What is an OPV and why does it matter for espresso?

An OPV (Over Pressure Valve) limits the maximum pressure in the brewing circuit — it should be set to 9 bar for espresso. Many entry-level machines (especially older Gaggia Classic models) ship with the OPV set too high (12–15 bar), which over-extracts and produces harsh espresso. Adjusting the OPV to 9 bar (or 8.5 bar per the original La Pavoni spec for some enthusiasts) is one of the first modifications home baristas make to improve shot quality.