The NPT (National Pipe Thread) plugs have tapered threads and sealing is done bythe threads themselves. Alwaysuse a small amount of AeroSeal(AF98-2023) on all NPT threadsand tighten until the it stops. These NPT plugs are ideal for sealing up ports andfittings that are unused such as unused holes in your intakemanifold, or any open vacuum, coolant, or oil holes. Each plug features an internal hex which provides an easy installation and a better appearance. These plugs feature a steel construction with a zinc coated or black coated finish.