Plug

A plug is a part that seals an opening, drains a fluid, or closes a threaded hole.

Screwed or pressed into a port, it blocks the passage to keep fluid in or dirt out; a drain plug is removed to let oil or coolant out during service.

Plugs are used on the engine, gearbox, axles, and fluid systems. The correct plug matches the thread or bore and any sealing washer.