Exhaust Flange Nut size for a 3.8 stock exhaust

Try taking the bolts to a hardware store. You should be able to match up the nuts and the bolts.

Try a metric size instead of an SAE size. Also think you are missing the lock washers which is why you can not tighten all the way.

Also, not a good idea to use low grade steel hardware store nuts for exhaust applications. Suggest going to Ford and buying the nuts designed for exhaust duty (bronze/copper I think).

Yeah, I would suggest going to Ford, or at least see if the company you got the headers from has a set of nuts to go with those studs. Yes, the nuts are more expensive, but that's because the nuts are torque AND heat rated.. for high strength and corrosion resistance. Exhaust nuts can easily become seized on, stripped or they can bust and/or stretch if the proper type isn't used. That is nearly the hottest section of exhaust, even hotter than the header to head nuts.

When I took my exhaust off one time, I was able to use my air impact wrench. That, along with 3 repeat and wait intervals of PB blaster. All the nuts came off with some work, except for one... in which case the nut never budged and the stud unscrewed instead. Thats what happens when you get overheated cats (red hot). Nut got cooked on there good. THIS is just one of many reasons why you NEED top quality nuts. Even some high grade nuts at the hardware store may not be enough. Those nuts were not designed for exhaust pipes where extreme high temps exist. IF you do use some hardware store nuts, make sure you slap on a good deal of anti-seize compound on the entire length of the stud threads.