Abstract: A ported piston for increasing the cylinder displacement of internal combustion engines which includes, in a first embodiment, an inertia-activated wafer provided in a cavity located in the piston, with one or more ports connecting the bottom of the cavity to the engine cylinder and typically provided with a check valve and one or more ports connecting the upper section of the divided cavity to the underside of the piston. In a second embodiment the cavity is annular in shape and receives a wafer ring and in a third embodiment the annular cavity and wafer ring, as well as the port or ports, are provided in an insert that seats in an opening provided in the top of the piston. The wafer and wafer ring move up and down by inertia in the cavity and cavity ring opening, respectively, to effectively increase the cylinder displacement and efficiency of the engine.