(94 LT1 )I am about to blow up this car! I am having a starter issue that is driving me nuts. I thought I was getting bad starters from Rock auto. About every six starts they would Thud and not spin the motor. If I cranked the alternator bolt and moved the motor the starter would engage and freely spin the motor. I bought a reman NAPA starter and swapped it out. I did measure the Rock Auto starter and it was fine using a paper clip. Did the same with the NAPA starter and it too was fine. I started the car 25 times last night with zero issues. Went to start it this am and Thud. Hit the key 3 times before it would spin. Car starts immediately after it starts to turn. Idles fine. Sits for a few hours and Thud, no spin. I am at my wits end. Was starting to think leaky injector flooding a cylinder but oil doesn't smell gassy, and if I turn the alternator bolt 1/16 of a rev. the starter will engage and spin freely. Should I add shim to the starter? Battery is good and cables are clean and tight. It seems like the motor will not spin after it sits. Could it be oil leaking down into a cylinder causing too much compression? doesn't seem to burn oil. I think I have ruled out the starter as being the cause.

 

Have you looked at ring gear? If you are slightly rotating engine and starter will engage. IMO it's something between the starter drive, flywheel, or clearance.between the two.

 

I would pull the starter and plugs, then have a helper spin the engine while you inspect the entire flywheel.

 

….. Have you checked and cleaned the battery to engine ground point (at the engine end) ? … this sounds like a connectivity problem to me