tdc mark is the longest mark,there should be shorter ones on either side as timing is set a few degrees off tdc, but you need tdc to start the timing process If you have a concern about TDC, and don't know of any other way to find it, try this: Pull the #1 sparkplug, put a long, narrow screwdriver or other clean, narrow device into the hole, then turn the engine BY HAND (don't use the starter) until you see the screwdriver at the top. You should be able to turn the engine back a little to seek the top. At that point there should be a mark and a reference point somewhere on the harmonic balancer. If you can't find a mark, make your own. It's possible that over the history of the engine someone may have removed the reference marker so you may need to do something yourself. Good luck. ** More ** The piston will raise twice, once on the exhaust stroke and again on the compression stroke, TDC is based on the compression stroke so following the above advice, when the narrow device starts to come up, remove it and insert your fingertip to feel pressure building up, when you feel pressure building up, the piston is on compression stroke, remove your finger and continue with narrow device as described.