ARP's handy thread cleaning chaser taps are designed with correct thread pitch and diameter to clean dirty blind or thru holes. Taps come in five USS sizes 1/4, 5/16, 3/8, 7/16 and 1/2, as well as metric. They are sold individually or in sets. Please note that these are strictly cleaning taps and are not designed to cut thread. They are a handy addition to the tool box of any serious engine builder and an essential aid to preparing any block for final assembly. Don't take a chance on weakening block and cylinder head threads. Use these handy thread cleaning chasers whenever possible Thread Cleaning Chaser Size 1/4in-20