Ed - be just a LITTLE bit cautious. The "mains" connection on the great majority (if not all) home machines is, in fact, 3/8". The Italians term it a 3/8" Gas thread and this typically means 3/8" BSPP threading. There is no problem with the 3/8" size and no problem with the BSPP threads.
The problem occurs in the transition from this 3/8" size to whatever is attached to the pump. In the case of the Vetrano, the 3/8" transitions to a M5 x 3 tubing - that's 5 mm OD and 3 mm ID. This works just fine as long as one is supplying the Vetrano a positive pressure - say 2.5 bar or around 35 psi. When one tries to operate the rotary pump in that machine simply by taking suction from a nearby stationary reservoir, the pump will cavitate, i.e. this DID NOT work:

My solution to the descaling adventure DID NOT WORK either because of the check valve incorporated on our machines at the entrance to the hx. My little pump could not overcome that valve whereas the machine's pump, whether it be vibe or rotary can easily meet the pressure reqts to pass through that valve.
However, I have not given up just yet - but keep in mind that the idea is to have a complete "kit" that ANYONE could EASILY use - not something requiring the Snap-On Tool truck and a regiment of espresso machine technicians.
