Description Repair damaged 1"mm spark plug threads by opening hole with a reamer/tap and screwing insert onto the spark plug. Use spark plug socket to install spark plug and insert. Remove plug and check that insert is properly seated. Set includes 1"each of 4 inserts: 3"8" 7"1"" 1"2 and 3"4_ reach and thread locking compound. Tap is designed for aluminum heads only.