The heat required to undertake the soldering process will cause distortion. You could consider removing the existing thread and fitting a sleeve to take the new thread, or possibly with the thread already cut/produced. The main body could be placed in a domestic freezer to cause it to shrink and the sleeve heated in a domestic oven to cause it to expand. The two part are then assembled producing a shrink fit. I appreciate that this still uses heat but the heating is far more uniform than with soldering and the temperature required is far less.

Assuming the top is the English one, and the other the Russian, are the focal planes the same? The threads seem to be in different positions along the tube as well as being of different pitch, or is there something I am missing?

As it stands, can't you simply machine off the Russian thread and cut a new English thread in the correct place for the English microscope, which seems to correspond to some solid material on the Russian tube.

There is some useful information at

Understanding Microscopes and Objectives | Edmund Optics

It implies that the distance from the object to the real image plane of the microscope is very standardised. May not be true of prewar Russian units, but looking at your two samples I would suggest that they are designed (optically) to be very similar. The only difference being the mechanical design of the focus mechanism.

I suspect that you will end up repositioning the thread in order to get the objective to focus.

Edited By Peter Cook 6 on 20/02/2021 14:22:44

On Howard’s theme, are you sure the thread is not imperial 36tpi?

Edited By not done it yet on 20/02/2021 18:33:15