Unions, couplings, boiler bushings, and plugs.
Machined from solid brass stock available in various pipe threads.
Click here for information on model pipe threads.
I purchased the 3/16-40 UST Union and this thing is a marvel. Well worth the price, particularly as I used it not only to bridge the Boiler to the Engine......but as a quick disconnet for tearing down my boiler.......(hybrid - Jensen #85 Boiler mated to a firebox and propane/butane burner of my own design).
very nice fitting......its nice to have a union with flats on it for wrenches instead of tearing it up with pliers......well worth the cost