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16 Once the holes are drilled, tap them at 1⁄4 x 20. For those who haven’t done this before, the 1⁄4-inch x 20 is a standard thread (it’s the “coarse” quarter-inch thread), and taps are readily available at most hardware stores. After affixing the tap in its handle, called a “tap stock,” simply push in and turn it. You’ll feel it bite into the metal; after a turn or so, back it out a half-turn to break the “chip” (the piece of metal you’re cutting out), so it’ll continue to cut cleanly.