Chuck backplate setup

I am at the stage where I am looking for more precision then just makin quick pins for ,y backhoes . I never really did all the machining to my lathe backplates when they got here that they really need for a good setup. one question I have is
how many turns am I looking for for when threading a chuck onto the spindle ? I have 2 imports , a 4" & a 6" both 3 jaws. the 6" backplate runs true +- .001 - .003 (dependin how hard I thread the plate onto the spindle) per rev .the unthreaded part of the backplate is just shy of .5" , that dosen't giver a lot of thread engagement .how much of that unthreaded area should I face off ?I was planing on making a 1.5 x .250 or .375 ring to place between the backplate & the mating surface on the spindle & installing the backplate backwards & facing the mating area to get more thread engagement but not sure how far to go . I would think I would want as much as possible
but not sure . I was also planning on resurfacing the back of the backplate to get rid of the wobble , the chuck side is fine . the 4" isnt as bad as the 6 " but I want to get them both tuned up as I am trying to get better finish's on steel.
I can get a mirror finish on aluminum but get chatter & the tear's ( for lack of better name ) on the surface of any steel I have tried . all I have laying rite now is some mystery metal & CRS & HRS
thanks
animal

good plan...it can only improve things....but i would make your spacer 1/2" thick...youll have approx 3 threads left for engagement....which is plenty for what youre doing.( thats all you have now anyway)

since the nose on a 9" is only about 7/8" total length, i would do exactly as you say and cut that spindle side of the plate down to about 1" TOTAL thickness.

at that point there probably wont be any counterbore left at all...make a new one about 5/16 deep.

then flip it around and true up the chuck side....but spin it on and off a few times before you do to "seat" it in so it repeats...youll end up with a near full thread engagement and also reduce the unecessary overhang at the same time.

good deal , thanks much . I know what I'll be doin tomorrow nite.
animal

Might want to check this thread out:

SB 10K Machined back plate for 4-jaw 6" chuck - How I did it with some GLOAT!