Harmonic balancer 340

To check the thrust, you use something to push/pry the crank as far forward as it will go, and then push the crank back as far as it will go. The movement is the thrust bearing clearance. It should not take a lot of force to move the crank forward and backwards; if you are not looking closely for this small movement, you will miss it. Have the damper's bolt and snugged in place when you do this, if you pry against the damper.

Was this engine just rebuilt? (Or am I confusing this with another thread?)

If you have .25" between the timing marks and the back of the damper, that is certainly sounds like an issue. But also check the damper to make sure the weight ring has not moved forward on the hub. The hub and ring are separate, and are glued to gether with a thin ring shaped layer of rubber between. On an old damper, they can separate and not stay aligned.

Some pix from the side showing your damper and the hub from the side, as well as the timing mark area on the cover, would help everybody.

At this point, I would be inclined to make a quick check of thrust and then pull the damper off to see what is going on.