TDC Indicator

The 10mm are used for the Hayabusa/GSXR1000/ZX-14 and most modern sportbikes, 12mm for GS1100-1150, and the 14mm are for the Kawasaki KZ900-1000-J Models

These are what you use when degreeing your cams, as a positive piston stop. Lube the threads before using, so you do no gall them in the spark plug holes