The valves are normally closed until the cam shaft acts on the valve train and opens any of them. When a valve is closed on a solid lifter , not a hydraulic lifter machine, there is valve lash. This is the space we measure with a feeler gauge between the tip of the valve and the tip of the rocker. The valve is closed when it will not flow and the valve face is on the valve seat. When the valve is open it will allow flow of gas and or liquid. When a valve is open there is no lash, a feeler gauge can not be put between the rocker and the tip of the valve, the valve spring is compressed.
On the 4 stroke engine there are two Top Dead Centers per combustion cycle. Only the one at the end of the compression stroke has both valves closed and both can be set. The intake valve will open with the movement of the intake rocker on the intake stroke, the piston is traveling down and the valve opens to let gas in. Just before the piston starts back up the intake valve closes, The intake valve should be entirely closed on the compression stroke and the exhaust will not open till the end of the power stroke so both valves are closed for most of 2 of the total of 4 strokes of the engine. That is they are closed for half of the time. It actual practice there is a lot of overlap and the valves take time to make all this motion but for the purpose of setting the valves use TDC after the intake valve has closed.
This is assuming that you are turning the engine in the direction it normally travels during operation. Looking at the front of the engine with the cover off the engine should turn clockwise.
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