chat desired

where do I get an accurate chuck backplate with 1 3/8 x10 thread.

Start a new thread called "WTB chuck with 1 3/8-10 thread"

Welcome to PM Frank .. hope you find what you are looking for.

Buck

you gotta make that one yourself to guarantee it will fit your spindle.

depending on your particular machine it may be possible to swap in a later 1.5"x8 spindle too.

Here is an old thread on the subject:
Looks like a rare part to find.
WTB: Old 1-3/8" -10 faceplate and/or backplate(s)

You might fond a back plate that your thread might be put into, then add chuck. Din't know if this would work.

NEW ATLAS CRAFTSMAN 6 INCH LATHE 1-8 3 HOLE BACKING PLATE FOR 4 INCH CHUCKS | eBay

Delta Rockwell Jet 6" Steel Face Plate 1-8 tpi Threaded Mount Wood Working Lathe 689829422103 | eBay

If you can’t find anything let me know, I’ve got some leftover Durabar that I can thread. I’ll leave everything else rough so you can finish it to the proper diameter and register to fit your chuck. What size chuck do you need it for?

3 wire check your spindle thread, then make a very good quality male thread that matches your spindle nose, good if it includes any features so it mimics your lathe spindle nose..
This is used to gauge your made up face plate or what ever.
You need this because your lathe is going to be busy making the new part.

May wish to make you new thing with the back face facing out toward the tail, so you can try and fit the gauge to the make-up side. Vert often a register diameter is important to just fit with a few tenths to perhaps a thousandths or two clearance depending on the size.

Here is some reference of a common spindle nose
Determining Your Lathe Spindle Thread to find the correct NOVA insert adapter - NOVA

*No, this is not about Nova stuff just about your lathe.
and....
TAKE THESE MEASUREMENTS:
1. Outside diameter of thread:
2. Pitch of thread:
3. Length of thread:
4. Register diameter:
5. Register length: