Crankcase overpressure relief valves include an internal flame trap and an external downward deflector. They are designed to immediately re-seal to prevent air from rushing back into the crankcase after an uncontrolled expansion of the gas in the crankcase. When using multiple valves on a frame, use valves of the same manufacturer, design valve area, and relief pressure. Crankcase relief doors are mandatory in Air Service when short coupled distance piece configurations are applied. This includes the standard guide configuration on the JGM:N:P:Q:I, JG:A and JGR:J:W frames. This is due to the higher potential for crankcase over-pressure conditions due to possible auto ignition of lubricants the cylinder, creating escaping gas pressure through the pressure packing.
