An air compressor having a piston driven via a crankshaft from an input shaft by means of a multi-plate clutch which is normally held in engaged condition by a spring device. The clutch and spring device are contained within a housing which includes a retainer placed, after the spring device has been pre-loaded to a desired extent, in a position determined by said pre-loading and secured in that position to maintain the spring device in pre-loaded state. The crankshaft is housed within a crankcase and passages interconnect the interiors of the crankcase and clutch housing, with the casing ends of the passages at a higher level than a predetermined permissible oil level for the crankcase. This permits crankcase pressure during operation to expel oil from the crankcase along the passage into the clutch casing, while limiting the height to which oil may accumulate in the crank case when the piston is stationary.