 An unusual form of four-stroke model engine that uses what is essentially a sleeve-valve format, is the British RCV series of "SP" model engines, which use a rotating cylinder liner driven through a bevel gear at the cylinder liner's "bottom", which is actually at the aft end of the cylinder; and, even more unusually, have the propeller shaft — as an integrally machined part of the rotating cylinder liner — emerging from what would normally be the cylinder head, which in this design is placed at the extreme front of the engine, achieving a 2