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Thread: K100RS fork rebuilt

  1. #1
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    K100RS fork rebuilt

    Hi All,

    Scored a set of used front forks off someBay and like to rebuilt them for my bike. Question I have is the cup that goes over the inner tube -- is that functional? It seems that it's just a piece that prevents dirt from getting in. At $21 a piece, I was hoping you all tell me it's non-essential.


    Also Should I order things that are labeled "gaskets" ? There are ones listed with "inner tube". some with "absorber" and some with "damper" on the microfiche. What do I actually have to replace? And I did a search in Progressive Springs and didn't see much. What's the latest opinion on them?

    Thanks


    Chuck
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

  2. #2
    A bozo on the bus deilenberger's Avatar
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    Chuck,

    Are these the same forks as are on your '85? Starting in '86 there were some changes made to the long-travel (and rather soft) K100 forks. As far as the parts, if you can provide a link to a photo or webpage using those terms it would help a lot in answering your questions.
    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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    Hi Don,

    Thanks for the reply. The set I got has the same part numbers as the pair I have on the 85 right now, so I'm assuming they are the same.

    What I was referring to as "cup" is the part labeled as 1. My set doesn't have the circlip inbetween the fork seal and the cup but has a ring washer.
    Attached Images Attached Images
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

  4. #4
    A bozo on the bus deilenberger's Avatar
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    Hi Chuck,

    The drawing you're looking at is of a later year set of forks, and they are different from the forks on your bike (and I'd assume the ones you bought, which do look like '85 forks.) The giveaway is the fork-bridge.. that started with the S series forks on the '87 K75S. The forks you have do not have a clip above the seal (it's just pressed in) - and the "cup" is the dust shield for the seals (keeps rain/munge away from the fork seals to some extent.)

    I'd suggest heading over to www.realoem.com - and enter your VIN# (the last 8 letters/digits) so you can see a diagram that is specific to the forks you'll be rebuilding. REALOEM can drill right down by year/model/month to the correct drawings if you give it a bit of info.
    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. That's why I was getting confused because the diagram isn't what I'm seeing and words used in the microfiche "cup" and "dust shield" are words describing the same thing. Thanks for the link to realoem.
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

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    My last 7 digits on the VIN are: 0042322. Realoem shows the same diagram, poss due to updates on parts designs?
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

  7. #7
    A bozo on the bus deilenberger's Avatar
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    Chuck,

    There are two sets of diagrams on REALOEM using your VIN#. You want to use the lower set: http://www.realoem.com/bmw/showparts...72&hg=31&fg=10

    That's the one for your forks - which are non-"sport"..
    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

  8. #8
    Benchwrenching PGlaves's Avatar
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    To answer the original question, the "cup" or dust shield is essential in my opinion. It is the first line of defense in keeping grit away from and out of the seal.

    Should you prefer (I do), a set of full length rubber accordian gaiters or boots that protect the entire exposed portion of the fork tubes could be substituted. They would provide even better protection. The top diameter of the boot should match the outside diameter of the fork tube and the bottom diameter needs to match the outside diameter of the fork sliders. It is possible that the gaiters (boots) for older Aiheads will match up, but you may need to go see a shop that is more oriented to dirt bikes.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

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    Took me a few days, but I finally got one of the springs out. There is one spring inside and it measured 18 inches or 460 mm. My Haynes manual and Clydes both say the free spring should measure about 400 mm. I have a hard time believing the spring can lose compression/elasticity by 60mm when they get old. What am I missing?


    I'm still contemplating a set of Progressive springs. Are those worth it?


    Thanks
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

  10. #10
    A bozo on the bus deilenberger's Avatar
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    Haynes and Clymer are often WRONG. They sometimes don't follow all the different parts BMW has used on the bikes through the years. You have the very early non-sport long travel forks. They're known to be a bit mushy, but give a comfortable ride. 460mm is longer than 400mm, so that would mean they grew 60mm with use. Highly unlikely.. more likely the manuals are simply wrong. There should be a spacer above the springs, IIRC it was about 1.5" long on the original springs on those forks.

    Progressives are OK - IMHO - Works dual-rate springs are a better choice if you're willing to spend some time tuning them. They are two springs, a short soft one, and a long stiffer one. You can change the changeover-rate of the springs from soft-to-hard by using different length spacers (provided with the springs) in the softer one. They also require some spacers above the springs to get the sag correct, but PVC tubing is cheap and works great for the application. I once made a bunch of different length ones so I could play with the settings.

    Good luck, and when it comes to accuracy, don't count on the aftermarket manuals. The factory manual (which I no longer have) was always my "bible" when it came to questions about stuff like this in the aftermarket manuals.
    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

  11. #11
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    thank you Don for the always valuable information. BTW, I should have listened to you on the rear main seal (if you saw my other post and the difficulties I had with that). Yes, there is a spacer on top, about 3-4 inches in length. And the fork it's a Brembo fork.

    As for tuning, right now I have 10w oil in the bike filled to about 330ml, volume which based on the information in Haynes manual is probably wrong for my early 85 fork. But so far, after about 500 miles of riding, the amount of stiffness is okay. I'm more annoyed by the handle bar rubber mount giving under braking than any dive from the forks. I have the rubber pieces on order.


    I think I'll leave my original fork springs alone for now and just "tune" with oil weight and volume till I blow out the seal since I have the spare set. I'll look into the Works Springs.


    Thanks again.
    Chuck
    Current: 1985 K100RS
    Previous: 2002 R1150GS, 1990 Suzuki VX800, 1993 Honda CBR600F2, 1988 Honda CBRF, 1978 Honda Passport

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