When you feel vacuum, the piston is pulling in mixture to compress, and therefore the next pressure you feel is that of the compression stroke. Since you don't know which, you are looking for vacuum. If you feel pressure the piston is either coming up from the bottom of the intake stroke or coming up from the bottom of the power stroke. You are feeling for a vacuum or pressure. Slowly crank the engine, using a remote switch or someone else using the key, but just a little "tick" at a time. If none of the above is practical, take out the spark plug on #1 cylinder and put your finger tightly in the hole. If you're lucky - and the hole is close to the #1 valves - you will be able to observe the valves as you crank the engine. If you aren't inclined to do so you might try a small mirror (dental mirrors work well) that can be inserted in the oil fill hole in the valve cover. The best way to know you are at TDC is to look at the valves, so if you're planning on replacing valve cover gaskets go ahead and take the cover off. However, if your engine is running poorly - or not at all - you can't make the assumption that the distributor is correctly positioned. Of course, if the engine has been running perfectly before removal of the distributor, just crank the engine until the rotor lines up with the position on the distributor cap where #1 spark plug wire is located. So how do you do this without exposing the valves? Carefully, very carefully. They stand out beautifully under the timing light's pulses. Tip: Engines are usually dirty up front, so when you find the timing marks paint them with a little correction fluid (White-Out, Liquid Paper, etc.).
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