An eccentric shaft for an internal combustion engine, especially for a rotary combustion engine, the working unit of which comprises essentially a peripheral housing, side housing plates, and a triangular piston mounted on an eccentric of an eccentric shaft, the tips of the piston moving along an epitrochoidal orbit inside the peripheral housing to form three separate working spaces, where the rotary combustion engine can have several working units. The eccentric shaft consists of several parts, which have radial teeth at their ends, by means of which these shaft parts can be connected positively to each other by the use of a tie rod. In rotary piston engines with more than one working unit, as many inner shaft parts of identical design as there are working units can be used in modular fashion to construct the eccentric shaft.