most engines have small offsets in the piston pin location that makes one direction more efficient than the other. between that and the port timing or cam timing, the direction of rotation is thus dedicated. it makes no difference which direction the engine actually turns as far as spinning a prop,..... the above details will produce the direction of crank rotation that is intended for the design of the prop,....in the case of our plane engines, the props are designed to be spun counter-clockwise, looking at the front of the plane, so the offsets are built into the parts to produce the best performance in that direction.. in some engines, these offsets are so small that the engine will run backwards, but it won't give nearly the performance as if it were running the direction it is intended to run. case in point,...the old Cox. 049. you could run that engine either way because the offsets were "o" and it didn't matter which way it turned. 4-cycle engines with valves and cams have a very dedicated direction of rotation and most of them won't even start in the opposite direction because valve timing is then reversed and not coordinated to the stroke phase of the piston. there are engines designed to run in reverse rotation,....this done by using a special cam and piston offset designed for reverse rotation. these are used on twin engine planes so that the torque of the engines are counter balanced. for these engines, prop pitch is also reversed. the twin engines used in PT boats were built "CW" and "CCW" rotation to facilitate a straight running boat, when run wide open.