A variable stroke crankshaft assembly includes a drive shaft surrounded by a sleeve. A crankshaft is disposed inside the drive shaft with the longitudinal axis of the crankshaft offset from the longitudinal axis of the drive shaft. A gear projecting outwardly from the crankshaft through a window in a drive shaft engages angled gear teeth on the inner surface of the sleeve allowing sliding movement of the sleeve relative to the drive shaft to rotate the crankshaft. A crankpin is attached to the crankshaft but offset from the rotational axis of the crankshaft. Rotational movement of the crankshaft with respect to the drive shaft thus pivots the crankpin with respect to the rotational axis of the drive shaft creating a crank stroke that may be varied by sliding the sleeve back and forth.