REPLACES STRIPPED SPARK PLUG THREADS IN ALUMINUM HEADS WITH 14MM SPARK PLUGS. INSTALLS A NEW SOLID STEEL SPARK PLUG THREAD INSERT IN ORIGINAL THREAD LOCATION. SET INCLUDES ONE-PIECE DRILL TAP, LONG TAP HOLDER, SHORT TAP HOLDER, INSERT INSTALLER, SWEDGER AND HANDLE. INCLUDES FOUR INSERT SIZES: 5/16", 7/16", 11/16", AND 1" (NEW SIZE). PER MIKE- THE TAP IS SUPPOSED TO BE 18MM BECUASE YOU NEED TO DRILL OUT TO A BIGGER SIZE TO SCREW THE INSERT IN. THE THREADS ON THE INSIDE OF THE INSERT ARE 14MM.