A 1/4 inch hex pipe plug for blanking a threaded port that is not in use. It is a common size for sender and switch bosses, so this plug often finds work sealing a spare oil pressure or temperature port, closing an unused branch on an adapter, or blanking a manifold vacuum take-off. Being a tapered pipe thread, sealing happens on the threads rather than a face, so apply PTFE tape or a thread sealant, starting a thread or two back from the end so nothing can shed into the fluid. Tighten firmly but do not over-torque, particularly into alloy castings where the taper can split the boss.