Abstract: An improved crankcase seal for air-cooled engines, such as the type utilized in aircraft and automobiles, comprises a groove cut into the planar mating surface of one of the crankcase component halves and a generally compressible sealing member disposed in the groove such that when the two crankcase halves are joined together the sealing member is disposed therebetween to seal the mating surface interface. Preferably, the groove is machine cut into the mating surface with a CNC machine and is provided with a pair of opposing upper edges that are spaced apart a distance less than the width of the sealing member such that the sealing member is securely held in the groove during assembly of the crankcase. In one configuration, the groove has a dovetail profile and the sealing member has a circular cross-section. The crankcase seal is particularly beneficial for horizontally opposed aircraft engines and the like.