Larry,
Glad you are getting the Fox fixed up.

The typical answer is that the crank counterbalance should balance out both the big end of the rod, plus somewhere around half of the reciprocating weight. That is, one half of the sum of the piston, wrist pin, and the small end of the rod.

Weigh the parts on a fractional gram scale. Add up to the total you want to shoot for. Make an equivalent dummy weight that goes on the crank pin. I have used concentric brass tubes trimmed for this. Then fiddle with the crank counterweight until you achieve balance.

If you don't have a fractional gram scale (I have a triple beam balance which only goes down to 1/10th gram), you can measure the parts and make approximations by calculations. Aluminum being around .097 lb/in3. (44.00 gm/in3 if you work in mixed units....) I don't have a Fox rod handy at the moment, but one end is probably around 0.8 or 0.9 gm.

I think for a stunt-Fox, you'd want to go for the full 1/2 recip mass. For a speed engine that doesn't get nearly the running time, you might shoot for less mass, because torsionally you have to accelerate the extra balance weight with each revolution and that takes power. Which is where all the arguing starts over the shaking, loss of power, and reduced mass moment of inertia....

Let me see if I archived one the engine articles.

Good luck,

Dave

Thanks for the input. I think that the crank will remain stock with the Aluminum piston as advised, at least at first. It would be tough to unbalance a modified crank! 

Actually, my ABC Fox seems to have some excess vibration, probably a modified crank, too. Otherwise, fantastic 2/4 break, runs with lots o’ power. When the Ceramic engine is back together, I’ll take a better look at the ABC internals.

I may need another stock crank.