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To big bore or to not big bore

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I called around the other day to price out boring my cylinder. I was thinking of boring my xr250r to 77mm. It needs to be bored anyway due to some damage. The machinist advised me not to, said it would throw off my crank balance? Also he would charge me more because he would take off so much material and it's hard on his bits.

 

Should I keep bore diameter to a minimum?

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It sounds to me like your machinist just doesn't want to do the job. I would find a different machine shop that does what the customer wants. People have been boring out cylinders for decades without incident.

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First.....the answer to your question is YES. Always big bore. Lol....

Second - Will making the bore bigger throw off crank balance? No. Dramatically changing the weight of the piston will. In some cases, big bore pistons weigh less than the OEM piston. Check you piston weights. 

Third - Hard on his bits???? Really? What does he normally machine? Cheese? 

Give us a call if we can help. www.millennium-tech.net

 

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You didn't say what year but FYI from around '87 on, the sleeve is removable on the 250.  77mm is a good choice for stock cylinder.  Avoid the first machinist.  Like most machinists, he would probably over bore, then you're using oil from the get go.  The clearance is less than two thousandths" difference in diameter measured per service manual on the 250, with forged piston.  

Stock cast piston clearance can be as little .0006".  Good luck getting a machinist to work to that tolerance.

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Have ran big bores on many of my bikes with no issues. If it has cylinder damage, then I would bore it.

Send it to Millenium. I wouldn't trust those other guys lol.

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A veteran pro rider over on the crf230 forum just posted that he's running .0007 clearance on a wr450 with JE piston.

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14 hours ago, Baja Rambler said:

A veteran pro rider over on the crf230 forum just posted that he's running .0007 clearance on a wr450 with JE piston.

Curious for how long???

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3 minutes ago, firstplacephoto said:

Curious for how long???

A lot longer and with better heat transfer than if you start out at .002.

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Different pistons, cylinders, materials, and applications require different piston to cylinder clearance. There is no magic number. If you are curious, give us a call and we would be happy to make a recommendation based on your specific application. 

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