What would cause repeated flywheel damage

[nameiztaken]

What would cause a 136,000 mile 4.0 ohv / m5od to repeatedly lose and chew up flywheel ring gear teeth?
I thought it was a random incident so I installed a new flywheel and starter after my last one lost teeth on startup immediately after changing to motorcraft spark plugs… (unrelated issues and bad luck? or somehow related)

It sounds as if the new flywheel I installed is already losing teeth. Someone suggested that I may have a bent crank, which seems most likely. The transmission and engine are flush together with the stock spacer plate, nothing is visually wrong. I used the correct manual trans starter (napa).

Any possible ideas are appreciated, I cant believe the amount of trouble I’m having with this engine.

Considering an entire motor swap, this motor has been a complete nightmare.

 

[boominXplorer]

Make sure you have a good engine ground or add one. I've seen weak grounds cause crazy issues with starters but not effect anything else because of the Amp draw.

Bent crank to throw the flywheel off would be massive. You can measure runout with dial indicator through starter hole. It won't be perfect but I'd say .02" or less is.

 

[Centaurious]

With all the damage to the Top of the teeth I'm going to suggest an improperly machined housing on the starter or the wrong tooth count on the starter pinion gear. I never trade in the old starter until I am SURE that the new starter fits correctly and works. Had a similar problem to yours and played hell trying to find the correct starter without the old one to match it with. Lesson learned!

 

[92exp4x4]

Wow, that's bad damage. From the wear on the remaining teeth, it almost looks like the starter gear is too big in diameter. Like there isn't enough room for the teeth to mesh nicely and get along. I'd verify that part number to a source like Rockauto. They have several options with pictures. You might be able to identify yours as something else or find some info as to what it should be.