Getting ready to put in starter #6. With all the bad starters you turned the key & you get a "click" - and then nothing. Tap on the starter w/hammer long enough & it starts. Tried new batteries & relay on the inside fender in '08, but still needed a new starter to get it started.
With the last bad starter, even with the tapping it won't start. The local electric motor shop had the truck for one whole day and could find nothing wrong with the electrical system. I've changed brands & models of starters from NAPA, but the problem remains. NAPA tested all of the bad starters and said that they were "good", but still honored my warranties.
I appreciate any help that you can give!!!!!!!

 

It is possible that you got 6 bad starters, but unlikely. Each time you removed the starter did you check out the flywheel?
Here are some other things I would check;
1. The ground connection at the engine block
2. The cables for possible internal corrosion
3. Low voltage at the solenoid not pulling it in hard enough
4. I can't think of any engine problem that you can get around by whacking the starter. You might let us know which one of the suggestions if any worked.
Last but not least--- try a starter from Ford!