Hi is TDC on the 400 just St the piston stops moving up when it stops coming down or somewhere inbetween

 

Somewhere in between, remember the piston hangs at TDC for a few degrees. There is a way to determine the "exact" TDC. When the piston stop shows TDC while turning the crank clockwise, make a mark on the damper. Then do the same routine turning the crank counterclockwise. True TDC is halfway between those two marks.

 

Open the valve cover and watch the intake valve close, you will then be on the compression stroke for a TDC reading. If you buy a piston stop - that threads into the spark plug hole, rotate carefully clockwise until you feel piston contact the stop - mark the harmonic balancer. Then rotate the engine counterclockwise until you again feel the piston contact the stop - mark the harmonic balancer again. Top dead center, TDC, is the midpoint of these two marks.