In this picture you can see I ground out the lip on the edge of plumbing pipe left here from the cutting and threading process. This way I can slide in the center rod. If you don't grind this edge away, then clearance the spacers on the center rod, the rod will rattle around loosely in the pipe ID. I clearances this on the end toward the air fitting, but not on the other end so the lip would stop the travel of the center rod so it wouldn't accidentally slide out during assembly or disassembly.

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