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Thread: New member restoring a Yamaha SR250, 1981

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    New Member
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    New member restoring a Yamaha SR250, 1981

    I have one complete, running bike and one for spares. The barrel from the spare bike has been bored to it's second oversize, so it is 74mm. Does anyone know if there is a reason why I wouldn't be able to bore it to 75mm? As I understand it, there are two different bore sizes used in these engines, 73.5mm/239cc and 75mm/249cc. the 75mm piston kit is easily available.

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    Member Crobbo's Avatar
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    Quote Originally Posted by Lizzie View Post
    I have one complete, running bike and one for spares. The barrel from the spare bike has been bored to it's second oversize, so it is 74mm. Does anyone know if there is a reason why I wouldn't be able to bore it to 75mm? As I understand it, there are two different bore sizes used in these engines, 73.5mm/239cc and 75mm/249cc. the 75mm piston kit is easily available.
    See if there are 2 different barrels. You might be lucky and and there is only 1. Otherwise if it's different you might start running out of wall thickness.
    In sterquiliniis invenitur.

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    Yeah, good point. as far as I can tell there is only the one barrel..The stock 73.5 goes up to 74.5 at max oversize, so going to 75mm isn't much bigger. it will leave 3.2mm wall thickness.

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    Member BusaSteve's Avatar
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    Are both machines bore's worn out? If the oversize 75mm is common and the piston sets are available then I see no reason why not.

    I'm assuming the running machines bore is either worn out and you want to go up to the lager size now or you want to bore the spare out to 75mm and fit it and a new piston to the running machine for the small increase in power.
    Smoke me a kipper...I'll be home in time for breakfast

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    Member Water pig's Avatar
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    Speak to Ray Easson in Bayswater, if it can be done he's the man for the job, and you can almost guarantee he's done it before if it works

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    Member Chuck Steak's Avatar
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    Quote Originally Posted by Water pig View Post
    Speak to Ray Easson in Bayswater, if it can be done he's the man for the job, and you can almost guarantee he's done it before if it works
    Might be a little difficult. She’s in Tassie

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    Member Water pig's Avatar
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    Fair call

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