Pass through pipe reducing bushing

Search on 'bulkhead coupling'.

I've seen those, but not quite what I need. I have atmospheric pressure on one side of the plate, high vacuum on the other, and I need to pipe through a different pressure, so it has to fit the existing pipe thread (the plate is too big to take back to the machine shop, and I'd rather not drill it out for a baseplate NPT adapter, which might be too small) and let me connect pipe fittings on either side. Thanks for looking, though!

Now that's a possibility. If all else fails, I'll go that route. Good tip, thanks!

How about this: Use a nipple a few inches long, and run a pipe die up about half way. Run a pipe tap into the hole far enough so the nipple will thread in and come out the other side. Use plenty of sealant, since you may not have a good taper-to-taper fit.

I would take a 3/4 to 1/2 reducer, bore it out so a 1/2 nipple will pass thru, silverbraze or weld the nipple in place. That gives you 1/2 NPT male threads on both sides of the plate.

I use a half of a 1/4 pipe coupling, turned down to fit the bored out inner end of a 3/4 - 1/4 reducing bushing. Silver solder the two pieces together and you have your fitting. I made a couple of these to adapt propane tanks for my compressor. Gerry :-)} London, Canada