A 1/4″ NPT socket plug for sealing an unused tapered pipe port. This size turns up on oil filter housings, adapter and distribution blocks, fuel rails, water pumps and cooler plates, so it is the go-to plug when a take-off point is moved or a spare port is left over from a previous setup. The recessed internal hex allows a flush fit where a protruding head would foul against nearby components. As with all NPT connections, use thread sealant or PTFE tape and tighten to finger tight plus one to two turns — the taper does the sealing, not brute force.