An apparatus for rotating a crankshaft is described. The crankshaft includes a main shaft and an axially offset crank pin connected to the main shaft. The apparatus includes a first piston that alternately drives an attached connecting frame in reciprocating motion. A hub is rotatably mounted within the connecting frame with the hub defining a channel and the crank pin disposed therein. The apparatus includes a locking mechanism with a locked state in which the hub is prevented from rotating and an unlocked state in which the hub freely rotates. The reciprocating motion of the first piston and the connecting frame imparts reciprocating travel of the crank pin between ends of the channel and consequent rotation of the main shaft in the locked state; while the crankshaft and the hub rotate freely in the unlocked state, with the first piston and the connecting frame in a stationary position.