bad starters 7.3 f350

[debsart4941]

I have a 2002 F350 7.3 powerstroke w/ approx. 200,000 miles. I replaced the starter with a brand new lifetime warranty starter approx. six months ago. It worked a short time and failed. I replaced it, then again, and again. This week alone I've purchased five starters from three different parts companies and they're ALL bad. I've tried everything from increasing my cranking amps, watching for voltage drop, shimmed the new starter, and I don't know what to do.
When I turn the key now it'll begin to roll over then spin free. I added another ground wire and made sure the bolts are tight and positioned properly.

I've gone through a total of NINE starters in six months, five in the past five days. Two tested bad in the box at the parts store even before I left with them.
They're manufactured in either China or Mexico.
The Ford Dealership wants more than $505 for a new ford starter with an additional $260 core charge saying they will not accept the off brand I have now. $765 for a starter is out of my league. Please help.

[gene c]

i'm just a little curious have you done a resitance check on your battery cables i would look someplace else its not a starter bad relay/solinoid you can;t have that many bad starters

[debsart4941]

No, I have not done a resistance test but replaced with new..... then added jumper cables from another truck to ensure voltage. What would you suggest?

[debsart4941]

When the starter begins to roll over... they sound terrible... like metal grinding in an echo chamber.... there's no marks or broken teeth on the flywheel and I've tried changing the position just in case... I've checked 360% of the flywheel and nothing appears damaged.

[debsart4941]

After that horrid noise... it only spins and sounds to me like the Bendix is not engaging.