Abstract: An air compressor has an inlet supplied with pressurized air, and is operable in a pumping mode and in a non-pumping unloaded mode. Unloader means is provided for placing the compressor in its unloaded mode. When the compressor is placed in its unloaded mode, diversion valve means operates for blocking communication of the inlet with the pressurized air supply and establishing communication of the inlet with atmosphere.

Abstract: A piston ring assembly having a pair of resilient split rings of identical cross section in a single piston groove in which the rings contact each other over a large surface to maintain the gap of one ring diametrically opposed relative to the gap of the other ring to resist passage of oil and compressed gases. The rings each have a separate line contact with the piston which accomplishes sealing without imposing frictional loads detracting from the radial expansion of the rings to produce a tight seal against the cylinder walls. The rings afford spaced seals with the cylinder wall to form an annular oil containing cavity for controlled lubrication of the cylinder wall.