O.K, hear goes on why your conversion factor works. The correct geometric formula (as Alexander Graham Bell would tell you) is pi x r squared x stroke. Now r is radius, which is one half diameter (d, same as bore) If you use diameter like you are, you need to divide pi by 4 because d squared is 4 times larger than r squared. This gives you .785398, which if you round it out to 4 decimal places gives you .7854 like you are using. If you have access to a scientific calculator (like all high school math students and available for $10) you don't even need to remember your conversion factor, just use the accepted mathematical version of the formula. If you don't have a scientific calculator, and either write down your conversion factor or remember it accurately you can use your formula (a modified version of the accepted formula). I admit that I did not think it out totally when I posted my first response (typical for me), but you obviously did not know why it worked and now you do. You have been using the formula that I am preaching about all along and didn't even realize it. Finally, remember to divide by 1000 to convert from mm cubed to cm cubed, since the measurements we are talking about are in mms.

 

zrwhats formula is right on the money with a 102mm piston you get 686.387cc's, exactly the displacement you should come up with a that piston. with a 105 mm piston you get 727.356 cc's. all as it should be, except why go to a 100.5mm piston?

 

ARRGH, my brain is melting........

 

Take have the bore (50mm) and square it. multiply by the stroke(84mm?)then you need to pie it(multiply by 3.14), and finally divide by 1000 to get to cubic centimeters.

 

As it turns out,the human brain is like an enormous fish.It's flat and slimy and has gills through which it can see.If one of these gills fails to open,the lungs do not receive any messages transmitted by the brain.
You have to remember,that most quad riders are like me,just average schmoes.If there is a simple equation with constant(.7854) that works,use it.Its the K.I.S.S principal.Keep it simple stupid.
Also,I am totally indepted to you for telling me I didnt know why it worked and since 1979 I have been using a formula that you have been been preaching.

 

Raptor720,
I took my motor appart w/ the original intentions of using a stock bore 11:1 compression piston, white brothers cam and white brothers valve springs. Once a
part, i learned that my cylinder walls were scored, and that i had to either go .10 or .20. I wish i was on this forum BEFORE i took my motor appart, and I would have asked which route i should take. Well, you live & you learn. At least i have them porting and polishing the head while it is appart to increase the flow. I wish i did this trinity kit i have been hearing about.
thanks guys
jason
666 (cc) !!! Mark of the beast!!!