QUOTE(Vacca Rabite @ Sep 12 2010, 07:28 PM)
OKay, the real first question. Are you sure that you are actually at TDC. An engine with move a piston to the top of its travel twice during, but only one of those times is TDC.
You are at TDC when the piston is at the top of its travel AND both valves are closed.
For completeness - sorta...
The piston doesn't know the difference between one TDC and the next - TDC is TDC. But one is more useful (i.e. we care about) than the other. (The cam position is different.)
The 'useful' TDC when both valves are closed is "TDC Compression" (the other is "TDC Exhaust").
When #1 is at TDC Compression (the basis for timing, although it's all arbitrary) #3 is at TDC Exhaust.
To know which is which, assuming no prior knowledge of the engine, yes, you've gotta pop a valve cover to know what the valves are doing.