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Thread: 1995 R100R engine into 1975 R90/6 chassis?

  1. #1
    Registered User zenduddhist's Avatar
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    1995 R100R engine into 1975 R90/6 chassis?

    Hey all,

    I have a very nice 1995 R100R Mystic and a 1975 R90/6. The R90 runs just fine, but the R100R engine is really sweet. Lots of power, runs very, very smooth, great response, etc. I prefer the ergonomics of the R90/6 versus the Mystic.

    I had this crazy idea of taking the R100R motor out of the Mystic and putting it in the R90/6. I know it's a nutty idea because I would be trashing a beautiful example of a Mystic. But just out of curiousity, is there an "easy" way to do it?

    Thanks!
    1969 Triumph Bonneville T120R, 1972 Commando 750, 1974 Vespa Rally 200, 1974 Commando 850, 1975 Commando 850, 2010 Triumph T100 w/sidecar, 2011 BMW R1200RT, 2013 Honda CB1100

  2. #2
    Registered User lkchris's Avatar
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    You'll need to make that engine AND transmission.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  3. #3
    Minnesota Nice! braddog's Avatar
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    Quote Originally Posted by zenduddhist View Post
    Hey all,

    I have a very nice 1995 R100R Mystic and a 1975 R90/6. The R90 runs just fine, but the R100R engine is really sweet. Lots of power, runs very, very smooth, great response, etc. I prefer the ergonomics of the R90/6 versus the Mystic.

    I had this crazy idea of taking the R100R motor out of the Mystic and putting it in the R90/6. I know it's a nutty idea because I would be trashing a beautiful example of a Mystic. But just out of curiousity, is there an "easy" way to do it?

    Thanks!
    See bold type.

    Seriously, they're your bikes, you can do what you want, but if you've got 2 airheads in good running condition, why mess with it? Sounds like you just want a little more "oomph" out of the R90 so you can be comfortable and have a little more power.

    Have you looked at what you could do to tweak the ergos on the Mystic to make it more likeable? That may be a direction you could go.
    -----------------------------------------
    Brad D. - Member #105766
    '77 R100RS - Black Beauty (big pipe, baby!)
    '94 R1100RS - Sylvia

  4. #4
    . AntonLargiader's Avatar
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    It would be easy to medium difficulty. Off the top of my mind, at the very least you'd have to:
    - find a '76-on flywheel (you could use the rest of your '75 clutch if in good condition)
    - change the ignition trigger setup to the '79-80 "points in can" system and rewire the bike to match it
    - modify or reconfigure the engine breather
    Anton Largiader 72724
    largiader.com bmwra.org

  5. #5
    Registered User b25bsaboy's Avatar
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    Quote Originally Posted by zenduddhist View Post
    Hey all,

    I have a very nice 1995 R100R Mystic and a 1975 R90/6. The R90 runs just fine, but the R100R engine is really sweet. Lots of power, runs very, very smooth, great response, etc. I prefer the ergonomics of the R90/6 versus the Mystic.

    I had this crazy idea of taking the R100R motor out of the Mystic and putting it in the R90/6. I know it's a nutty idea because I would be trashing a beautiful example of a Mystic. But just out of curiousity, is there an "easy" way to do it?

    Thanks!
    The comment that I would have, is that anything is possible providing you have really deep and I mean deep pockets. I see from time to time where guys and gales have this vision of what if? Dive into it and all of a sudden the piggy bank went dry. Thus the project gets abandoned....

    The 1995 R100R Mystic and 1975 R90/6 are beautiful pieces of machinery just the way they are. If you prefer the ergonomics of the R90/6, that's where you want to invest your money wisely.

    Just my two (2) cents worth! Gotta quite saying that as here in Canada, the Government has now started to phase out the penny....
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 1976 BMW R75/6, 2009 R1200RT

  6. #6
    Superkraut typ181r90's Avatar
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    Why don't you try putting the R90 tank and seat on the mystic instead? Probably would need a little modification, but,ess work than an engine and drivetrain swap
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

    http://symphonyofshrapnel.blogspot.com

  7. #7
    Registered User RINTY's Avatar
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    Quote Originally Posted by zenduddhist View Post
    Hey all,

    But just out of curiousity, is there an "easy" way to do it?

    Thanks!
    You might want to look into installing some big Siebenrock barrels and pistons on your R90.
    Rinty

    "When you don't know where you're going, any road will get you there."

  8. #8
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    My guess, and the operative word here is guess, is that what makes the R100 engine sweeter than the R90 are a lighter flywheel and lower compression pistons. (My guess comes from owning a '76 R75/6, an '84 R100 and my current R90S, in that order.) IIRC, the R100 also has smaller diameter carbs. As such, it probably makes less power but runs more smoothly and revs more freely. It would be easier, and cheaper, to replicate that in the R90. I'd start with a lightened flywheel and see how that feels. Then, if that's not enough, you could either lower the compression or dual plug the cylinders (either would let it run better on the lower octane gas currently available). I'd bet that the flywheel mod would get you 80% of what you're looking for.
    61 Gold Star, 76 R90S, 03 CBR600RR, '13 690 Duke, '14 Street Triple R

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