When repairing water valves in an old house, you may need this size of screw for a faucet washer. Newer valves use smaller washers, and home repair centers often do not carry the large sizes.

When replacing faucet washers, if the existing brass screw is difficult to remove, and the metal begins to chip under the screwdriver, STOP! The metal has become brittle and has a good change of breaking. Instead, remove the stem from the valve, remove the remnants of the old washer, put the stem in a vise, and heat the washer sear with a propane torch. This will expand the seat, and lossen the screw. When it is hot, place a properly sized, sharp, flat blade screwdriver into the slot of the screw and gently, but firmly turn the screw. Being careful in this way may save you the chore of replacing the entire valve.