This universal thermactor cylinder head plug set is designed for small block V8 engines, providing a clean and secure way to block off air injection ports on performance or rebuilt cylinder heads. Perfect for applications where the A.I.R. (Air Injection Reactor) system has been removed or replaced. Manufactured from high-quality alloy steel, each plug features both 5/8"-11 external and 7/16"-14 internal threads for an exact fit. The slotted head design allows for easy installation or removal using a flat-blade screwdriver. These plugs effectively seal air passages located at the front and rear of the cylinder heads, preventing exhaust leaks and maintaining proper back pressure. Sold in packs of 5, they're ideal for rebuilders and high-performance engine specialists who demand precision and durability in every component. Engine Quest produces high-quality OE-style replacement parts. Each part undergoes rigorous testing and inspections to meet EngineQuest's exact specifications. Designed for precision manufacturing and durability, our products ensure easy installation. With nearly 500 SKUs available, the company specializes in hard-to-find, in-demand parts, including those no longer available from the OEM, difficult to salvage, or with prohibitively high OEM costs.