Cylinder Overboring - R90S

Discussion in 'Old's Cool' started by hank0614, Jan 10, 2011.

  1. hank0614

    hank0614 TurtleGuy

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    A friend of mine has begun the process of restoring his 1975 R90S (built 10/75). He was advised that both cylinders required a .20 over re-bore, and he acquired the appropriate pistons along with other required parts in a single purchase several years ago prior to any machine work. One cylinder cleaned up nicely, the other did not. He now has a .20 over piston looking for a cylinder. He has two potential donor sets of cylinders available, one from an R60/6 and one from an R90/6.

    The question is: can a cylinder from either set be re-bored to .20 over the R90S spec and be used on the R90S? Alternatively, if he can acquire an R75/6 cylinder, can it be used?

    He would prefer to use an R60 cylinder, since it comes from the remains of a parts bike. The R90 set is from a bike that is a fairly minimal project that he would prefer to keep intact.
    Any advice or observations would be greatly appreciated.

    Hank
    ’72 R60/5
    ’95 K75S
    #1
  2. wirewrkr

    wirewrkr the thread-killer

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    Okay, here ya go.
    The bike is a 10/75 production, which makes it a 76 bike not a 75, which means it has the large bore spigots and the 18mm diameter push rod tubes rather than the 16mm.
    The cylinder used has to be of the same era, basically from 9/75 to 8/80.
    If the R90/6 jug has the same size spigot and pr tubes, use that one.
    The R60 jug will not have enough meat in it as the walls are around 5.5mm thick or so at the thinnest point and a bore size of 73.5mm. You are trying to go up to 90.5mm. Do the math.
    Another option if the R90/6 jug is a no-go is to use an R80 jug.
    They went into production in the middle of 1977 so there is no issue with the spigot being too small.
    There is just enough material in there to go to the second overbore, provided the boring person knows how to and has the correct jig for the jug.

    The walls are also about the same thickness at the thinnest point.

    Robert

    Attached Files:

    #2
  3. datchew

    datchew Don't buy from Brad

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    besides the immediate issue... I love seeing pics of chopped up parts cutaways.
    #3
  4. djkilmer

    djkilmer Adventurer

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    Curious about the prospect of using the 750cc jugs as well for a 900, I have a mint set of R90S pistons that I want to use on my current project and was planning on using my stock 750 jugs and boring them to the 900 specs, will they work as well?
    #4
  5. wirewrkr

    wirewrkr the thread-killer

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    Don't count on it.
    I don't have a dead R75 cylinder to split.
    Anyone else wanna donate?
    #5
  6. PaulRS

    PaulRS Dutch fool

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    I had my 75 (16 mm tubes) bored to 90 std a few ages ago without any trouble.

    Still holding after years of (ab)use. :D

    Paul.
    #6
  7. wirewrkr

    wirewrkr the thread-killer

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    :eek1
    Wow, that means that you possibly have 1.5-2mm of wall thickness in there, unless the R80 jug was just a bored out R75 from the factory.
    Nice to know it isn't distorting.
    #7
  8. Xcuvator

    Xcuvator Justa Venturer

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    I think hte OP wants to bore to 90 2nd over though and wonder if that would be a different kettle of fish?
    #8
  9. djkilmer

    djkilmer Adventurer

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    Thanks for letting me jump in on this thread with a few questions of my own, will someone mind giving me the O.D. of the 900cc cylinder, the top where it mates with the head, thanks!
    #9