Jimmi, on the relatively few engines I have assembled I have checked the factory TDC marks and found that they are not always accurate. That check is done by making a positive stop that contacts the #1 piston before it reaches TDC and rotating the engine gently by hand until the piston contacts the stop. A mark on the flywheel or harmonic damper is then made at that point.
Then the engine then is rotated in the opposite direction until the stop contacts the piston and another mark made. Actual TDC is midway between these marks.
Most I have checked have factory marks that are close but it's nice to know where actual TDC is. When degreeing in a camshaft it is essential to know the location of TDC to verify that the camshaft is in the correct pocition in relation to the crankshaft.
To some that may be an unnecesary step. However the camshafts I have bought from Crane Cams and Competition Cams for my daily drivers and street rod came with instructions that strongly recommend degreeing the camshaft in rather than relying on marks on the timing set and harmonic damper. So far the camshafts I have installed checked within a fraction of one degree of the desired position.
It's easiest to make the piston stop for a short block. With the heads installed I knock the porcelain out of a spark plug, thread the hole and install a bolt to create an adjustable stop.
