A hex-headed pipe plug in 1/2 inch, used to seal off a threaded port you are not using. Typical jobs include blanking redundant sender bosses on a block or manifold, closing the spare port on a fuel rail, filter housing or oil cooler adapter, or plugging an unused outlet on a tank or swirl pot. The 1/2 inch size is one of the larger common port sizes, so this often ends up sealing coolant drains and larger sender bosses. Being a tapered pipe thread, it seals on the thread itself, so use PTFE tape or a suitable sealant and keep it clear of the leading thread to avoid contaminating the system.