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RD250 Cylinders heads interchangeable with RD350

Discussion in 'Yamaha 250cc 2 Strokes' started by David Hanke, Nov 24, 2013.

  1. David Hanke

    David Hanke Member

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    Hi i have recently bought a 1975 RD250B (first cafe racer project/first bike). She can't idle without stalling so i am pulling her apart slowly, attempting to diagnose problems. On pulling apart the top end i have found that the cylinder heads are from a RD350 and the cylinders are from a RD250.

    This may be a silly question but are the cylinder heads from the RD250 and RD350 interchangeable? Could this be causing sinister issues with how my RD250B is running? Any help is much appreciated.
     
    David Hanke, Nov 24, 2013
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  2. Phil

    Phil Senior Member Contributing Member

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    Hey Dave
    Welcome to the forum, nice choice of bike. Perhaps a photo or two wouldn't go astray.
    Now to dilemma with parts. I'm no expert but a quick check has revealed that an enormous amount of parts are interchangeable between no just 2 but 3 bikes. Seems that the rd250/350/400 share many parts.
    Does this include cylinder heads, have no idea??? Perhaps comparing manuals on the 2 bikes might reveal some similarities. That's my 2 cents worth, perhaps other may know more than I.:thumb_ups:
     
    Phil, Nov 24, 2013
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  3. mboddy

    mboddy Well-Known Member Premium Member Contributing Member

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    RD250 is 54mm bore standard and RD350 is 64mm.
    You need to have heads set up for the 54mm bore.
    This could easily be the source of your problems.
     
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    mboddy, Nov 24, 2013
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  4. David Hanke

    David Hanke Member

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    Cheers Phil and mboddy! Will have to source the 2nd hand RD250 cylinder heads I've been stalking on ebay. At the moment she's in pieces on my shed floor! Will post some pics soon.
     
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    David Hanke, Nov 24, 2013
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  5. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    I am no expert when it comes to RD's, but I would think that the 350 heads would give a lower compression on the smaller motor. I was reading an article yesterday in the English magazine 'Classic Motorcycle mechanics about the LC versions and the heads look to have different squish bands, so would give different burning rate of the fuel and different power delivery.
     
    Murdo, Nov 25, 2013
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  6. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    No offence to anyone here, but the place you want to go for parts and info is the RD RZ Forums here
    http://www.rzrd500.com/phpBB3/

    There are some incredible projects on there - one guy Smurph IIRC, even managed to make his RZ 350 fuel injected

    peace out
     
    ruckusman, Nov 25, 2013
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