The plastic plug probably just deformed around it. I agree, file it out, but then you have shavings, etc to deal with. Or, just put a plastic plug in w teflon tape and call it a day?

 

Maybe try using magnets to catch the shavings. Attach one to the file or tap. Place a few under the hole. The shavings should settle down in the bottom anyway, so not much to worry about in my book. Definitely get it tapped out if you plan water excursions.

 

Unfortunately a piece of metal in there is no good no matter how you come to realize where it came from.

There are 2 very little rolled compression pins holding the center carrier pin in its place. It is possible to loose one of those pins with mileage (use) -

Are you considering removing 3rd member & inspecting? Those carrier pins are a known Achilles heal.

 

use brake cleaner to clean the differential - i use about a half of a can for each diff after i drain before refill

 

So does anybody know the tap size I should purchase to clean out the threads?

 

Don't overthink it. Remember, they came from the factory with plastic fill plugs. Those first two threads will hold the new brass plug just fine, even if you bugger them just a little taking out the metal in the back with a rat tail file to get it out of the way for the new plug.

 

I'd use a spiral flute tap (pulls the shavings out of the hole unlike a straight flute tap) and a shop vac placed next to the hole. That should keep all the metal out of the diff. That, or just grind down the fill plug so you've got fewer threads. There is no pressure on it, so you don't need much thread engagement.

 

Yeah, Alex and are basically saying the same thing, I think; just different ways to approach it. Two threads (two full rotations) and it should be fine.

 

Solved.

GTD 1/2 - 14 NPS is the correct tap size for the differential fill holes. This worked perfectly and corrected the problem.