Top Dead Center. When the piston of an engine reaches the top of its stroke it is said to be at top dead center. This piston position happens to be the point at which the air/fuel mixture is most compressed. In most engines, depending on their RPM, the spark plug will fire a few degrees before the top dead center in order to start the burning process before the down or power stroke. One of the great benefits of Viscon is its ability to control the propagation of a flame emanating for a spark plug. By reducing the number of very small droplets of fuel, Viscon promotes a more even burning flame, which makes ignition before top dead center work more efficiently.
Top Dead Center -- when a piston is at the top most position during the compression stroke. TDC can also refer to the top most position of the piston in the exhaust stoke, but "true" TDC is on the compression stroke.
This mark on ALL crank pulleys refers to the compression stroke of the #1 & #3 pistons. Set it to the seam in your case, then pull off the distributor cap and note which plug wire it's pointing to. That's the piston ready to fire.