David,
The problem you're having is due to the distortion of the tube when you remove it as well as when you reinstall it. If you were to make yourself a mandrel that fits snugly into that tube and use it during the removal and the later reinstall, most of the problem would be solved. There is one aspect though of the repair that you need to be careful with and that is the wire you solder to the coil itself. It doesn't need to be any heavier than 22 to 24 gauge. Plastic coated wire works, but the fabric covered wire currently available works better as you can solder it more easily to the coil and use a good grade of paper tape to insulate the work as opposed to the white tape you use. Why? You're adding to the diameter of that coil, and when you put the coil back into its housing, you're squashing it just a bit which distorts it and puts pressure ont he tube.
You can fashion new tubes as the material is available in aluminum and brass, although doing so is a pain. Over the years I've done hundreds of these couplers and there's never been a need to file the tubes, grind a point on the plunger or add some type of spring. These were designed to operate dry, no oil, WD40 or anything else.
Dennis
