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Thread: Rear drive compatibility

  1. #1
    Mark Green
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    Rear drive compatibility

    I have a question for you MOA smart guys.

    Is the final drive from an 81 RT compatible with a 78 RS?

    The attachments appear the same. Both are 32/11. The case on the 81 has fins while the 78 case does not.

    Thanks,
    Mark Green

  2. #2
    Polarbear Polarbear's Avatar
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    Drive shaft side check;

    The splines may well be the same, but a look see will confirm this on the drive shaft side of the differential. No other issue, except shock mount to rearend must match, also. '81 was the first year for single sided swingarms on the GS800, but did the other models get it later? I think so. You cannot use a rear from a single sided swingarm on the '78, without some huge mods to the '78's frame, I think. I have a '78 R100/7. Randy13233

  3. #3
    Liaison 20774's Avatar
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    All the R100s as well as the R80RT remained twin shocked thru 1984. The G/S and ST models went single sided in 1981. Everything was single sided in 1985.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  4. #4
    John D'oh
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    rear-end, final drive, differential, hinterradantrieb...

    Quote Originally Posted by 77158 View Post
    Is the final drive from an 81 RT compatible with a 78 RS?
    The attachments appear the same. Both are 32/11. The case on the 81 has fins while the 78 case does not. Thanks, Mark Green
    Mark, I can speak to the compatibility of drum style twin shock style final drives. I am running an 83 R100 final drive on my 78 R100S while I get its original (same ratio), 78 final drive re-sealed. I'm not a fan of rear disc brakes on these bikes so I can‘«÷t speak to the interchangeability of the disc style final drives. I would have to assume they cross-fit and function simply because there are no other changes to the components that also interchange with the disc modles too like wheels, brake components, axles and so forth.
    John D'oh

  5. #5
    Mark Green
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    rear drive compatibility

    Gentlemen,
    Thank you for your resonse. I am going to purchase the later year rear drive for my 78 RS.

    The starter quit working on a trip and I bump started it approx 5 times. That was curtains fro the original rear drive with 78k miles.

    Mark Green

  6. #6
    Registered User AntonLargiader's Avatar
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    According to a BMW service bulletin, you'll have to use the brake shoes, cam and shaft O-rings from the later drive. So if they don't come with it, you'll have to buy them.
    Anton Largiader 72724
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  7. #7
    RRDavis
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    Exactly What Failed ?

    Quote: "The starter quit working on a trip and I bump started it approx 5 times. That was curtains for the original rear drive with 78k miles."

    Do you know exactly what failed? I'm curious about this because it doesn't seem to me that "bump starting" would be that much additional stress on the final drive and would cause it to fail. Does anyone who knows more have an opinion on this? I "bump started" a Volkswagen all one summer with no apparent consequences and drove the car for years after that.

  8. #8

    Rear Drive Compatability

    Assuming your bike still has a rear disc brake and that the '81 RDU is off an R100RT, you may have a problem with the cast-in "bumps" on the right side of the hub, and you may have to remove them. Or, you could buy a later rear wheel, too.

  9. #9
    Registered User AntonLargiader's Avatar
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    Disks! Right. I was thinking drums. Disks, you will have to machine off the bumps of the wheel. That was also on the service bulletin.
    Anton Largiader 72724
    Tech articles - YouTube
    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  10. #10
    Huckleberry, Gilera &Toad kstoo's Avatar
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    Quote Originally Posted by RRDavis View Post
    Quote: "The starter quit working on a trip and I bump started it approx 5 times. That was curtains for the original rear drive with 78k miles."

    Do you know exactly what failed? I'm curious about this because it doesn't seem to me that "bump starting" would be that much additional stress on the final drive and would cause it to fail. Does anyone who knows more have an opinion on this? I "bump started" a Volkswagen all one summer with no apparent consequences and drove the car for years after that.
    I am gonna guess that the final drive to wheel splines looked something like my 1980 R100 at 80k. From lack of maintenance or what (no history of PO) the splines looked like needle sharp triangles and I can thoroughly believe that if I had tried to bump start it they would have all sheared off.

    ---/\---/\---/\---/\---
    1980 R100T (Gilera), 1982 R100RT (Toad), 1975 R60/6 (cern?°calo)
    Adventures at the Cave

  11. #11
    Mark Green
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    rear drive compatibility

    I bump started the bike and rode the 90 miles home. When I got home I noticed oil leaking out the top vent. I opened the drain plug and metal bits came out. I have not diassembled it.

    When I asked the BMW shop about bump starting they said "it's not recommended."

    Can you comment more on the fins. The fins from my recollection of the pics are on the shaft side of the drive. The BMW microfiche lists the same part number for both bikes for the final drive.

    I appreciate the info!

    Mark Green

  12. #12
    BMWRich58
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    Hey Mark if it's any help,give a hollar over to Jonny the owner of Mr Moto in Chicago at around 19th and Western Ave. 312-738-4269
    The former owner Bob had ton's of used parts all over and some of the old "airhead"tech's still work there. He does a lot of BMW work.
    They should pretty much be able to give you an answer.

    And are you a member of the Chicago Region Club??
    if not PM me and I'll hook you up with the club info.
    Here's the web site for the club.
    http://crbmw.com/

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