OPV, 9 to 12 bar
The over-pressure valve
Bezzera's manual says: if the machine is malfunctioning, do not try to repair it yourself; call technical assistance. Every BZ10 carries an anti-tamper seal that cannot survive opening the case, and Bezzera grants no warranty on a machine whose seal is removed or damaged (12 months from the invoice). Treat this as technician work.
The manual: regulated to 12 bar. The 2020 catalogue: assembly 5965212.01AL, adjustable from 9 to 12 bar, with a 16 mm hex inlet fitting and a 12 mm hex gasket holder. Bezzera gives no adjustment method.
Owner practice, not Bezzera's: one owner says you change it by turning the part with the waste-water hose; another turned an older valve down to about 10 bar. A whistle at full pressure often follows a valve turned down too far (home-barista.com).