diameter and lighter taking less power from shaft 20 the piston heads 38 are also prevented from twisting within the matching bores 26 by virtue of their non circular cross sections meaning that the ball and shoes assemblies 42 interconnecting the pistons 36 to plate 24 do not have to resist such twisting action in conclusion while basic operation is conventional several operational advantages result from the unique structure the primary objective of which is increased compactness variations in the embodiment disclosed could be made a conventional round piston could be flattened off on two sides in planes that lay along or parallel to radial planes of the cylinder block this would give each piston a shape similar to the pie shape of piston head 38 though not as pronounced as well as the cylinder bores this modification alone in conjunction with arraying the cylinder bores in a circular pattern would create flat radiating webs between the cylinder bores that were bisected by radial planes of the cylinder block essentially like the webs 30 this would eliminate the empty interstitial spaces between the cylinder bores and could be easier to manufacture the pistons would also resist twisting within the bores but the cylinder bore to block volume ratio would not be quite as favorable as in the preferred embodiment with its more pronounced pie shape of the piston heads 38 different configurations for the cylinder block slots like 34 that allow vapor entry to the crank case cavity 54 could be used such as simple round drilled holes more than one such drilling per.