I'm looking for a formula I saw years ago in an R/C Modeler Magazine that you use to determine the actual engine size. It used piston diameter and stroke. Anyone know what I'm talking about?? Can't find it and need it! Any help appreciated!!!

Something like bore x stroke = displacement
(if measuring in .cm then you'd get CCs, if measuring in .in then you'd get .CI)

3.14x(diameter/2)x(diameter/2)xstroke=displacement(measure bore and stroke in inches, get cubic inches, measure in CM get CC), I don't think they take combustion chamber volume into account on these... round to the nearest normal sounding number and you get what it is...

3.1416(Pi) times half of bore diameter squared times stroke all in inches for cubic inches or centimeters to get cc's
Example 20mm bore x 20mm stroke=2cmx2cm. 1/2 bore=1, 1 times 1 is 1. 3.1416 times 1 is 3.1416. 3.1416 times 1 (stroke) is 3.1416CC. Bigger example 100mm bore 100mm stroke is 10cm by 10cm. half 10 is 5. 5 squared is 25. 25 times 3.1416 is 78.54. 78.54 times 10 (stroke) is 785.4cc One last one, a real engine. A DA50cc bore is 42.6mm stroke is 35. So in cm it's 4.26 X 3.5
Half 4.26 is 2.13. 2.13 squared is 4.5369. 4.5369 times 3.1416 is 14.25312504. That times 3.5 is 49.88593764cc which rounds up to 50cc. It's actually pretty easy to do on a calculator

Or:

(Bore x Bore) x stroke x .7854

If measuring in inches, that will give you cubic inches. If you want cc, divide that result by .061.