A 3/8 inch hex pipe plug used to blank off a threaded port that is surplus to your setup. It sits between the common small sender sizes and the larger drain ports, and typically ends up sealing spare ports on oil filter and cooler adapters, block galleries, surge tanks and larger fittings. The tapered pipe thread seals on the thread form itself, so PTFE tape or a thread sealant is needed, applied a thread back from the end so none of it can find its way into the oil or fuel. Tighten it firmly without going overboard, especially in cast alloy where the taper can wedge and crack the boss.