The valve leads the piston by 90 degrees. You can advance this by up to 10 degrees or so to get the best performance, but we did not bother with fine tuning. The piston crank length should be no more than half the full stretch of the balloon, and the valve crank length should be at least half the diameter of a straw, but we just guessed and our first crank shaft worked fine (about 3/4" for the piston and 1/4" for the valve