if a head or heads are already on, set the dampener slit at 15BTDC & screw the TDC tool into the #1 or #6 plug hole till it contacts the piston snug, then turn the crank CCW till it contacts the stop again & if no slip/bad machining of dampener (somewhat rare) or even crank keyway machined off (rare tho) & yes it is at 1:30, you want the slit to be at 15 ATDC. repeat one time to confirm it is equidistant, same # of degrees on either side of "zero" on the tab . A degree or less off is acceptable. this will confirm zero on the dampener/timing tab matches the actual TDC position of the piston.