A 1/16″ NPT socket plug used to close off an unused tapered pipe port. The internal hex drive means it seats low and flush, which is useful in tight or shallow locations where a hex head plug would foul on something nearby. At this size it typically blanks a spare gauge, sender or vacuum port on a manifold, block, pump body or adapter block. Because NPT is a tapered thread that seals on thread engagement rather than a flare, use a suitable liquid thread sealant or PTFE tape and tighten to finger tight plus a wrench turn or two — over-tightening a small plug can split a casting.