/5 fork refurbish

Discussion in 'Airheads' started by oldroadie, May 1, 2012.

  1. oldroadie

    oldroadie Two wheel addict

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    So there I am in my little shop stripping the front end and refurbishing the rubber bits and I come across four bits that I can't account for: two half-thickness donuts that resemble the bottom stops and two clear/white rubber rings. The exploded diagrams don't seem to have these bits in any kind of logical assembly order so I've come to the collective to obtain wisdom. I'm thinking the thick one is the bumper stop, maybe the thin one is an alternate?

    For what it's worth the existing rubber (at least that what I think it was) was mired in brown goo at the bottom of each leg and totally unidentifiable.

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    #1
  2. oldroadie

    oldroadie Two wheel addict

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    Anyone? Bueller?
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  3. Renner

    Renner combustophile

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    two different-sized pairs of black compression bumpers. that's new to me, maybe appropriate to different models?
    If I recollect correctly the taller pair are right for the /5.

    The white translucent ring is an extension bumper, slips loosly over the long rod of the damper assembly.
    The diagram (not /5 specific) is not particularly clear about this (and the part you have circled there appears to be the aluminum ring sandwiched between threaded rings at the bottom of the fork tubes, the damper guide ring... or probably not as I look again... got to see the whole image).

    Don't let the drawing tempt you to place the extension bumper where it will be in contact with the spring.
    Think of that bumper as a bump-stop for the dampers when they reach full extension.
    #3
  4. Renner

    Renner combustophile

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    seen here, I believe items 9 & 10 are interchangeable, so though both are shown you need only one each side and it'll be aluminum... but you're not asking about that.

    Item 7 is the white extension bumper, but it's not in direct contact with the spring, as it appears here.
    Spring 6 sits on top of item 14 and #7 slips up #18.
    IOW the complete damper assembly fits "between" #6 and #7.

    This part of the illustrated parts catalog leaves a lot to be desired, IMO.
    (I copied it from MAX BMW under R75/5: http://www.maxbmwmotorcycles.com/fiche/DiagramsMain.aspx?vid=51885&rnd=05012012 )


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    #4
  5. oldroadie

    oldroadie Two wheel addict

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    Thanks for that. The white one had me stumped because I tried the order listed in the fiche and knew immediately that it was wrong. Between the top of the damper and the bottom of the stanchion makes sense. So did I get excess parts? The smaller of the bumper stops is still a mystery.
    #5
  6. Renner

    Renner combustophile

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    yes.

    and yes: still a mystery but of the two I'd go with the taller bottoming bumper.

    Maybe someone will offer more insight...
    #6
  7. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    This is the first time I've seen/heard of the bigger bottoming bumper; I'm bumfuzzled.

    The thinner one is 31 42 1 230 751, which I've ordered from Hucky before. MAX BMW lists another bumper as 31 42 1 232 426, which might be the thicker one. Riding on pitiful Walker County roads I'd be inclined to hit the thicker one.

    Check around, and I'll check around too.
    #7
  8. oldroadie

    oldroadie Two wheel addict

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    I went for the thicker one since the roads here in DeKalb county can get pretty rough. Once Renner set me straight on the bushing being out of assembly order in the fiche it was full steam ahead. The front end responds so much better now it's obvious the springs are weak or worn out....

    I have the two thinner bumpers left over....anyone? Anyone?
    #8
  9. Boxer Metal

    Boxer Metal Mad Scientist

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    The taller one on the left is for a 74-80 fork and the middle one is for a 70-73 fork.
    #9
  10. oldroadie

    oldroadie Two wheel addict

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    Ah, too late but thanks for clearing that up; I won't be wasting 560cc of fork oil to gain that 1/2". I'll just keep them hacndy for the next time I change out the fork oil.
    #10
  11. Renner

    Renner combustophile

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    thanks for clearing that up... good to know.
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  12. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    And the part# on the later bumper is:

    31 42 1 231 314 rubber bumper - bottom thread plug 1974-1980
    #12
  13. oldroadie

    oldroadie Two wheel addict

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    AHA! So there's a proper order for the front brake springs, too! Well that answers the new shoes worse than the old shoes question :-)
    #13
  14. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Yep. there is an upper and a lower sproing. And if the stronger spring weakens (with age) the braking can be off. Reason is these are "self-energizing duplex drum brakes", and all has to be in order.

    Duane Ausherman has a good section on these brakes:

    http://www.w6rec.com/duane/bmw/
    #14
  15. oldroadie

    oldroadie Two wheel addict

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    Yes sir it was Duane's article that made me realize my error. And, for drums, the front brake now works pretty good; once it's bedded properly it should be just fine. The rear drum is still not so keen but I'll keep running it in until gets better.
    #15