Starter grinding

I am on my third new starter in 12 months on my 1970 Mach 1 351 C . All three will randomly grind against the ring gear. Not everytime just now and then. Usually when the car is warm and often at a car show (never in my garage)! Each time we have inspected the ring gear for a missing, warn or bent tooth but see nothing. The starters are aftermarket gear reduction starters and bench test fine. I have checked the wiring and both wires are on the correct posts both at the solinode and starter. Any thoughts? Thanks  

there are 2 different starter lengths...  

the automatic transmission cars have a starter pinion that sits farther into the bell housing..

the manual transmission cars have a starter pinion that does not sit as deep into the bell housing.

since you did not mention which transmission you have.. i hope you will .. and also post a part number..   you can post it to a private message by clicking on my user name..

this way i can cross it ..   i have a chart someplace that does just that... but i will have to search it out..


can you also check for crankshaft end play...  usually this is done at the front damper..  i don't know what kind of tools you have..  dial indicator is the best way..  pry the flywheel back.. take a measurement at the front damper or pulley..  

0.06 to 0.018 i think are close to what there should be..   i personally have seen some engines with almost a quarter inch of crank end play.. there was NOTHING left of the thrust bearings and the crank thrust faces were totally wiped out..  

Thank you. I should have mentioned my transmission is a C6. It is not the original. Both the motor and transmission have been rebuilt as part of the restoration. Motor is a 1972.  

just because it has been rebuilt... does not mean the thrust surface has not eaten away the thrust bearing..

 if you stand beside the drivers side fender.. looking straight down the belts.. reach over and bump the throttle a few times while watching the belt alignment with the crank pulley...

see if the crank is moving forward when you rev it...    it should NOT move any visible amount..