Decided to dump and refill all the fluids on my new-to-me '97 K2500. Got the transfer case drain plug out, there was only about half-a-quart of fluid in the thing, and it was brown-black when I expected red ATF to come out. Smelled like burnt ATF, thin like ATF. Anyway, I go to take the fill plug out...and the Allen socket just spins in the rounded-out fill plug. I had to weld a bolt in the Allen hole, then turn the plug out with a socket on the bolt head. I've got a bunch of pipe-thread plugs of various sizes, so I grab one that looks the right size, and stick it in the drain hole. The NPT plug threads more loosely that I'd expect, and it goes in way deeper than I'd expect. It also leaks even though I used a good pipe thread compound on the thing before it went into the case. I really don't want to have to re-use the reamed-out, bolt-welded plug to seal the case. WHAT THREAD SIZE IS USED ON THE DRAIN PLUGS? Metric? It sure doesn't seem to be standard pipe thread.