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Thread: 75 R90/6 New Rear Main Seal Leaks

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  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
    Rally Rat
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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?

  5. #5
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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

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