Oil Gallery Plug
Threaded plugs that seal the high-pressure oil passages machined into the engine block.
Oil gallery plugs (also spelled oil gallary plugs) are threaded fasteners or expansion plugs used to seal the ends of the main oil galleries (passages) machined inside the engine block. These galleries run throughout the block, carrying high-pressure engine oil from the oil pump to the main bearings, camshafts, lifters, and cylinder heads. The plugs keep the pressurized oil contained inside the block.
During engine manufacturing, long oil passages must be drilled straight through the block casting. To seal the entry holes where the drill entered, the manufacturer taps the holes and installs threaded plugs, often using thread sealant or pipe dope. If an oil gallery plug leaks or backs out, it causes a severe drop in engine oil pressure, which can starve critical bearings and lead to sudden, catastrophic engine failure.
During engine rebuilding, the gallery plugs are removed to allow the galleries to be thoroughly cleaned of carbon sludge and metal debris. When rebuilding the engine, it is critical to reinstall the plugs with fresh thread sealer and torque them to spec. Threaded plugs are preferred over expansion-type plugs for high-performance builds to eliminate the risk of blowouts under high oil pressures.