OK, we might be getting somewhere. @RustyRatRod was correct on shimming, using a washer or as it turn out screwing down the disc. The threads are 10-32. After cleaning the loctite off, the disc threads on easily. Did a quick test on a disassembled 4.5” Hg PV. Disc at original height, flush with stud, then screwed it all the way down, then back to flush. Used a thin wire that would show deflection. Crude yes, but clearly shows it takes more force to close the valve after the preload is added. The disc screwed all the way down does bunch up the spring and restricts or blocks the valve somewhat, but with some testing might only need to be screwed down 1/2 way or at a point where still unrestricted or closed up too much.What do you think??
Tester on the way to more precisely measure, hopefully.