Originally Posted by John Healy
To do a twin, ...
Ah, right, he has that extraneous cylinder. The equation is fine if only he would install his rods first which, I realize, no one does when balancing a twin, if for no other reason than the friction of the plain bearings in the big ends. Of course, when balancing a twin the flywheels are much further apart than on a single so the potential of a big rocking couple is there which is why dynamic balancing should be used (I know you know this, John, but it's worth mentioning in case the OP doesn't).