The only accurate way to find top dead center. CNC machined billet aluminum.
Note: NGK plugs starting with "C" are 10mm. Starting with "D" are 12mm If you use another brand, you will have to remove the plug and measure it .
TDC10 10MM TDC TOOL
TDC12 12MM TDC TOOL
TDC14 14MM TDC TOOL