when I replaced my RMS in 2004, it leaked a bit, but not as much as you indicated. It stopped after a while. I guess in my case it just needed to set in abit.
    Jeff in W.C.
    1988 R100 RT (the other woman)
    "I got my motorcycle jacket but I'm walking all the time." Joe Strummer

  7. #7
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    Don't put any kind of lubricant on the teflon lip. I know-it sounds crazy, but this technique was suggested by BMW, who probably got it from the seal manufacturer. BMW has supplied this seal since about 1986, and I have installed dozens of them this way.

  8. #8
    Stage Crew beemerPhil's Avatar
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    Lostboy is correct- never heard of oiling a teflon seal. Also installed dozens this way with no problems- the only time I've seen these leak, when installed correctly, is if there's a burr or severe wear on the flywheel stub.

    I'd try cleaning the oiled seal off very carefully and re-installing it, but a new seal is a sure thing....they're not as expensive as they were when they first came out!

    Just curious- how do you check the crankcase breather by unscrewing the dipstick? Sounds like a good theory, but I don't think it'll pass. The change in crankcase volume through each crankshaft revolution is equal to the engine's displacement, plus blow-by volume; there will always be some pressure, even in a very tight engine. There is no pcv vacuum connection like a car has- the vent hose dumps upstream from the carb throat, where the pressure drop is far less than downstream.
    Ring sealing is critical; nikasil was the solution, and the teflon seal was essentially the last word on rear main seal complaints.
    Phil Keppelman #20331
    MOA Rally Stage Manager
    The shortest distance between two points.............
    ain't how I got here......

  9. #9
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    Rear Main Seal Leak

    I just got a new seal from Bob's.. and it looks different from the one I installed originally. I see the "BMW" on the outer rim. I'll check for it on the leaky seal. It will be interesting if the bad one turns out to be a poor aftermarket copy.

    I found you could unscrew the dipstick and let it lay on the top thread. A good crankcase vacuum will hold the cap in place. I guess it was a long shot, thinking a positive pressure could push oil out the rear seal.

    All that to say I will be replacing the seal, with a dry one. Thanks for the comments.

  10. #10
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    R90/6 Rear Main Seal Leak

    Well, I installed the seal today, dry. Light buff on the flywheel surface to make sure it was smooth. Light oil on it too before re-assembly. Went in without a hitch. No leak observed during the run time. I checked the old seal and sure-enough, it did not say BMW on it. The new one did. I'll be sure to check this if I'm every around this way again. Now it's time to put the rest of the bike together. Many thanks for the help!

  11. #11
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    R90/6 Rear Main Seal Leak

    Here's a pic of the new seal, as installed...




    The quick run-setup, without transmission. I needed to jumper the neutral switch cable for the starter circuit to work.


  12. #12
    Administrator 20774's Avatar
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    Sounds like you've turned a corner on this one...great! Man, you can eat lunch off the inside of that bell housing! Yowsers...that's clean!!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  13. #13
    Polarbear Polarbear's Avatar
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    Good seals:)

    I've got at least 250000m on a teflon replaced seal in my R100/7, I did myself a loooong time ago. And, yes it went in dry! They were very stiff, as I recall and an overnight shaping of the seal on the flywheel was recommended then at the time, before final installation. I have had NO sign of a leak since that day, very long ago. Randy and still have the bike, since new@370000m now...

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