Originally Posted by
Duragrouch
Washers: Non-existent ones. I suggested, did not say for certain, that it might be ideal to adjust the radius at which the tool functioned (with different diameter washers in contact with the derailleur hanger), in case derailleur hangers bent at different places. I have not seen the tool. Others (perhaps you, I don't recall) here have said such washers, at different diameters, are not necessary. I accept their statement......
To nail it home.....
No washer is needed because the wheel's mounting faces serves that purpose.
Also consider the nature of the force applied. It's not straight across as would be with a press, but a twisting force applied as a torque, which helps move the hanger in an arc.
In any case, those familiar with bike repair know that this job is very common and easily done (in steel) with no complications.