In this video, This Old House plumbing and heating contractor Richard Trethewey and host Kevin O'Connor cut threads for a new air vent.
1. Enlarge the existing air-vent hole in radiator with 7/16-inch-diameter high-speed steel drill bit.
2. Put a ¼-inch tapered tap onto a ratchet wrench and cut new threads into the 7/16-inch hole.
3. Spread pipe dope onto the threads of a ⅛-inch to ¼-inch bushing, then thread bushing into hole and tighten with a ratchet wrench.
4. Apply very little pipe dope to the threads on the air vent and thread the vent into the bushing; hand-tighten the vent until it's in an upright position.