Abstract: A semicircular packing comprising a metallic portion and an elastic portion sealing the pierced hole of the cylinder head when setting, having thick portions at both ends of auricules and a thin portion between thick portions at both ends, at least on the metallic portion partly contacting with the outer surface of the pierced hole. This semicircular packing is easily conformed to the curved peripheral surface of the pierced hole and makes it unnecessary to coat it with sealing liquid as well as able to permit the clamping torque of the cylinder head to be small and yet maintain its sealing state over a long term of years without being subjected to excessive or premature aging.