Putting a 350 top end onto a 250 bottom end
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  1. #1
    Senior Member shoom's Avatar
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    Putting a 350 top end onto a 250 bottom end

    Hi all.

    I have a 1968 CB250 supersports.

    I was just wondering if it would be possible for me to install a 350 top end (eg cylinders. pistons, cylinder head etc)

    Was the 350 crankshaft any different from the 250? will it be fine, or will it result in mass vibrations?

    is the 350 combustion chambers even any different?

    Thanks for any help.

    Tom in Australia.

  2. #2
    Junior Member leifarne2007's Avatar
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    Hi Tom

    You will have problems to get the cylinder sleeves down in upper engine case. Holes are to small. Exept from that should an 350 top end be bolt on.

    Leif Arne, Norway

  3. #3
    Senior Member TOOLS1's Avatar
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    Someone on here was doing this a few years ago. It was discussed in detail. I am sure a search will find the thread. However now, I am wondering if the cylinders could be resleeved with the 350 sleeves? I know that the holes in the case can be bored out for them.
    TOOLS

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  5. #4
    Senior Member Erwin's Avatar
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    The crank will be fine. It has different bearings. I have a 250 crank in my 350 engine and it has been working perfectly for many 1000's of miles and more than 4 years now. I am also using a 250 head but that has to be modified to suit the cilinder. The combustion chamber is larger on the 350 but other than that, the head is identical.

    To mount 350 cilinders on a 250 engine case, you'd have to enlarge the holes in the case to fit the sleeves of the 350 jugs.
    Last edited by Erwin; 06-21-2018 at 08:33 AM.
    1972 CB350K3 General Export | 1976 CB200T K0

  6. #5
    Senior Member shoom's Avatar
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    Does anyone know if the combustion chambers in the heads are different? As if they aren't, that means i don't have the added expense of a cylinder head also. The 350 cylinder blocks are a-plenty on Fleabay.

  7. #6
    Senior Member Erwin's Avatar
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    Read the comment above... your answer is right there
    1972 CB350K3 General Export | 1976 CB200T K0

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