Oversize Piston ?

[jasenr]

I was wondering what the correlation was between .xxx in oversize pistons to displacement? If you put an .020 oversize piston into a 250 engine, what does that give you? Is there anywhere that has the math detailed? Thanks!

[ATCnut]

Here is how you calculate displacement

R = Radius of the piston, which is half of the diamater
if it is a 66 mm piston, then R =33 mm
for it all to work right, you have to do it in cm, 33mm = 3.3 cm

S = stroke, again use cm

displacement = R x R x 3.1315926 x S


For example, Stock 350x bore = 81 mm, stroke = 68 mm

4.05 x 4.05 x 3.1415926 x 6.8 = 350.4 cc

[jasenr]

Thanks for the tip! This has been bugging me all day

[short4stuff]

So I have a 225DR and it needs an engine job. now this is orginal .....

Yamaha225DR

3.5 x 3.5 x 3.1415926 x 5.8 = 223CC

Now if I put a .50 oversided piston is this correct (is this to big of a piston)


3.75 x 3.75 x 3.1415926 x 5.8 = 256.xxx

Is that right????????

Also will more cc mean faster or more power if I am not using a high compression piston say a 9:1 ???

[short4stuff]

I hate to "bump" this but I really need to know.

Will I get more power/speed?

[TimSr]

If bore is 70mm, .5 (mm I assume) overbore would make it 70.5 making your radius 3.525 for a total of 226cc.


Wiseco uses a different formula which will give you the same number

.7854 x bore x bore x stroke= Displacement

In answer to your question, boring is not a performance modification. Any noticeable gains would be from regaining compression from a good ring seal by smoothing out irregularities, and not from the small increase in displacement. If it doesnt need bored, your throwing your money away, and needlessly throwing away the number of future bores left on your cylinder before it needs resleeved or replaced.