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Thread: Rear main seal replacement help '85 K100

  1. #1

    Rear main seal replacement help '85 K100

    I am replacing the rear main seal on an 85 k100. I have the motor on the bench/ out of the bike.
    I am on my 2nd seal, I boogered up the first one.
    Any tricks, or tips b4 I try again?

    Thanks in advance for any help.

    This is my project bike, I bought a 90 k100lt Parts bike (because I needed the cooling fan to go to the Tenn. Rally a couple years ago) Figured I would part it out. So I took it all apart and started selling parts. Well I found another parts bike, an 85 k100rt I bought for the 22 lt. top case. I decided to make one bike out of the 2, so I bought most all the parts I sold, still need a couple small parts.
    So it is time to take that pile of parts I have and make a bike out of it.
    I will post progress, but expect it to be a slow process. Winter is coming, my favorite time of the year Snowmobile season.

  2. #2
    Rally Rat CATHDEAC's Avatar
    Join Date
    Nov 2006
    Round Rock, Texas
    Good luck..

    Setting seals is pretty easy..
    - measure and document the "depth" the seal is set/driven prior to removal.
    - clean the location, remove seal, verify clean area.
    - put clean lube on outside of seal and inside the install spot.
    - Put some "engine rebuild lube" on the inside of the seal to retain the spring if there.
    - try to keep "square".. as driving... and do not "dent" the seal.
    - use a deep socket that is close to the size of the seal.. or use a dedicated "seal driver"
    - if installing seal over the shaft, ensure it is clean and free of sharp edges... then lube for install.
    - be careful and drive "deliberately".
    - drive to correct "depth"... and be sure it is "square"/level.

  3. #3
    A bozo on the bus deilenberger's Avatar
    Join Date
    Mar 2003
    Spring Lake NJ, USA
    cathdeac (real name??) had some good suggestions.. but a few comments:

    - BMW has changed the seal design several times on the K series bikes. What is important on installing a new seal is determining where the lip on the seal will ride on the clutch housing (aka flywheel sorta) boss when it's all installed. IF you set one of the new seals to the same depth as the original '85 K100 seal was - you will have a leak.

    Since you're on your second go-around, I'm assuming you no longer have the seal you removed kicking around. If you did - I would have suggested that you use a vernier-caliper to measure the seal height from the outer edge height.. then compare that to the new seal. Having those two dimensions would give you some idea of how deep to mount the new seal. Not having those makes for more of a challenge.. You'll have to take measurements from the output shaft height on the engine, to where it mates up on the clutch housing, and the outer edge of the boss where the seal seats. I'm sure it can be done, but I'm at a loss as to how to describe this in words...

    As I remember (memory is often faulty - and it's been about 15 years since I did this job) - the newer seal design sits higher (somewhat "proud") in the recess then the original (which sat flush with the case..) BMW had/has a special driver that pushes the seal in the correct amount and keeps it square with the bore it fits into. If the engine is out of the bike - it might be worth taking it to a dealer and asking if they can install the seal for you (with their special tool..)

    - Also - with newer teflon seals - some are designed so the sealing lip is supposed to face in toward the oil surface (which shouldn't be a problem in this case) and that the lip is supposed to be pre-formed for what it seals on before installation. The pre-forming can be done by putting the seal over the boss on the back of the clutch housing overnight.

    - You also want to make sure the boss is absolutely clean and smooth. Using a bit of 1000 grit paper on it to provide a tiny bit of "tooth" to help the seal break in might not hurt also. Use the paper in the direction the seal seats on (ie - around, not across the boss.)

    Don Eilenberger
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

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