Here is a link to an easy way to find TDC.
http://www.wikihow.com/Find-Your-Eng...ad-Center-(TDC)
A variation is to put oil in the tube and then mark the high point going forward, then go past TDC and mark the high point coming backward. TDC will be exactly in the middle of the two marks. They will be about 1/8" apart, depending on the size of the tubing used.