A 3/8″ NPT socket plug used to close off larger unused pipe ports. At this size the ports are usually found on oil filter and cooler housings, distribution blocks, larger pump bodies and tank or sump adapters, where a spare outlet needs sealing after a line is relocated or removed. The internal hex drive keeps the plug flush in the boss, useful when clearance around the port is tight. Apply a suitable thread sealant or PTFE tape before fitting, and tighten to finger tight plus a turn or two — tapered threads wedge as they go in, and over-tightening risks stressing the surrounding casting.