Several years ago, on ebay, I bought a couple of German military truck
surplus air reservoir tanks.

These tanks have 3 threaded holes. Two are at one of the heads and
the other is mid-way along the cylindrical portion.

I would like to plug these holes and drill and tap the plug so that it
will take standard American air fittings. The objective is to create
a dry air tank to run a plasma cutter and/or air tools.

I'm not a machinist and I don't have any measuring tools. So I'm not
sure about the diameter of these holes but I'm pretty sure they're
M20x1.5 or M21x1.5. I have found European sources for blind plugs in
these dimensions but have not yet found a source in the USA.

A 7/8" bolt will not fit in the hole and a 3/4" bolt will not touch
the sides. This is my basis for thinking the holes are 20 or 21mm.
The thread pitch seems fairly fine so I'm guessing it's 1.5mm.

I'm open to suggestions as to other approaches to plumbing the tanks
so that I can use standard compressed air disconnects with them. I
weld but I don't consider myself competent to weld on pressure
vessels.

Any thoughts?

Thanks,

Vernon