v~~~ timing mark @ 10 deg BTDC ----------------------::------------- ====== <--- spin ==================< crank pully ----------------------::------------- 16 12 8 4 . 4 8 12 16 20 24 . . . . /|\ . . . . . . /|\/|\/|\/|\/ | \/|\/|\/|\/|\/|\/|\ | ATDC TDC BTDC TDC = Top Dead Center (piston in cylinder is straight up at ignition) BTDC = Before Top Dead Center (advance; ignites when piston on way up) ATDC = After Top Dead Center (retard; ignites when piston on way down) * Each tooth point is 4 degrees, so each mark would be 2 degrees. * The timing mark represents the piston and spark for cylinder 1. * The specs in the books and stickers call for timing at 10 deg BTDC.