These spark plugs are for use in VW heads that have a special casting allowing the use of “long reach” plugs. Some head beginning with the part number 041, 043, and 044 for example may use these plugs. You should check the thread length of any performance head to insure that you are using the correct length plug. These plugs have a thread length of about 5/8″ (or 16mm.) Set of four.