the fly wheel is balanced but not to alleviate engine vibration, no. the fly wheel is balanced to keep it from adding to the vibration. when you cut the fins down if you do it by just cutting them with a cutoff wheel you have to rebalance the flywheel. but if you use a lathe to cut the fins down you dont even have to balance. removing the flywheel alltogether doesnt add vibration. inside the crank case there is a weight on the crank to counter balance the piston when its combusting. DA engines makes a 50 that vibrates horribley but i think its due to the counter balance on the crank is too small to cope with the size of the engine. to see what i am talking back if you take the backplate off the engine you will see a solid almost triangular shaped dodad on the crankshaft. that is the counterbalance. it is the sole source of crank case balance to reduce engine vibration.