a crankshaft 4 the automatic valving of the cylinders accomplished simply by rotation of the cylinder block relative to valve parts in the cylinder head results in an appreciable saving in weight 5 for an engine of given stroke the guide rail configuration can be essentially standardized regardless of the number of cylinders used since but two peaks and two valleys are required in 360 u00b0 of guide rail for a four cycle engine regardless of the number of cylinders 6 as the number of cylinders is increased the number of such cylinders acting simultaneously on a power stroke on the guide rail is increased the embodiment disclosed in fig1 20 is presently preferred due to the positive guiding of the piston rod in combination with anti friction rollers for resisting and counteracting centrifugal force on the piston rod also due to the improved coaction between the piston rod and guide rail obtained from the conical roller followers and the arrangement of their axes of rotation relative to the axis of rotation of the cylinder block and further due to the sealing means between the rotating cylinder block and head which acts individually for each cylinder but with compensation for each adjacent cylinder and for.