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Thread: '85 K100 I'm confused, what is the "rear main seal" and what is causing my leak

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  1. #1
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    '85 K100 I'm confused, what is the "rear main seal" and what is causing my leak

    Hi all,

    Thanks in advance for the info. I started out just intending to lube the splines, but now found oil in my clutch housing. Don recommend that I replace the clutch-nut O ring, which I am going to do. But looking thru Haynes Manual, I can't see what exactly is this "rear main seal" that people purport to be a major cause of oil leaks into the clutch housing which dribbles down the opening onto the gearbox bottom which I also have.

    Are the clutch nut O ring the same as the rear main seal? If not does any one know where on the microfiche will I be able to see what the part actually look like?


    Can I just spray brake cleaner to get the oil out?


    Thanks again
    Last edited by deilenberger; 03-28-2013 at 11:06 PM. Reason: added year/model to title

  2. #2
    Registered User ANDYVH's Avatar
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    If the friction plate is oil soaked you "may" be able to clean it with brake cleaner, but I doubt it will provide full clutch grip. Once oil soaked a friction plate needs to be replaced. The rear main seal is the main oil seal around the engine cranckshaft output flange. The clutch basket/carrier/flywheel mounts to this flange and the main seal is what the flange rotates against. When it leaks hot engine oil seeps past it and the spinning action of the clutch basket flings it onto the friction plate.

    Look for the main seal in the back of the engine.
    Woodenshoe to Cheesehead

  3. #3
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    Get a Clymer manual for your bike. You'll want to reference it while you're working on your bike.

    Once you remove the clutch assembly to clean it, you'll see the rear main seal and O-ring behind it.

    A leaking seal doesn't necessarily "fling" oil onto the spinning clutch plate.

    Read about clutch repair before you remove it. The pieces need to be marked first so they are replaced exactly the same.
    Tim
    Olympia, WA
    1991 K75 - 121k Miles

  4. #4
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    Thank you all for the quick reply. I have already took off the clutch housing, pressure plate and the clutch. I saw the mark on each and will make sure the marks go back 120 degrees apart as per Haynes Manual and other info found here.


    On further search (I think the forum archives old posts so they don't take up too much room here and now), I got a part number on the teflon seal and Don's previous suggestion using piece of thin polycarbonate from packaging wrapper to form a sleeve over the shaft to avoid damaging the seal.

    I think my confusion comes from not having a 32mm socket to remove the big M20 hex nut to actually remove the rear clutch housing to see what the back actually looks like. Since the seal costs about $30, any suggestion on how not to screw up is still greatly appreciated.

  5. #5
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    I think I bought a 1.25" 1/2" drive socket at my local Autozone for $7.99.
    Tim
    Olympia, WA
    1991 K75 - 121k Miles

  6. #6
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    to remove the nut? or is it to drive the seal? thanks for the reply.

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