On the 4.5" hybrid motor I'm currently using NPT fittings in the injector to inject propellant and measure chamber pressure. However, there are many reasons to avoid using NPT fittings in injector heads - packaging, leakage, cost just to name a few. Unfortunately, there are not many good options if you don't have a welding machine, especially working in aluminum.
I am considering starting project which is too small for NPT fittings. I would like to use Aluminum brazing materials for attaching the propellant tubes directly to the injector heads. I bought the brazing material from Durafix some time ago for another fabrication test that didn't work out. McMaster-Carr also sells aluminum brazing alloys. Before I machine the injector, I thought I would test out the joining method with a hydro-test - if it doesn't work I'll have to modify my design.
This is a picture of the finished joint on the test piece. The test article is just a 3/8" aluminum tube brazed to a round rod which was left closed on the end. Both the rod and the tube are made of 6061T6.
The test procedure is simple. I fill the apparatus shown below with water and attach the free end to a standard grease gun. I fill the the grease gun with water as well and then pressurize the entire system. I got the grease gun at an Ace Hardware and its really nothing special. The box says its good to 6000 psi, but I've only ever had it up to 3500. It is very important when doing this kind of test to fill the entire apparatus with water - don't use air!
The end result was no leaks, so it looks as though brazing could work as a low cost method for joining aluminum tubing to manifolds.