Replaced four starters. whats making them go bad. on 1996 Ford Bronco

I've had 4 starters replaced in as many months. What could be making them go bad. I when I return them and have them tested they work. I've checked for 12 volts going to the starter and have that. Solenoid replaced already.

3 answers 2 comments
Do you have oil dripping from the valve cover from above on it. Did you alter the exhaust and it getting too hot.
how are your positive cables? if they look green in color replace them. they could be bad or have a break in them. check your ground cable while you are at it. a bad ground can cause problems to for your starter. make sure they are tight on the starter
if you have a meter you can check to see if it is getting 12 volts.
FIRST things first, did you remove the HEAT SHIELD or is it missing the heat shield..., make sure all of the cables are good, and clean... make sure you put a good quality starter on the vehicle, and make sure you have a good heat shield around it, I use the wrap that they use on race cars... it costs about 20-20$ but is worth it to me, I used to fry starters and have only replaced one in the last 3 years now
did you read the ad! when they returned them they worked just fine! sound like a cable problem to me.