grinding noise

Alright, ive now had 3 different starters on my 74 chevy 350 pickup and it keeps grinding when i go to start it. The first time i took it off and saw the teeth on the starter shaft were chewed to hell so i bought a new one and again it would grind. I added shims, it would run fine for a day or 2...

1977 sbc rebuilt, engine rotates for about 5-6 seconds and then makes a grinding noise. Starter is brand new. Sounds like its coming from starter area? Engine was never primed and has timing issues. I dont want to keep trying to start it because Im worried about causing damage. Any ideas what...