starter grinds

Hello! i have an 87 ranger 2w. 2.9L standard, my problem is my truck eats starters!
in 2 years i've rebuilt a starter 3 times and had 3 brand new ones. they all start off ok, then get gradually noise, untill they grind like crazy and fail to start my truck the second last one lasted a month and this last starter lasted a week, i've checked the flywheel and all the teeth are there, what can it be? i'm at my wits end please help!!!!!

Hey. I had the same problem, I went to the junk yard and got a bellhousing, starter, and flywheel from another ranger at the junk yard, and threw it into my truck. the problem was that the starter is a different size than what is needed, and is coming up short of having a correct mesh with the teeth on the flywheel. cost me 16 dollars to fix this. you should try it......good for a sunday