I want to remove the rear differential fill plug on my 03 Trailblazer. I had it out probably 4 or 5 years ago when I changed oil and at that time I put anti-sieze on the threads and torqued it to the proper level (30 or 35 ft. lbs I believe). So, it shouln't be too frozen. Guess I should check it more often but with absolutely no leaks, it's something that's easily put off. Went to take it out the other day and it didn't want to move. I didn't get a lot of power on it though. I was just using a 3/8" extension but it felt like it was starting to slip so I just quit before doing damage. Anyway, my question is this... is that hole in the plug actually 3/8" or is it 10 mm? 10 mm is about .020" larger than 3/8" and using just a little bit too small of a wrench for that could be bad. It would be surprising to me to find out that this was actually a 3/8" hole when every other nut, bolt, fastener and drain plug that I know of on the vehicle is metric. Anyone know for sure? Either way I will get a proper pipe plug socket. Do they really grip a lot better than a ratchet extension? Would it be wise to get one that could be used with an impact wrench or not necessary? What is the overall diameter of the plug? Will maybe get a new one if I get the old one out. Thanks in advance. I searched for the answer here before posting, but didn't find this informmation.