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Thread: Will a 87 K75 motor fit a 85 K100 frame?

  1. #1
    Motomark motomark's Avatar
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    Will a 87 K75 motor fit a 85 K100 frame?

    Does anyone know if it is possible to fit a 1987 K75 engine/trans to a 1985 K100 frame?Seems like it would bolt right up.
    Mark
    Mark

    1971 R75/5,1990 K75RT,1994 K75,1999 KLR

  2. #2
    rabid reader dbrick's Avatar
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    Back when the K75 was introduced, I remember reading that the frames were different because the engine lengths were different; the frames' front "arms" (which reach down to the top front of the motor) have different angles from the steering head.
    David Brick
    Santa Cruz CA
    2007 R1200R

  3. #3
    Registered User
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    different motor lengths = "no way".
    Ride Safe, Ride Lots

  4. #4
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    3 inchs different

    Plus 2 for dbrick. The distance from the front motor mount to the back motor mount is "about" 3 inchs longer on the k100. "About" because I couldn't tape in a straight Line.
    Harold In Kansas
    1985 K100RT Bullit
    1985 K100XX/EML Bemel

  5. #5
    Registered User beemerguru's Avatar
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    Nope...you're one cylinder short in length for the front mounting bolts.
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

  6. #6
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by motomark View Post
    Does anyone know if it is possible to fit a 1987 K75 engine/trans to a 1985 K100 frame?Seems like it would bolt right up.
    Mark
    That all depends on how good a person is with a grinder and a welder.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  7. #7
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    They put big block chevies in motorcycles, so anything is possible. I don't have a K100 to look at, only my K75 so I am trying to visualize what it may take. The rear of the engine and transmission should bolt right in and correctly position the engine. At this point , you could possibly use flat iron and make an extension of the downtubes that you could bolt to the downtube and to the engine. The big problems are going to be all the little things. If the radiator is left in the original position, the hoses are not going to reach. You will need the airbox for the 75 as well as the ignition system with coils. It could be an interesting project. Keep us posted if you tackle it.

    87 K75C
    81 Yamaha XJ550 Seca
    80 Yamaha XT 500

  8. #8
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by MWIECK View Post
    They put big block chevies in motorcycles, so anything is possible. I don't have a K100 to look at, only my K75 so I am trying to visualize what it may take. The rear of the engine and transmission should bolt right in and correctly position the engine. At this point , you could possibly use flat iron and make an extension of the downtubes that you could bolt to the downtube and to the engine. The big problems are going to be all the little things. If the radiator is left in the original position, the hoses are not going to reach. You will need the airbox for the 75 as well as the ignition system with coils. It could be an interesting project. Keep us posted if you tackle it.

    87 K75C
    81 Yamaha XJ550 Seca
    80 Yamaha XT 500
    Don't forget that these bikes do not have a cradle frame. The engine casing is a stressed member that completes the bottom of the frame. It takes a little more than affixing the front of the engine in a frame. There are structural issues when the engine is part of the frame.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  9. #9
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    Yes Paul, I realize that the engine transmission assembly is the main structure of the cycle. I should have been a little more specific in my front engine mount. I would consider something like 3/8" plate cut into a triangular shape with holes on the bottom edge to bolt to the downtube and to the engine. It would need a 3/8" offset in it to allow for the plate thickness . The front leg of the triangle would follow the angle of the downtube and extend several inches up the tube where it would be welded to the tube. This is just a thought. I have built enough things in my shop to realize that what you first perceive in your mind is not what the final product looks like. It is probably easier to buy a good used K75 in the model you want and go ride, but we all need to do a little pipe dreaming now and then.

  10. #10
    Registered User beemerguru's Avatar
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    Course you could just buy the right frame for around $200 with a clear title..5 bolts, triple, and some wires.
    Greg Hutchinson
    R80G/S (4) 633CSi with 450K mile
    '68 R60/2 '88 K100RS Special Edition
    http://gregsgssite.shutterfly.com/

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