Setting base timing without marks on Pulley

Thanks Andre. I did this as you suggested. I welded about 25 mm metal bar onto an old spark plug. Screw in the plug and rotate engine until piston 1 comes to a stop by the spark plug. Mark it. Rote opposite direction and mark it . I get 2 marks about 70 degrees apart. However, I can't understand how the TDC be between the 2 marks.

Also I noticed when I repeat this process again, the piston stops don't come exactly at the same marks I made before. They are slightly offset.

You need to be a little careful with engines like the LS since the plugs aren't directly above the piston. This means that you can end up bending the rod that you've welded to the spark plug if you put enough force into turning the engine over. In this situation I'd use a large diameter steel rod and weld it to the plug very thoroughly to ensure rigidity. Make sure when you're finding TDC that all the other plugs are removed to make it easy to spin the engine by hand. Carefully bring the #1 piston up and stop as soon as you feel resistance from the positive stop.

If you think about the way this test works, you're stopping the piston a small distance from TDC on both sides of TDC. You're then making a mark on the pulley corresponding to these points. It stands to reason therefor that true TDC is exactly in between.