'68 Tach Drive distributor problem

The cable going to the tach stopped spinning because the gears on both the main shaft and cross shaft were ground down. I suspect this happened because of a gear misalignment caused by a missing thrust button. I replaced the main shaft, cross shaft gear, washer and thrust button with parts purchased from Dr. Rebuild. The thrust button did not seem to fit correctly but I got thing put back together. This fix the problem for a short time but the now the gears are nearly ruined again. Does anyone have an idea what I am doing wrong. Thank for the help.

Did the distributer turn easily when you got the shaft and cross-gear back in?
Are you certain the tach cable is not binding, which would accelerate the gear wear?
There seems to be a LOT of replacement cross-gears that are not surviving very long...I'm wondering if they are not being properly made/cut. The cross gear should be bearing grade bronze, not brass, as I suspect some of the replacements may be...
There are also some stainless steel shafts/cross gears out there, with needle bearings in the cross-shaft housing...

The cable is not binding.  The cross gear turned smoothly when I put it in but as it worn down it developed problems.  I replaced the main shaft and cross gear at the same time.

Sounds like it might be soft material.

You may have bushings that are worn out in the dist. housing. That would let the dist shaft move away from the cross-shaft, and cause the wear to occur.
If the shaft/gear turned smoothly after you installed them, then the button that didn't seem to fit properly would prolly not create the problem.
Too much end-play on the dist shaft(up/down) could do that, but it would also effect the way the engine runs.

See if there is any side-side movement of the dist shaft at the top, where the rotor sits. There should be none.

Other than those things, it would have to be a quality issue with the gear/shaft...

I second the suggestion you check the distributor's bushings.  But there are some very poor quality gears on the market.  We shopped around a lot before finding a good source, but now I can't remember where it was.  I will try ot find out and post again.