M137A – for fixed or electrically adjustable or autonomous propeller, oil system modified for full aerobatic operation and unlimited inverted flight, kicked aerobatic maneuvers permitted, specially modified crankshaft, air intake to the engine from the front of the propeller, operating overload +6g, -3.5g

M137AZ – for fixed or electrically adjustable or autonomous propeller, oil system modified for full aerobatic operation and unlimited inverted flight, kicked aerobatic elements permitted, specially modified crankshaft, air intake into the engine from the rear of the engine with the option of connecting to an air filter, operating overload +6g, -3.5g