After I rebuilt the last lever machine (Rancilio 2-group) for a friend, I kept it for a few months (my price for doing the work at cost - with my La Cimbali M32 standing in for him).
I liked the mechanics and feel of the machine and I liked the quality of the coffee it made. One thing I could never figure is how to adjust the group pressure? When I checked it with a manometer it was a constant 8.4 bar static pressure (~7.5 bar dynamic pressure).
There didn't seem to be a way to conveniently, or even safely, change the preload of the spring. Other than changing the springs, how are these groups tuned for brew pressure?