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Thread: Rear Main Seal replacement mid-90s K75S

  1. #1
    Registered User 88bmwjeff's Avatar
    Join Date
    Aug 2004
    Walnut Creek, CA

    Rear Main Seal replacement early-90s K75S

    I may need to help someone replace the RMS on an early 90's K75s. I'm curious if replacing the RMS on the K75s is simialr, more difficult or less difficult than on my 88 R100 RT.

    Thanks in advance.
    Last edited by 88bmwjeff; 04-25-2013 at 02:55 AM.
    Jeff in W.C.
    1988 R100 RT (the other woman)
    "I got my motorcycle jacket but I'm walking all the time." Joe Strummer

  2. #2
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.
    If you want to borrow a driver for the seal, PM me with your phone #.


    Ride Safe, Ride Far, Ride Often

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

  3. #3
    Rally Rat
    Join Date
    Jun 2012
    Olympia, WA
    Replacing that seal took me 3 minutes.

    Removing the back half of the K75 takes a good two hours, as does reattaching it all.

    It's all work though; no math or edumacation is involved besides setting the torque wrench.

  4. #4
    Don't even consider replacing the seal without replacing the O-ring on the crank. That's probably the source of any oil you see.
    Anton Largiader 72724
    Tech articles
    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  5. #5
    I just did the RMS on an '89 K75 restoration. The bike sat outside uncovered and unstarted for 10 years!

    I've done a couple K1200 seals too. Other forums suggest using a Viton O-Ring instead of the BMW butyl o-ring. Viton apparently holds up better to the heat than the OEM rings. The K75 and K1200 use the same size ring, and I suspect the K100 and 1100 do too. But a little motor oil on the ring when you install it.

    There were a couple different seal models and the installation instructions are different. If you borrow a driver, make sure it drives the seal to the correct depth. The original seals were set flush to the cases while the new seals are set about .5 mm proud of the cases and go in dry.

    When pulling the transmission, you need to disconnect the clutch cable. Be careful. With the cltuch cable disconnected it is easy to tear the rubber cup on the back transmission. When pulling the trans off the bike I put a small/flat "piano" dolly underneath and blocked it up so I could slide the trans straight off the motor. I wish I had though of that when I was muscling the K1200 tranny on and off.

    I find it a lot easier to install the seals straight if I can get directly behind the motor, so on the K75 I pulled everything off the back of the bike--fender, case mounts, even the intermediate "clutch" housing. (It gave me a chance to give the clutch housing a good cleaning too.) On the K1200 I actually pulled the motor completely out the last time I changed the seal.

    You'll need to make a plate to hold the clutch cover while you remove the nut holding the basket--I used some aluminum stock and did some cutting and filing so it would clear the nut and fit snug to the sides of the intermediate housing, then drilled hols so I could bolt it to clutch housing.

    When you have the trans off it's a good time to check for leaks around the seals. While I didn't have leaks, I replaced my k1200 transmission seals as a precaution.

    Before disassembling the clutch I filed a small spot on the clutch cover, basket and spring so I could reallign them on reassembly. It's a balanced unit. The problem with using paint or a marker is that those marks wash off. When reassembling the clutch follow all the lubrucation instructions. I've been using a mix of the recommended BMW lube with some Honda Moly 60.

    BMW recommends using a special tool to align the clutch cover and friction plate so the clutch actuator rod can slide right in when you reinstall the transmission. I used a appropiate size punch to line up everthing except the friction plate which I eyeball. Before sliding the tranmission back on, I cut the heads off a couple long bolts that I threaded into the clutch housing. It make it much easier the slide the transmission on while lining up the clutch actuator rod with the headless bolts in place.

    There is a lot of good advice out there, and most of what I suggest was culled from other's experience.


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