I think I may have tamed my 1984 La Pavoni Professional and the classic group overheating issues. I modified the group from a steam heated group to water heated group for temperature stability. While it's not exactly the same as 1st generation and Millenium models it is similar and gives similar heat stability with a little practice.
NOTICE: HERE IS THE FINAL MOD POSTING. MUCH IMPROVED OVER THE ORIGINAL DESIGN. GREAT TEMP CONTROL.
Heat control of 1984 La Pavoni Professional



READ THE REST OF THE THREAD IF YOU ARE INTERESTED THOUGH.
These things heat the group using group metal to boiler metal conduction, but even more so with the steam heating of the group. I've used the classic 2mm Teflon heat break for the group to boiler connection, but that didn't do enough. I've tried using wine corks around the dipper tube to block air from the group, but they get wet, swell and end up popping out when I lift the lever. I've used corks with extra holes and grooves cut, but eventually it swells and pops out again.

Now that I have to use this machine as my current daily driver, pulling 5-8 back to back shots every morning, I wanted a solution to the overheating. I was dipping the group into a cup of cold water, pulling the cool water into the group while raising the lever, doing the dance to keep the group heating under control.

Finally, I decided to give the cork another shot, but this time with some kind of plastic instead. Luckily a bottle cap had a ridge inside that perfectly fit the group opening to the boiler. I cut the cap down to leave that ridge and a flange around it fitting nicely to the back of the group opening. Drilling a hole in the center of the cap, enlarging the hole with a screwdriver until I had a tight fit with the dipper tube. Then I drilled 2 tiny holes on opposite ends of the cap to allow for some airflow and condensed steam drainage. Fit the dipper tube through the opening, position the tiny holes vertically, screw in the dipper tube to the group.
Put it back together with the Teflon heat break in place and started the heatup. It took a bit longer to heatup, required a few more pumps of the group to get up to full heat, but it got there. The second day of use it started boiling shots out of the PF, and an inspection of my "fix" showed it had popped loose.

Next mod was to wrap some uncoated copper wire around the dipper tube, firmly against the bottle cap. This seems to keep the cap in place for over 1 week without issue. Once I get the group up to heat, I'm able to pull up to 8 shots without any need to cool the group. After a shot is pulled and decompressed I unlock the PF, give it a brief flash of steam & water to flush the group screen, reload the PF, lift lever until steam comes out of the group, lock in the PF and pull the shot. Repeat, repeat, repeat, but not one shot boils out of the PF!

The narrow part of the group just above the flare tops out at around 103c, usually around 101.5-103c. The shots themselves come out quite a bit cooler per the infrared thermometer that I aim at the PF with about 93c being my top range there. I might want to open up the holes a little to have more heat, but at this point I'm also very happy and don't see too much need for it. I'll probably make another cap adapter so I can swap between the 2 and see if my shots come out warmer and improved.

Here are the pics:
Teflon heat break, trimmed bottle cap, copper wire keeper.


View of the bottle cap inside and 2 vertical holes.


Bottle cap removed from dipper tube.

Just quickly eyeballing your photos, have you compensated in length of group screws for the extra width of the heat break gasket? Not doing so poses the danger of the group getting loose while the machine's under pressure.

The group bolts are long enough for this system per measurement some time ago, thanks for asking. Nobody wants a group breaking loose and explosion of steam!

That top photo has the bolts backed out quite a bit.

Oh good. That was scary. This is a good mod experiment, BTW.

I am wondering about the material you are using for the steam shield. Is that plastic sufficiently robust or are you better using something like gasket material backed by a wide brass washer?

Right now it's just a white plastic cap from a water bottle. Actually, tea bottle in this case. The plastic is PolyPropelene (PP) with a melting point of around 160-170c (320-340f). Seems to be stable and solid enough to hold its shape when heated to 117c/242f. We'll see how it holds up long term, but a solid backer like a washer might be a good consideration.

Today's shots will be pulled without the teflon heat break in place. I'll see how that effects the temps and shots.

The Pavoni is really nice when you don't have to keep dunking the group in water to cool it between shots!

The REAL problem with overheating second generation La Pavoni machines is the condensation of steam above the piston. There was a thread some time ago about the possibility of attaching a tube to the small hole above the water inlet and bending the tube to run parallel to the intake tube so that it emerges beneath the surface of the boiler water. This would make give second generation La Pavonis the same group heating systems as Creminas have. There is not much space there and the hole would need to be enlarged to receive a threaded tube. It takes a very long time for a second generation la Pavoni to warm up to usable temperature when the space above the group is filled with air. However, if you do not bleed the air temperature control is so erratic as to be hopeless!

Another approach would be to take off the inlet tube and bore out the space where the plastic piece is being placed to make it smooth. Then take a piece of brass stock turn it to the correct diameter to fit this bore, and put a groove on it for an O ring. Drill a hole in this round piece and take a die and cut threads to match the original inlet threads. Thread the inlet tube far enough so that it not only screws into the group but into the brass disk just described. You could drill and tap a second hole in this brass piece and attach a tube to it that runs to below the surface of the boiler water. Water would run into the space above the piston, but steam would no longer be able to get there with it filled with water. Each shot would bring in a fresh shot of hot water, but only during the shot.

This solution of Robert's looks very good. I'm glad both of you are trying to find a better way to stabilize that group than the toggling on/off I've done when I had a 2nd gen machine.

rpavlis wrote:Another approach would be to take off the inlet tube and bore out the space where the plastic piece is being placed to make it smooth. Then take a piece of brass stock turn it to the correct diameter to fit this bore, and put a groove on it for an O ring. Drill a hole in this round piece and take a die and cut threads to match the original inlet threads. Thread the inlet tube far enough so that it not only screws into the group but into the brass disk just described. You could drill and tap a second hole in this brass piece and attach a tube to it that runs to below the surface of the boiler water. Water would run into the space above the piston, but steam would no longer be able to get there with it filled with water. Each shot would bring in a fresh shot of hot water, but only during the shot.

I did something like that once, although not quite the same, and blew the cork out with the top of group water pressure when lifting the lever. There was quite a bit of resistance in moving the lever, but it's worth trying again as it could also be a good fix.

If you size the above piston tube large enough it will not have excessive resistance and still only allow group filling above piston when the lever is cycled. Length below surface should more or less match present tube. The tube diameter is only an issue under dynamic conditions.

Today's shots were done without the Teflon heat break, and the group temp showed 103-105.7c. Shots were a little improved, although the "funnel" on the bottom of the naked PF shot was larger than I like. These mods make me really glad I brought the La Pavoni to Japan, as it's shaping up to be a reliable machine shot after shot.