A typical model engine crankshaft acts as a rotating valve. It has a window or port cut into it and it is timed with the travel of the piston.

To determine which direction a crankshaft is designed to turn compare yours to the photos below. Aero engines are normally designed to run counter clockwise when looking at the front of the engine. But clockwise rotation crankshafts are available from some manufacturers. Many times we receive calls from customers that have acquired second hand engines and they are not sure if they have the correct crankshaft.

Some times an engine can start in reverse and run fine at an idle but as the throttle is opened it will usually stop or backfire and run the proper direction. On very high performance engines, like the marine engines shown at the bottom of this page, the crankshaft timing is very radical and the engine may start easily in the wrong direction.

A counter clockwise rotating crankshaft shown here in the K&B Screamin' 48

With the piston at top dead center (at the very top of the stroke) the crankshaft port or window is positioned to the left. The crankshaft valve has just allowed the intake fuel/air charge enter the crankcase and is closing.

A clockwise rotating crankshaft shown here in the K&B Pro Outboard engine.

With the piston at top dead center (at the very top of the stroke) the crankshaft port or window is positioned to the right. The crankshaft valve has just allowed the intake fuel/air charge enter the crankcase and is closing. Notice how much more open this engine is compared to the aero engine above. This marine engine has much more radical timing to produce the high rpm's and performance expected from a marine engine.