I am following you activity with interest. I have a similar issue on the 10” 4J that came with my MK Enterprise lathe (it was near new in 1983, when I bought the machine, the chuck had been fitted and is serial numbered to the lathe - still the axis did not line up to the lathe bed). It didn’t really cause that much trouble for me, for the first 30 years of ownership I just used it and didn’t worry about it. The chuck is branded Mysore Kirloskar (MK), they probably did make it. Strictly speaking it is not a premium chuck, the body wobbles around.
I have been through the back plate setup described here on HM else where (a few years ago). It is more relevant to a 3J, but still some learnings for any chuck. I have cut the jaws on my 3J. I mostly followed the approach of Robin Brenz (3J will have hard jaws, I used a TBC insert, it worked out great). Robin goes through a presentation on how much the chuck face deflects - thus the jaws will come outward. Ideally you want the jaws to be straight when under load (thus they will be coned in smaller at the nose when not loaded).
I somewhat tuned the 4J when I tuned the back plate. The next step was to put my 14” 4J on and turned a piece of 2” bar (which was then true to the lathe), and then mounted the 10” 4J backwards onto that 2” bar, and skimmed the rear mounting surface of the 10” 4J. It addressed about half of the axis misalignment (got it down to about 0.006” per foot).
I took the approach of using cash to solve the problem. I recently purchased a direct mount D1-4 Bison 4J. It is very pretty. I have only just started playing with it. The first finding was that it seems to pull up funny on the D1 spindle mount - to get the face to run true I need to run a dial on the face as I snug the 3 cams. It also sticks pretty hard on the spindle mount (I think the chuck should stick, but not to the point of needing a smack with a dead blow hammer - just a fist bump and it ought to come free). My guess is that I am not getting good contact on the flat face of the spindle nose. Regardless I’ll play around a bit more before doing anything drastic.
Back to the issue of fixing the off axis grip on the old 4J. I still want to fix the old MK chuck. I intend to hold it in the 14” chuck, dial in the body and recut the back mounting surface, then using a jaw loading ring (looks sort of like a 4 leaf clover shaped center that slips over the jaw tips so the load is near the end of the jaws (as you pointed out). Then I’ll run a boring bar through, the jaws are not hardened. The jaw ID will be concave, probably not a big deal, but I have a dozen chucks across the fleet and all are concave, so I’ll keep it that way.
Please continue posting your progress, seem the off axis issues is not uncommon on a 4J.
