Trying to add a valve to the smaller bung fitting on a 15 gallon plastic drum. I can't figure out the thread.

The male thread diameter on the plug is 1.02" OD, straight thread, not tapered, uses a rubber gasket to seal. None of my standard thread pitch gauges match the thread but the 1.75mm metric thread pitch gauge matches perfectly. So, I'm assuming it's a metric thread. 1.02" OD = 25.9mm, almost exactly 26mm.

Must be M26x1.75. I've looked several places and can't find that thread in a valve or any pipe fitting. I've found that thread in a thread tap, commercial coffee maker spout, refrigerant expansion valve, and a few other really weird things but not a plumbing fitting. Any idea where I could find something like this?

I may just tap it to something else but I wanted to be able to reuse the original bung plug if possible.

Hydraulic fitting sounds like. I would check a hydraulic repair shop.

Any farm supply store (at least around me) sells valves that fit bungs holes for all sizes of plastic barrels

I looked at a couple of industrial suppliers I use online for that and couldn't find that thread. I'll stop at Tractor Supply today to see what they have.

check out the dimensions for british standard pipe thread -- they use alot of those on the water pumps that we used to import and those dimensions sound about right -- but is has been a few years since I have had to deal with.

NPT threaded?

I buzzed the top off a 55 gallon drum the other day for a burn barrel I noticed it had a smaller bung then other 55s I’ve messed with it was about a third smaller then the older type barrels. If that sounds like the one you need send me a PM I’ll send it to you.

I think this is correct. Look up BSPP or "G" pipe fittings. McMaster-Carr has some fittings I think.

"JB Weld" is your friend.

You could always use a thru hull fitting.You would need to cut top off.

A small bulkhead will pass thru a 2 inch hole. Use a piece of wire to pull into drilled hole.If you need a small amount, you can push a tubeless valve in from outside with rite tool, or get an external installed tubeless valve. They work good, but are dear