The La Pavoni plastic pistons used on some models utilise an extremely expensive polymer, polyphenylene sulphide. Its high price results in its not being used too often, even though it has many very good properties. It must have been at least as expensive to make pistons from this material as brass.

I suspect that La Pavoni used it because of the material's low conductivity and relatively low heat capacity.

Many have problems with these pistons unscrewing. There have also been cases of their failing.

There are thread locking compounds for plastics. Has anyone tried these? Polyphenylene sulphide has excellent thermal stability. I thought of trying to use a piece of it to make the base for a decorative boiler cap, but found it so expensive that I could almost purchase a new Europiccola for the price of the minimum order of rod with a large enough diameter to make a cap!

Has anyone tried any other techniques to prevent the pistons from unscrewing?

yes, I bought the brass piston to replace.

rpavlis wrote:I suspect that La Pavoni used it because of the material's low conductivity and relatively low heat capacity.

Yes - most of us agree with you that the change to 'plastic' was about reducing thermal mass as opposed to being a cost-saving measure. Your comment about the high cost of that material is very interesting and provides more credence to that assumption.

Thread lock for that material, if available, may be mechanically helpful but I'm not sure if people really want to lock the piston that securely onto the shaft, and there may be some food safety issues given any boiler water contact (although the shaft is on the other side of the piston from the actual shot in progress). What we all want is something that stays screwed in when it's supposed to be screwed in, and something that unscrews normally when we want to unscrew it.

With so many LP owners (at least the ones who care) now controlling the temperature via portafilter cooling and using the thermometer strips for guidance - or some alternate method - any thermal disadvantages of the brass piston have been rendered more or less irrelevant. So while the brass replacements are not cheap, they remain the best response I know for those of us experiencing the unscrewing problem.

I've seen recommendations to use a drop of PLA Turbo-Lock High Temperature Thread Lock, available from Orphan Espresso. My millennium machine has a plastic piston, and has never come unscrewed in the year that I've had it. I suspect it may be locked in with this stuff.

[ Edit addition - Looking at the OE site, I don't see a specific recommendation of this thread lock for the Pavoni plastic piston. I just sent orphanespresso a PM inviting them to this thread.]

Mine broke after 7 years so had it recently replaced with brass. Didn't dislodge as far as I knew but I had mine serviced every 2-3 years. Not sure i can tell much difference with the brass one.

rpavlis wrote:
Has anyone tried any other techniques to prevent the pistons from unscrewing?

Yes. Take a suitable thin-shaft and hardened screwdriver and insert the screwdriver through the piston rod where the shaft hole is drilled for a pin to secure the lever, and crank down on it. Not knowing cold characteristics of how exactly PPS shatters, perhaps there's safe torque ratings for lessening a chance of striped threads or dislodging that inner metal thread-guide ferrule. Presuming it's there -- I can't offhand recalling the ferrule insert when I last "cranked" on mine. Luck or some experience with irreparably over-torquing materials, nonetheless I did manage for now that the piston shaft is very much mated and secured. So much, if it loosens I'd suspect design deficiency. A teflon coating I first gave the threads, I didn't feel appreciably benefited the torque I was applying. Perhaps better, as with a Locktight brand sold in autostores, would be to use the teflon across an upper range of piston rod threads, farthest from the piston, partial to an added barrier to potentially unsuitable compositions leeching out from within questionable assembly aides. I don't believe it's coming loose, and for all I care it stay that way forever. Provided no reason it would loosen unless the piston were defective or due to cold eventually were to shatter, as pictures show, against and through the same (Millennium) PPS insert-walls guiding the piston over its shot strokes. (A very hard inner right-angle edge to the insert, aside. . .that, as the thinner, outer -V's to the piston rings expand during normal wear, manges the top ring unless the piston when reinserted, lacking a touch for care or ring-compressor band. Think I'll take that edge to the lip down with Dremel and fine sandpaper for a slight, shallow bevel.)