For the above purpose a groove, recess, or channel is formed in the external periphery of the piston immediately below the one or more of the piston spring rings, the groove, recess or channel being situated between the point between which the oil for lubricating the cylinder walls is supplied and the space beyond the piston. From this groove, recess or channel holes are formed through the body of the piston, or other means are provided whereby such groove, recess or channel is placed in connection with some portion of the engine or compressor pump, into which the oil can be discharged without detriment.
By this device the walls of the cylinder which have become coated with lubricating oil during the upward stroke of the piston are scraped by the spring rings during the downward stroke. The oil so scraped by the spring rings passes into the grooves, recesses or channels round the piston, whence it may be drained away or collected.