GM uses sand to cast its engine blocks' external structure and internal oil passageways (galleries, aka "gallies"), and incorporates a number of external holes in the passageways to get the sand out after casting. GM uses brass plugs to seal these holes. While the plug and block materials expand at roughly the same rate while heating and cooling, over time the plug may begin to loosen and leak. The big block Chevy has about a dozen of them front and rear, and vary between 1/4- and 1/2-inch in diameter. Plug replacement takes only a few minutes, and is de rigueur for any quality rebuild.
Identify the affected oil gallery plug and remove it with a Allen-head wrench. If the plug is leaking, then oil has likely already lubricated the threads, and it should come out fairly easily.
Clean the block threads with brake cleaner, a rag and cotton swabs to rid them of oil. Try to keep the brake clean and oil out of the gallery.
Twist the appropriately-sized pipe tap into the gallery plug to remove any residual sealant and de-burr the threads. Use a gentle back-and-forth motion, twisting the tap about 90 degrees into the hole and then backing out by 45 degrees to remove any cut material. Keep the tap well-lubricated with penetrating oil at all times.
Clean the newly tapped hole with brake cleaner to remove any trace of penetrating oil or metal shavings. Clean the new plug's threads with brake cleaner to remove any oil or wax. This step is crucial to a good seal. Allow 15 minutes for any remaining cleaner to evaporate.
Coat the new plug's threads with enough high-temp sealant to cover them completely. You can't use too much sealant here, since any excess will squeeze outside of the block instead of into it.
Screw the new gallery plug into place and tighten it with an Allen-head wrench or to 15 foot-pounds with a torque wrench.
