A 1″ NPT socket plug, the largest size in this range, used to seal big tapered pipe ports. It suits fuel and oil tanks, surge tanks, sumps, large housings and heavy adapter blocks where a substantial outlet needs to be closed off permanently or during a build. The internal hex keeps the plug flush in the boss instead of protruding. A drive of this size requires a large hex key and, given the thread area, a fair amount of leverage to seat — but still work to finger tight plus one to two turns with sealant or PTFE tape rather than forcing it, as tapered threads can wedge and crack a casting.