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

  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
    Nickname: Droid
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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.

  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.

  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.

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

  7. #7
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    Oh - I see what you're saying....

    I just checked and it's actually a 1-3/16" socket you need to remove and reinstall the nut (if you don't have the metric equiv).

    I think I used a small block of wood to install the seal, tapping gently. Pretty sure you want to let the seal protrude a tiny bit and not set it entirely flush. I think I used a thin washer to measure that protrusion distance. That was 9000 miles ago on my bike and it seems to have worked.

    Have somebody strong holding your bike up when you torque that nut back on so your bike doesn't tip over on you.

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    Hi Tim,

    Thank you for the info. Can you also tell me what I should use to pull the seal? Also will oil leak out when the seal is pulled or do I need to drain the engine oil first?

    Thanks again

  9. #9
    3 Red Bricks
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    An automotive seal puller can be purchased from most car parts stores for around $10. No oil should leak out.


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

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    Hi Tim, thank you for the link.

  12. #12
    A bozo on the bus deilenberger's Avatar
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    Cy7878 (real name? We're friendly here..)

    Please read: http://www.bmwmoa.org/forum/showthread.php?46057 - I'm going to add that info to your thread title.

    The two parts are completely different.

    The "rear seal" seals the crankcase to the hub that protrudes from the rear of the clutch carrier (aka - basket, whatever..) The O ring seals the splined shaft that drives the carrier, and is located under the nut holding it on, and a special spacer that lives under the nut. You need to protect the O ring on installation - tape around the splines works.. just something so the sharp edges of the splines don't cut the O ring.)

    The rear seal design has changed at least 3 times that I know of. The new one (if you even need one..) will not seal if set to the depth in the original factory manual. In general - it sits a bit "proud" of the case (protruding a bit.) The dealer has a special machined driver to install it. Most of us just wing it. If you check the actual seal LIP position in relation to the front surface of both seals, you'll get a hint where it has to be.

    My WAG - the rear seal is just fine. The O ring is leaking. If the O ring leaks - it does NOT tend to oil the clutch. The rear seal can oil the clutch, as can the transmission input seal. Some people replace everything when it's apart. I don't. I'm a believer in "leave well enough alone" (or don't fix it until it's broken - which has a dual meaning, think on it..)

    Might I suggest you open your user control panel here on the forum and add a "SIG" to it.. something telling us (1) who you are (2) where you are (3) what you ride. That makes it lots easier on other members to help you. Making it easy usually means you'll get more and better help. Just a thought..
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

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    Thanks Don. I have edited my sig.

  14. #14
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    Quote Originally Posted by deilenberger View Post
    Cy7878 (real name? We're friendly here..)

    Please read: http://www.bmwmoa.org/forum/showthread.php?46057 - I'm going to add that info to your thread title.

    The two parts are completely different.

    The "rear seal" seals the crankcase to the hub that protrudes from the rear of the clutch carrier (aka - basket, whatever..) The O ring seals the splined shaft that drives the carrier, and is located under the nut holding it on, and a special spacer that lives under the nut. You need to protect the O ring on installation - tape around the splines works.. just something so the sharp edges of the splines don't cut the O ring.)

    The rear seal design has changed at least 3 times that I know of. The new one (if you even need one..) will not seal if set to the depth in the original factory manual. In general - it sits a bit "proud" of the case (protruding a bit.) The dealer has a special machined driver to install it. Most of us just wing it. If you check the actual seal LIP position in relation to the front surface of both seals, you'll get a hint where it has to be.

    My WAG - the rear seal is just fine. The O ring is leaking. If the O ring leaks - it does NOT tend to oil the clutch. The rear seal can oil the clutch, as can the transmission input seal. Some people replace everything when it's apart. I don't. I'm a believer in "leave well enough alone" (or don't fix it until it's broken - which has a dual meaning, think on it..)

    Might I suggest you open your user control panel here on the forum and add a "SIG" to it.. something telling us (1) who you are (2) where you are (3) what you ride. That makes it lots easier on other members to help you. Making it easy usually means you'll get more and better help. Just a thought..


    Thanks Don also for the reply. Yes, I see now after taking the Hex Nut (30mm BTW) and saw the O-ring you mentioned with oil seeping out. AFter taking off the clutch housing/basket/whatever, I see the large brown main seal. I did buy a replacement main seal but was wary of replacing, especially when I see the 2 parts looking very different. How long do the seals last? Is it a age thing or mileage thing? Now that I poked around, I'm also worried that I messed up the lip and now it's going to leak....

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