I returned to the section on balance in Mr Irving’s book to re-read the calculations. I will need to run them for my engine to help decide on the balance factors. At the moment I know the bore and stroke, but without a Piston I can’t weigh the reciprocating mass.

Unlike a conventional (poppet valved) engine, I also need to consider the weight of the sleeve. This is moving at half engine speed and through less distance. Around one third of the distance the Piston travels. However, the weight of the sleeve is significant compared to the piston, which at times will be moving with – or against it. Interesting engine to balance!

Rpm is definitely a factor, with imbalance forces rising in proportion to the square of the rpm. I know that the max rpm of the Hercules was 2880, and I considered that 3500 to 4000 would be my peak rpm. I guess the calculations will tell me once I know all the masses!