TDC and cranckshaft position

[TwinFast]

I have to put a new timing belt on a CS1 Fiat Spider 78. The engine is open and on the floor. I start to get directly the TDC with the cylinder head removed. It should be easier, but when i turn the cranckshaft pulley and reach the TDC, i see that i can continue to turn the pulley few degrees without moving the piston. The piston start going down after rotating the pulley 15 or 20°.

My question : is it better to set the TDC using the visual position of the piston at beginning of reaching the top or simply the mark on the pulley ?

[carl]

Yes, there are a few degrees of rotation when the crank turns but the piston seems to stay at top dead center. If you were using a degree wheel you would turn the crank one direction and note where the piston tops out and then turn the crank the other way and approach TCD from the other direction and note that position. You would then mark the crank in the center of the difference. In other words if there was a 20 degree rotation between TDC from clockwise and TDC from counter clockwise then you would split the difference and set it 10 degrees from either initial point. Hopefully this would be the factory reference point.