So im retimming my 4.3l tbi cuz i had to replace the intake gasket anyways i was gonna put the engine to tdc and then drop in the distributer putting at the number one spark plug, well i just figured out that there are two timming marks so now i have no idea which one is tdc. They are about 15 to 30 degrees apart from each other on the harmonic balencer. So im gonna start over but i need help, one how to make sure im tdc and two which timing mark? Im guessing one is when the timing advance is working and the other is when it is disconected.