I have been trying to bring a 85'atc250r back to life. It sat in a garage for ten years and the cylinder had to be bored to a 67.50mm piston to get the rust out. So would that change the cc's, if so how do you figure it? The stock size was 66x72, I think are these numbers used to figure cc's?

um i think it is pi x radius squared.x hieght
so 3.14 x half the borex half the bore x stroke.

Yeah, or (bore)(bore)(stroke)(.7854).

Boring your 250 1.5 mm oversize yields a little over 11 more cc's displacement, 257 cc vs. 246 cc.

Diogenes

Do not foget to dived by 1000 if you are using mm for your bore and stroke.

Thanks for the responses, what do you x by 1000? the bore? and what do you mean how fast do i want the spend? is such a oversive bore going tear the motor up? I no experiece with boring 2 strokes. Thanks

you DIVIDE by 1000 because you want to know the cubic centimeters, since most dimensions are givin in millimeters you end up with an answer in cubic milimeters...

EG. using diogenes formula & 66mm bore, 72mm stroke...

66 x 66 x 72 x 0.7854 = 246326.57 MM^3

Now divided by 1000 = 246.32657 CM^3

See the difference?? Hehe, You should not wreck your motor in anyways (although I do not know what the maximum allowable bore is on your bike) The "how fast do you want to spend" is just my sig.. I should change it your are like the 5th person to ask me :)

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Thanks for the breakdown, now I get it.