[clayman]

Out of curiosity, are we still trying to make this feasible? I do like the concept of spring returns, but the only advantage I see in them would be not having to power them back - which is canceled out, of course, by the necessity of pairing them with more pistons (I checked, 15 ibs is about the max force you can get for a spring return, and that's with a 2" bore - pretty sluggish). Swapping around bore and stroke may give more power for the same air volume, but I'd begin to worry about the chances of such a smaller shaft offset just causing the engine to lock up (not to mention that reducing the torque arm defeats the purpose of increasing the bore).