At TDC of the compression stroke for cyl #1, the cam lobes will be in a position such that they are putting no pressure on the valves, and valves are closed.\n.
\…nAt TDC of the exhaust stroke for cyl #1, the lobes will be in a position such that intake and exhaust valves may be open. Exhaust will be closing, and intake will be opening.\n.
\nThere are two revolutions for the crank for one of the cam. TDC usually refers to TDC of the compression stroke, when valves are closed. This is when combustion/spark occurs, and is when the valves are adjusted on most engines.. (MORE)