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Thread: R65 Front Master Cylinder

  1. #1

    R65 Front Master Cylinder

    So I need to rebuild the front brakes on a 1981 R65 with duel Brembo front disks, the MC has a 13 stamped on the bottom would it be a 13mm??
    It's a relative low mile bike (29K) and very stock.
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    Last edited by 3457fl; 02-03-2019 at 10:11 PM.

  2. #2
    Registered User Anyname's Avatar
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    Looking at photos in the Max BMW fiche, it appears that the number cast into the master cylinder is indeed the diameter.
    BMW R bike rider, horizontally opposed to everything...

  3. #3
    Liaison 20774's Avatar
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    Use your VIN in RealOEM or MaxBMW to identify the specifics for you bike. I was looking at RealOEM and for a dual disk front end, I thought I saw 16mm. If you have twin disks, don't you need to push more volume?
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
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  4. #4
    Yes, but not sure mine is stock from the factory with the duel disk. Might of been a owner upgrade from the first owner, I bought it from the second owner.

  5. #5
    got, got, got no time... rguy's Avatar
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    The original dual brakes for 1981 should have been ATE calipers with 16mm Magura master cylinder. If they are indeed Brembo calipers they were probably replaced once upon a time. ATE replacements have not been available for years and I don't think there are rebuild kits available either. New Brembos are the only official option AFAIK. As far as the master cylinder is concerned, I *believe* the 13 mm was for single disk but I cannot say for sure. I bought my R65 new in 1981 and confirm that the ATE dual disks used the 16mm master cylinder.
    Neal - '16 R1200GS / '81 R65
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  6. #6
    Registered User GTRider's Avatar
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    The dual-disk upgrade from BMW used a 16mm MC. It’s possible a dual-disc conversion was done with minimal individual parts purchased, and the OEM 13mm MC left in place. IIRC there were complaints about the 16mm giving a “wooden” feel to the brakes. So long as the 13mm MC is in good condition it may yield better feel and more effective braking at the expense of greater lever travel.

    I hope the OP has carefully inspected the MC for corrosion or pits. Even the slightest will result in an unsatisfactory finished job.

    Best,
    DG
    DGerber
    1983 R80ST — 1984 R80 G/S PD — 1993 R100GS — 2004 K1200GT w/Hannigan S/C — 2010 K1300GT
    BMWMOA#52184, AMA#271542, IBA#138

  7. #7
    Registered User GTRider's Avatar
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    Here’s a good reference on the MC sizing: http://www.largiader.com/articles/mc/

    and a source for complete master cylinders in various sizes: https://www.motorworks.co.uk/vlive/S...BA_15_20_HA_35

    I’d be tempted by the 14mm if one could be sure it would fit the ‘81 perch.

    A complete new MC is a beautiful things-no rust, no leaks, and a nice clear reservoir so one can easily see fluid level. Just sayin’

    Best,
    DG
    DGerber
    1983 R80ST — 1984 R80 G/S PD — 1993 R100GS — 2004 K1200GT w/Hannigan S/C — 2010 K1300GT
    BMWMOA#52184, AMA#271542, IBA#138

  8. #8
    Thanks for the link to the new MC, think I will go in that direction about double of a rebuild kit but new.

    The bore of mine looks very good without any marks or corrosion, but new is nice.

  9. #9
    Motorworks responded to my questions and I thought it was good information:

    HI
    If your existing master cylinder is the rectangular type as per BRA10785 then BMW originally fitted 16mm for twin disc and 13mm for single disc R65 between 09/1980-09/1981
    Then they changed to 12mm for single disc and 15mm for twin disc. It will physically fit no problem and work fine.

    The differences are how the lever feels. Larger the cylinder the more wooden- less feedback.
    The 14mm one will work for twin disc- Bmw used it after 1985 for twin disc. So, the choice is yours but for me 14mm is the smallest I would fit for twin Brembo calipers- and 15mm the largest, Bmw clearly found the 16mm was slightly too large as they changed it after only one year,

  10. #10
    Registered User GTRider's Avatar
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    Sounds like you are ready to make a selection and get the bike back in service. Please do yourself and any future owner a favor and fit new, correct BMW crush washers on the lines, and insert into your owner’s manual a card or sheet outlining the non-standard brake parts and information as to their source. Years and miles down the road that info will be valuable should the MC need attention again.

    Happy wrenching and riding!
    DG
    DGerber
    1983 R80ST — 1984 R80 G/S PD — 1993 R100GS — 2004 K1200GT w/Hannigan S/C — 2010 K1300GT
    BMWMOA#52184, AMA#271542, IBA#138

  11. #11
    I have the 16 mm master cylinder on my 1981 duel disk with original a t e calipers but I changed them to Brembo, much better brakes. although i haven't been able to get perfect seal/feel yet so yes I think that the 15 mm master cylinder, a new one would be great! Plus 2 SS brake lines from the MC would be an upgrade? Or 1 line into 2 lines is better, similar to modern brakes better? Please offer your opinions/experience!!

    Sent from my Nexus 6P using Tapatalk

  12. #12
    Registered User Anyname's Avatar
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    My 1986 R80 (originally and RT), has what I believe is original dual Brembo disks. It also has a 13MM master cylinder. There was some discussion of the effect of changing the master cylinder size. A larger MC will move more fluid with less lever movement, but will require more force as it loses some hydraulic advantage. Conversely, a smaller MC will require more motion at the lever, but it will require less force.
    BMW R bike rider, horizontally opposed to everything...

  13. #13
    I think all will work, just depending on the feel that you are looking for. I went with the 14MM which seemed to be in the middle of the road, I like to have a bit of feel when braking.
    2 day ship from England to Florida which is quite nice. Have the calipers all disassembled and soaking for a couple of days before cleanup and rebuild.

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