With the very close fit I have on the spindle spigot it's a bit tricky screwing it on and getting it to "slap" up to the mating face, but nevertheless with a bit of jiggling it goes on reasonably easily. You can see that I have machined the ring face and also the ring diameter. The diameter is about 0.3mm(0.012 inch)less than the chuck recess. This nice long ring had to be fitted so that my "set-true" screws will have something to bear on. During this op I also faced the mounting face and a very small cut off the outside diameter of the body. The length of the spigot has to be a tad less than the depth of the back of the chuck, I aimed for about 0.2mm(.008 inch)
Drilling the chuck as close as possible to the inside cover plate. I need to drill as far away from the mounting face as possible so I can fit M6x1 grub screws. I didn't want to go to a smaller dia grub screw as the allen key then starts to get a bit on the dinky small size. There will be 3 equispaced screws around the periphery. Four adjusting screws would probably make for an easier adjustment but it looked a bit iffy to get 4 without clashing with one of the other holes etc. so I opted for the easier route and went for 3.
You can see the threads just cutting into the cover plate.
I removed the plate and with a 1/4 inch dia mounted point just ground a bit of material away.
I hardly ever use a three jaw so it's all in good nick in there. I put a drop of sticky gearbox oil here and there where there are bearing surfaces and smeared a bit of grease on the teeth.
15mm long grub screws with the serated cups ground from the end fitted in nicely with the other end just a smidgen under the outer periphery.
All fitted up now and using the 3 grub screws I have trued the chuck to a piece of ground tool steel. Now of course, depending on the accuracy of the chuck it may or may not be true on other diameters. With this piece of tool steel still in place I removed the chuck assembly from the spindle and then refitted it, I did this a few times and each time the chuck was still true, so now I know that the spindle to backing plate fit is good for repeatability. When adjusting the "set true" grub screws one has to (of course) slightly looosen the 3 chuck to backing plate cap screws and I found that gentle adjustment on the grub screws was better than overtightening them, after adjustment tighten the capscrews, recheck the job and if still true then all is good. During all the setting up and checking I used one particular chuck key socket for final tightening, that is now the "master" socket for this diameter job and possibly for other diameters.
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