Most of the time, it is the guarnizione sottocoppa, the group rubber gasket that has become stiff or worn. Sometimes, it is the filter basket that has been flattened or nicked from not using a proper battifondi, or knock box, and as such is damaged and will not form a good seal.
Last on the list is a worn handle, specifically the "ears" that you guide into the group to make the seal. When connected to the group, the handle should point to 5 or 4 O'clock. If it points to 3 or maxes out, this may be remedied by ordering paper spacers (shims) that are to be installed first, then the new gasket. Usually 2 shims do the trick.
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Most of the time, it is the guarnizione sottocoppa, the group rubber gasket that has become stiff or worn. Sometimes, it is the filter basket that has been flattened or nicked from not using a proper battifondi, or knock box, and as such is damaged and will not form a good seal.
Last on the list is a worn handle, specifically the "ears" that you guide into the group to make the seal. When connected to the group, the handle should point to 5 or 4 O'clock. If it points to 3 or maxes out, this may be remedied by ordering paper spacers (shims) that are to be installed first, then the new gasket. Usually 2 shims do the trick.