The heart of the design is the valve body. I chose to make these of threaded brass rod, 3/8-24, rather than thread the 3/8 section. I cut blanks 1.25” long. Leave enough extra for cleanup if your cuts are rough or out of square.
I have a collet lathe, and for making fittings nothing is better but, you can get great performance out of a 3 jaw chuck by using a threaded split bushing. This is a straight bushing threaded and then split with a fine saw down the side so that it tightens when clamped in the chuck or in a collet. I used one for making the valve stems. Holding in 3 jaw chuck without a bushing has the liability of damaging the threads due to the higher pressure of the jaws line contact.
So now that you have a valve body in the lathe, turn it down to .248” for a length of .875”. The under ¼” size is so that there will be room for the braze material in the manifold assembly.
Then turn the blank around and turn down the other end to .312” for .125”length. Now on the same end center drill with a ¼ inch diameter center drill until the diameter is just starting to raise an edge. This is to support the next operation. Grind a ¼” drill for a flat bottom and be sure to dull the cutting edges to keep it from hogging in. Use this special drill to make a square bottom recess to a depth of .125”.
Next use a #21 drill and drill .75” deep measured from the front surface. This is where the 10-32 threads go, but before threading drill the rest of the way through with a #36 drill.
Then thread 10-32 down about .625”. Now the valve seat is made by drilling to a depth of .813” from the front surface, with another modified drill bit. The drill is a #21 but the end is ground to make a flat bottom. It should just remove enough to create the flat surface or more importantly the square edge at the # 36 passageway we drilled earlier. You don’t want to go to deep and being a little shallower could be useful if you need to clean up a seat in the future. I put a sleeve over the drill so I could drill to the same depth reliably.
This completes the valve body, excluding deburring. The front recess should have smooth surfaces at the edges and the remaining 38-24 threads should be clean with the start point softened. The ¼” ends of the valve bodies are intended to get threaded or connected with compression fittings, and if you have it all figured out it may be easier to thread them in advance.