I have a piece of 6061 AL tube, 2"OD, 0.125" wall, 1.75" ID, 1.25"
long. Lets call it Part A. I want to machine a second piece from
1.875" OD solid round 6061AL rod that will screw into the first piece.
Call that second piece Part B. The rod, Part B will have a 1" hole
through it and will be 1" long, and will screw into the tube, Part A
to a distance of 0.25".

Two questions.

First, since Part B will be around 1.75" diameter but only screw into
Part A for 1/4", does that suggest any specific thread pitch? I
arbitratily chose a 1.75-20, thinking that a fine thread will hold it
more snugly (less movement side to side) because of the higher thread
count. I want the joint to be as secure as possible with as little
slop and play as possible.

Second, since Part A is already 1.75" ID, the threads will be cut into
that. This means the OD of my Part B will need to be 1.75" plus
whatever the thread depth of Part A is, plus/minus a tolerance value.
I haven't done threading before. Is there a standard thread depth for
a given pitch? What is it? And how much extra (if any) should I
allow for to be sure that when the parts are made, machining
tolerances don't cause them not to mate correctly?

In other words, given Part A is 1.75"ID, what should I machine the
diameter of Part B to, and what thread depth should I machine to get
them to mate cleanly without a ton of slop and side-to-side play?

Thanks!
LP