From an non mechanically-minded perspective, I'd say if the difference between a 1.5mm and a 1.75mm thread is the difference between the thread working or failing, then you're too close to the edge. I'd go with the tooling that you already have, and if you strip the thread use that newly purchased lathe that you just know that you need to turn a tapped steel insert into the newly reamed hole in the ali pressure plate. Or re-make the whole plate from steel.

Preload?, if ease of turning is an issue make a bigger hand wheel - but I doubt that it is. Finer control?, not really much of a difference to worry about.