Over the past two or three years I have probably repaired or renovated over 50 ZW transformers and have yet to come upon a situation where the clearance between the primary and secondary coil is an issue.
I totally agree with John in his above reply that you don’t want to operate the transformer with worn down or broken rollers. Having the roller arms alone against the secondary will give you the voltages, however in a short period of time the arm can scratch through the secondary coil insulation and cause a short circuit. If and when this happens about all you can do is strip the ZW of reusable parts and put them in your parts bin.
Replacing the rollers is quite easy to do and can be done in just several minutes. If someone is not familiar with how to do it, the first thing they do is remove the roller arm, grind off the head of the roller pin, remove the pin and remove the roller. Then they replace the roller and pin and figure out someway to peen over the bottom of the pin to secure it in place. Then they reinstall the roller arm. Very, very time consuming.
Here is how I do it. First take a pair of needle nose pliers and crush the roller. The roller is made of an easily crushable material. Then I take a pair of diagonal wire cutters and snip the pin in the middle and then it is easy to remove. Then install a new roller and pin and then take a pair of needle nose pliers and squeeze the bottom of the pin. The pin is hollow and it’s easy to squeeze and if you squeeze it flat it will never come out of the roller arm holder.
You want to get all of the crushed roller and the old pins out of the bottom of the transformer so I tip it over and work them to one edge then turn it upside down and shake everything out.
Good luck on bringing your ZW back to life and if you have any questions that I can answer I will be happy to. My email is in my profile.