TDC means Top Dead Center.When the piston is at it's highest point in the cylinder & valves are closed.As stated in a 4 stroke engine the piston goes up & down 4 times for every cycle.Intake(piston going down) Compression(piston coming up) Power(piston going down) Exhaust(piston going up) Hence 4 strokes.Where the "T" is not as important as you think.As long as the both valves are closed, (nose of the cam facing 180 away from the stems)You can go ahead & adjust your valves.No need to worry about the timing marks.Hope this helps.
