[AZKen]

So logic says it's NOT the battery and NOT the starter because you tried different one's. Logic says it's not switches or solenoid if each new starter had it's own solenoid.
Logic says it's not ground because you cleaned and checked and tightened.
Logic says 90% chance it's electrical, 10% it's mechanical. Starter pulls +/- 450 amps on battery!!!!!!

These are my logic. Logic has to be tested.

We are in a brainstorming mode now after all you have reported. I'm blasting out thoughts. Brainstorming is an actual engineering technique for meetings and large groups to solve an issue. It may trigger the guys and gals here on 6066club to offer a new idea or test for you. Or see a missed point.

Not wiring because you bypassed with remote starter switch.
Remember I said check oil for milky. Don't want it to be hydraulic lock. I busted three starter noses before I caught on to that once.

Take ALL plugs out and take off dizzy tower wire and crank to see if you can get it to clunk once. Watch for something shooting out plug holes.

Other than that, could be
1. a bad spot on flywheel teeth.
2. You missed something.
3. Two bad batteries.
4. two bad starters.
5. battery cable broken inside the insulation or at clamp and can't see it.
6. Clutch sticking. Whatever that means.
7. Piston sticking, rotating component sticking like bearings
8. Oil or water in cylinder
9. Sticking valve causing compression/back pressure
10. Timing off causing compression/back pressure
11. Those internal compression/check valves in head are stuck
12. Fan belts too tight. Naw, can't be that.



This last batch of things are sketchy ideas I know but it's pointing to something very odd happening.
It does crank good and start good with no starter or grinding noise. Fires 10 x 10 on the bench. Do that plugs-out test and report. Gotta get some change to happen to get a clue.
Intermittent stuff is a pain. You may have to run it til it fixes itself or breaks more.
I have a headache now.

You haven't done anything to anything you forgot to say have you?