Thought I'd share what happened to me and what I discovered. The solution might be obvious to some but remember... I've only had my Cremina a real short time.

As I was making my morning, afternoon, or evening espressos, more specifically... as I would be on the down stroke and coming close to the bottom of the stroke, I would feel a "click" and would find the dispersion screen laying on top of the puck when I pulled the PF off. The adjusting nuts on the piston rod were "bottomed out" so what could it be? As I typically do with a puzzle, I was pondering it one night while watching TV with my wife. Then, voila! If there were threads on the top of the piston rods, there were probably threads on the bottom of the rod (where it screwed into the piston). I rushed to the machine and with one hand 'jiggled' the lever up and down while (with the PF & dispersion screen removed) making clockwise pressure with my thumb & fingers against the bottom of the piston the piston did in fact start moving up as it screwed onto the rod. I noticed a marked change in the location of the piston in relation to the group cylinder. Eventually, I will remove and disassemble the group and probably use some type of removable "loctite". Any suggestions as to a 'food grade' product?

I don't have any experience of the Cremina to offer any insights, but others more knowledgeable than I may be helped if you mention what year/model of Cremina you have. There have been variations in piston design over the years.

Cheers

Mike

Thanks Mike, the only insight that I was really requesting is not model/make/year related. I'm wondering if there is a 'loctite' product that is food safe. But, I have a '76 Cremina 67.
Of course it is possible that the last person to service my Cremina, before I got it, did not sufficiently tighten the piston body onto the rod.

Food grade is not so much an issue with an anaerobic threadlock as high temp in this case. You are going to be putting one small drop in the last thread of the rod to piston joint and tightening it all up so there is not a lot of possibility of water contact with the threadlock. You just want a high temp substance so it won't loosen at 220F or so. It is completely inert after a 24 hour cure. We use a threadlock that is rated to be removable at 500F, so it is pretty stable. the food grade certified stuff costs an arm and a leg and is used where the applicator puts it on with a trowel and not with a toothpick. Thread retaining compound is a different story since it never completely sets and cures. Avoid the basic blue stuff as it is not high temp rated and will soften. Teflon tape is another solution. Good that you figured it out so quickly....nothing like some good TV time with the wife to get these things sorted out.

If you want to pursue the food-safe/high-temp combination:

http://www.engineeringtalk.com/news/hec/hec115.html

Before I started using non-food grade to the group internals, I would ensure that you were doing regular maintenance and cleaning on the machine. Both the Olympia and the Pavonis have a habit of unscrewing the piston over time. On the Olympia it will kick the screen off and on the Pavonis the piston will start to block the incoming water inlet so both give good signals when this is happening. If your goal is better coffee, then it is good to get in a routine of cleaning the grouphead about every 6 months, clean the oils and deposits off the dispersion screen, lube the piston rod...with food grade lubricant, and ensure everything is tight. With this approach you have the machine working properly and are not introducing any additional products that may lead to off flavors in the coffee or trace elements in you.

Thanks to all who have responded!
In terms of regular maintenance... Steve, you said, "...it is good to get in a routine of cleaning the grouphead about every 6 months... I do, even before Ms. Cremina started doing in by herself, regularly pop the dispersion screen off, scrub the bottom of the piston, and slightly lube the cylinder walls and all of the piston rod that I can get to. Steve/Doug, I'm curious as to how often one should actually pull the piston assembly out for a 'total' cleaning?
Doug, I like the idea of Teflon tape. Non-chemical, easily cleaned/removed and readily available.

Thanks,