I never gave the "threads showing" point any credence. Put that wherever you want it on whatever carb you wish to select as master, (provided #2 has an adjustment feature).
While backing off the main idle knob, watch the slide travel among carbs, the first slide that fully seats to the bottom of the bore is designated as master (assuming #2 is also adjustable in the set, if not, #2 is the master, by design).
Lock down the "master's" carb adjuster, as you won't touch it again during any sync procedure.
For bench sync the task now is to mechanically align all the other slides to be in the same relative position inside their carb bores as the master carb.
I prefer a light source as the measuring gauge and then adjusting the other carbs to also seat at the bottom of their respective bores, closing off the light.
But, if you prefer the visceral feel of poking something into a hole, then select something in the 1/8 diameter range, and turn the idle knob to open the master's slide so the measuring tool just fits between slide and bore.
Now adjust each of the other carbs slide height adjusters, so that your test tool just fits into the slide openings of the other non-master carbs.
That's it. They are bench synced. And the carbs will deliver equally if in good condition, clean, etc. and if the cylinder draw is equal among the 4 cylinders sucking on them.
But, since perfection is not in the same realm as us mere mortals or machines, the 4 cylinders will NOT suck equally among them. So, you still have to vacuum balance the carbs (never touching the master carb adjuster) so the cylinders draw equal charges of mixture into them.
Also this may result in relatively low sync readings, like 16-18 cmHG rather than 20-22 cmHG.
This is what's known as a red herring in the vacuum syncronizing world.
The peak vacuum strength sourced by the engine is determined by mechanical factors of the engine itself. The slides only control how fast that pressure can be equalized from the atmospheric source (air intake). The pressure test device (vacuum gauge is not placed at the engine's intake valve but along the inlet duct path near the carb. On and individual basis, you can artificially raise or lower the equalization rate by adjusting an individual carb slide height. (This is like opening your mouth wider to suck in more air easily, close your mouth to a smaller opening and your cheeks will hollow (or bulge), causing reduced (or raised) pressure inside your mouth, closer to what your lungs are producing, pressure wise).
So, for vacuum sync purposes, the vacuum reading is-what-it-is for what the engine can produce, and that they are equal among the four is the goal. If you want the average cumulative reading among the cylinders to change, you will need to change bore, stroke, cam timing/valve timing. Or, simply block the single air intake providing equalization pressure to the engine's 4 cylinders. So, don't expect "book" vacuum pressures, if you have a modified the induction path away from stock.
Cheers,
