I'm currently completing a full rebuild on a Gaggia America Lever 1954 ( 2 groups plus a hotel group ) for a customer.

I had one of the group heads in the vice and was tightening the nut to compress the spring when there was an almighty bang and suddenly no tension. The piston and rod had shot out with such force that they had hit the garage door 5 meters ( 16 feet ) away ! . Fortunately no damage to the piston but it was lucky I wasn't standing in front of it or I'm sure I would have been in hospital.
When I eventually found the nut ( Brass ), the thread had been stripped from it letting the rod/piston free.

Lesson - Don't stand in front of the piston when working on tensioning springs with brass nuts. Also, if reusing brass nuts check them carefully for wear before doing so.

I think in future I will be substituting stainless ones.

John