by crfsonly » Mon Jan 31, 2005 6:47 pm
good questions! yes, to simplify things, displacement is a function of bore and stroke. there are other factors at play but we'll keep it simple for now. the bore is the size of the cylinder and hence determines the piston size. the stroke, as you mentioned, is the total distance the piston travels in the cylinder. these two things largely define the displacement of an engine...like 100cc in your case. if you increase the bore you increase the displacement. if you increase the stroke you increase the displacement. obviously, if you increase both you also increase the displacement.
on achieving more power from a motor it's been said, "there's no replacement for displacement" this is especially true on small bore motors.
a big bore kit gives you a larger than stock piston which means you have to "bore" out your stock cylinder for the new, larger piston to fit. this will increase the displacement allowing the motor to make more power. so, you typically have to bore the cylinder, and replace the piston and rings at a minimum to install a big bore kit. you'll also have to adjust your jetting to match the new found displacement.
ken