Thanks Jim!
Mike, it pulls drinks just fine so doesn't really need a new piston. But you can look up my other thread on Londinium seal replacement a few months ago. It has relevant information and a lot of users who have had the machine a lot longer than I were chiming in on niggling problems some folks were having. I did have a couple instances of seal slip, just a touch, and then it went away and has not come back; not too worried about it, but its always at the back of my mind. Also, the catch point when I pull the lever is about normal, far as I can tell, with other Londinium I owners.... roughly 10:30 ish. But that doesn't leave very much headroom; I have zero problems pulling a tasty single; but if I want a double, I usually just turn on the Strega instead because it has so much more volume. By the time the Londinium's lever goes to the top, the amount of liquid in the cup is a bit on the sparse side for a double, and I'm never sure if the pressure gradient favored a good pull or not. So, Paul Pratt was kind enough to show some pics of a prototype piston he had made, and its not a difficult thing to do to make a piston out of brass; and I actually have a hunk of brass lying around here that I was going to use to make a Pavoni cylinder, but that project fell through because there isn't enough interest, so its going to go into a piston instead
If I didn't do ridiculously time wasting, semi useless projects like this, what else would I find to spend the time on??
notes on new piston; the lands (grooves for the seals) will be tighter, and not quite as deep as original, to hold the seals better and hopefully provide a touch more outward pressure. First ring will have holes into the chamber on the perimeter to puff out the seal under pressure. That's the easy part. The new piston will have to be 2 part, to afford a little movement so the shaft can rock a touch. Probably the shaft is metric, meaning I have to order a metric tap to duplicate
I already made one mistake, I didn't measure the height of the piston when its mounted tight on the shaft.... need this measurement so the new piston is same height relative to the water inlets and chamber.

chappcc wrote:While the sping is under tension it creates so much force on the threads, at either end, that breaking the threads loose is next to impossible. At least is my case, that is what I experienced. I did try using the group head as the jig but I could not get the piston shaft to loosen. Also, I was afraid I might do damage to the group head liner, when the spring released its energy after being loosend.

Removing the piston with a short pin wrench is really difficult. A long one will give you the torque to unscrew and break loose the threads. I take your point on potentially damaging the inner sleeve but you could easily remove that beforehand, making it slightly easier to unwind the piston. It's just much safer than building jigs and compressing springs.

Once you remove the rod you will note how deep the thread runs inside the piston. If you then reconnect all the parts and use the group head as the jig, compressing the spring back to working tension is easy and safe.

Yes. IF you still have a piston solidly mounted to the shaft the design playing around in my head has a small football shaped finger screwed onto where the piston goes... and a chamber hollowed out in the piston to accommodate the football, then a closing plate to hold it together. But such an assembly would not need any radial alignment and would spin easily, so could not be disassembled in traditional way. Spring compressor would be mandatory. hmmm. maybe the football should be squashed flat so that you CAN unmount it by twisting. Dunno. the CAD/CAM program in my head works in spurts. What d'ya expect for free?

A wobble piston is likely to be locked fairly tightly by spring pressure.

piccolo espresso wrote:Removing the piston with a short pin wrench is really difficult. A long one will give you the torque to unscrew and break loose the threads. I take your point on potentially damaging the inner sleeve but you could easily remove that beforehand, making it slightly easier to unwind the piston. It's just much safer than building jigs and compressing springs.

Once you remove the rod you will note how deep the thread runs inside the piston. If you then reconnect all the parts and use the group head as the jig, compressing the spring back to working tension is easy and safe.

My piston did not disengage from the shaft. The shaft unscrewed from the lever end. I think there is a lot of loctite holding the piston tight to the shaft. I reassembled by screwing the shaft back into the lever end. Without compressing the springs, there is not enough expose thread to start engaging the shaft with the lever. Thus my approach to making a jig and compressing the springs and keeping them compressed for re-assembly.

Rich! was hoping you'd chime in. Always appreciate your views
Agree, the spring puts pretty good pressure on the piston... but it only has to rock slightly; rather, the piston stays straight in the cylinder, while the shaft rocks slightly as its mounted in an eccentric up top near the lever base. I don't know the forces involved, but the other machines that Paul Pratt mentioned have this feature, so I guess it must work well enough. I'm too lazy to measure the spring perch force and do the calculations. Will just try and see if there's any difference.

I think it is one of those things that looks good on paper. I do not have any hands on with that machine and group but I understand it is the same basic group used by Bosco and they do some proprietary mods to it. I would think that linkage is such that there is no/minimal lateral deflection on the shaft. if the top and bottom spring perches are not completely parallel the piston will be forced to tilt or be off center.

All of that being said the Gaggia has a rack and pinion lift and the piston is loose pinned to the rack but the lower spring perch is below the pin sufficiently that the pin joint is effectively solid and it does not allow anything but slight hinge motion.

Finally got it all the way off... can't believe how long the threaded portion is!

Placed the piston next to the threaded part, with the rear shoulder about at the end of the threads.... the piston is internally threaded down to about the partition between 1st and 2nd seals!

Threads are: 12mm x 1.5

Is that a 12x1.5 stud threaded into the main shaft?

I'm not at home, will check when I get there but it looked like a one piece solid shaft cut down to 12mm for the threaded part. The small striations barely visible on the shoudler is coffee gunk from the piston, I think; it butts up against the shoulder where the shaft gets big.