Set-true Buck Kalamazoo 7-1/2 chuck help

[Froneck]

I purchased on ebay two 7-1/2" Buck Kalamazoo set-true chucks from a seller I purchased quite a bit from. Top quality and very accurate descriptions and always pleased at what I get! In addition to the chuck that didn't have back plates was a 7-1/2" L0 spindle set-true back plate. (back plate nor chuck listed as set-true but I knew from the photos) I received the items and again well pleased at what I got! Chucks seem to have very little use, no dents, dings or even scratches! Same with back plate. 3/8" mounting screws (6) were missing as well as the 1/2-20 set screws (4) to set-true. Ordered needed screws from McMaster. To be sure the back plate ran true with my LO spindle (Hendey Model 1 lathe) I mounted it and took a slight face cut (few thousands). Ran a tap to clean-out any dirt in the 1/2-20 holes (holes were clean no metal chips visible on the tap that easily screwed in) Screwed in the 2" long Dog point set screws that fit perfect, not loose! Mounted back plate after checking bolt center distance, back plate (threaded) and chuck (hole size 7/16") both 6-3/4". Assembled and mounted the chuck, while on the spindle I tighten the 3/8 socket cap screws. Threads are about 1/8 longer than needed, I will trim later. Set-true set screws were loose not contacting the inner hub. Before tightening I removed the jaws, all was clean and in excellent shape, inserted them being sure jaw number matched the number on the chuck body. Jaw fit great all was like new! I then tightened the mounting screws. Jaw were now difficult to move, movable but with much greater force than before tightening. Loosened bolts, inserted ground round rod and adjusted so it ran true. Tightened mounting bolts, same results! Loosened set-true screws, no change but screws turned easily when loosened. Thinking adjusting hub as too long, I removed the chuck from the back plate and checked length with depth mic. Hole was .015 deeper than plate projection! To be sure I skim cut a few thousandths off the projection. Remounted the chuck with 3 cap screws and tightened, same results! Again removed chuck, double checked hole depth, were as measured! Put set screws in the other chuck, had same bolt hole dimensions. Mounted the chuck, tighten the screws well, no change in jaw wrench required torque! All was perfect, adjusted jaws so needle on Starrett .0015 last word indicator hardly moved, at the most a line width.
Any ideas why the other chuck, the wrench torque needed to move the jaws increased quite a bit when mounting bolts were tightened? I checked the rear of the chuck body, it's flat, no dents dings or even scratches! Both chucks seem to be like new, no sign of wear! Back plate too! Chucks were far better than expected, I plan to put D1-6 back plate on the problem chuck but need to find reason for the torque change.

 

[DTB]

Will do and thank you very much.