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Thread: K75 Water Pump Question

  1. #1
    Registered User argent brick's Avatar
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    K75 Water Pump Question

    Hi everyone,

    It is my understanding that BMW used two different water pumps on their K bikes. Can someone please explain why the change, the differences between them, and if one is better than the other. I don't have problems with my water pump. I am just trying to gain a little more knowledge and wisdom.
    Lynn
    MOA #57883
    Current Ride: 1995 K75 Standard
    Past: 1978 Yamaha XS 750, 1976 BMW R60/6

  2. #2
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    I wasn't aware of 2 different pumps. However the seal between the water and oil side has changed several times, possibly leading to new part numbers.

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    K water Pump

    What you probably heard is that there was a shaft and seal change after 1985. To rebuild the early pump required the updated shaft, seal and impeller. The economics of rebuilding the early pump didn't make sense. DAHIK. I believe that all Classic K pumps the interchangable. Harold in Kansas
    Harold In Kansas
    1985 K100RT Bullit
    1985 K100XX/EML Bemel

  4. #4
    Registered User 58058D's Avatar
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    The Reason

    Argent,
    The reason for the change after '85, or whenever it was....was to correct the Bullet Through the Cover syndrome. I purchased my K100RS with about 4,500 miles on it in '91. I rode about 500 miles on it and was on the freeway in Sac when it happened to me. I pulled up to the curb at our office softball game and began to show off my new to me bike.....Oooh, Aaah....hey, did you know you are leaking over here?

    The nut on the end of the shaft that held the impeller in was prone to come loose and mine actually shot through the cover plate like a 22 round. Really wierd. Drained all the coolant in a very short time. Even though the bike was out of warranty, BMW paid for all the parts and I covered the labor...all this two weeks after buying the bike, my first intro to BMW pricing! Tim at A&S (still there in charge of service) explained it all to me. Since the design had changed specifically due to this problem, that was why BMW was picking up the parts after 6 years.
    Jim Douglas '00 K1200RS >135,000 miles my primary bike again,
    Gone: '09 K1300S sold @ 22k mi, '93 K1100RS traded @ 78k mi, '85 K100RS sold @ 44k mi
    '06 Kaw 650R track bike sold
    http://www.seagullbb.com/

  5. #5
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    Water and Oil pump 85 K100RS

    I rebuilt my '85 pump on the day I was to leave for a 8 day lap of New England, but had water spewing out and oil in my water 30 min. from home. So I topped up the water and was home by 8am. I read the K bike tech page, took my pump apart and drove in my car the 90 highway miles to Bob's by 11, bought the new style shaft and seals, impeller etc by noon and was home by 1:30, had the new parts in, radiator and overflow bottle cleaned out of the oily sludge (gulf of mexico spill stuff) and ready for a test ride to my son's adult baseball game at 6. It worked fine. I did my lap of New England starting 24 hours late without any more trouble.

    The older style shaft used a nut to hold the impeller onto male threads on the shaft, well the small bolt part of the shaft stretched after a 100,000 miles in my case and oil gets in the water. The new design is better because a bolt holds the propeller on the shaft that has a tapped hole. I think the steel of the bolt is stronger/harder and doesn't stretch, the shaft no longer stretches because it is now a big shaft with a small hole for the bolt.

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    There is a further variation. Pumps prior to approximately 1990-91 had a main shaft/gear with 19 teeth; those after that had 32 teeth. The two gears have a different radius, and are not interchangeable. If you're ordering a replacement part, you must match the one you have.

    If you buy a complete used pump, either variant will fit your K75.

    Additionally, the pumps from the "brick" K1200 (1999-2009?) will bolt right on, though you'll have to use your existing cover on the front. My K75 is proof of this. Interestingly, these pumps have the earlier, 19-tooth gear.

    More discussion here: http://www.motobrick.com/index.php?t...26505#msg26505

  7. #7
    Registered User argent brick's Avatar
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    Quote Originally Posted by JamesInCA View Post
    There is a further variation. Pumps prior to approximately 1990-91 had a main shaft/gear with 19 teeth; those after that had 32 teeth. The two gears have a different radius, and are not interchangeable. If you're ordering a replacement part, you must match the one you have.

    If you buy a complete used pump, either variant will fit your K75.

    Additionally, the pumps from the "brick" K1200 (1999-2009?) will bolt right on, though you'll have to use your existing cover on the front. My K75 is proof of this. Interestingly, these pumps have the earlier, 19-tooth gear.

    More discussion here: http://www.motobrick.com/index.php?t...26505#msg26505
    Any idea why they went from 19 to 32 teeth?
    Lynn
    MOA #57883
    Current Ride: 1995 K75 Standard
    Past: 1978 Yamaha XS 750, 1976 BMW R60/6

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    Quote Originally Posted by argent brick View Post
    Any idea why they went from 19 to 32 teeth?
    No. As best I can tell, it would pump both oil and water more slowly, with fewer rotations of the large main gear and the attached impeller. I suppose it would reduce the pump's drag on the engine; whether there's any inherent advantage to reducing the fluid flow I have no idea.

    I read on one UK vendor's site, though without other confirmation, that the "Authority" (police) models during the period all had the 19-tooth gear, for better cooling in slow traffic. And interesting that they went back to it for the K1200.

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