A scotch yoke type fluid device where the or each piston (32) has its own slider (18) and its own pair of sliding surfaces (30) disposed on the same side of the big end bearing (16) of the crank (12). The main crank axis (14) is movable along a path such that the pistons are neither substantially retarded nor advanced. The effective centre of mass of the crank, piston and slider remains stationary relative to the crank axis. Guide means (2030) constrain each piston to move along its piston axis. The guide means is transverse of the sliding surfaces (30) and engages crankcase mounted guides. An intermediate member may interconnect the piston and the slides capable of adjusting the intermediate member to change the compression ratio. For a device having opposed pistons, the yoke (6060) attached to the two pistons is split into two parts (6062a, 6062b) releasably held together.