A little local coffee birdie who is much handier than I am swears by mcmaster carr and they have several o rings that can stand up to boiling water.

http://www.mcmaster.com/#metric-o-rings/=rbn7bx

I went to check my seals today after I felt a loss of pressure and sure enough the bottom seal was missing a chunk, similar to the OP's. Not sure how this happened. I installed the seals a couple of weeks ago. Inside of the cylinder seems smooth.

SimonPatrice wrote:They seem to be safe for use with water but would they be suitable for use with boiling water?

Grainger (and I would assume McMaster Carr) has the 4mm x 2mm o-rings in Viton, which is food-safe and takes boiling temperatures just fine.

The u-cup seals are a little more difficult to source. Francesco has the dimensions on his website (28 or 27.5mm ID x 38mm OD x 6.5mm height) and I haven't been able to find those dimensions anywhere yet in a stock seal in any material, much less Viton or other food-safe high-temp material.

Thanks Italyhound and GVDub! I'll have to see how easy (or hard) it is to order from these companies once I go through my last o-ring. I still have to figure out how to put the piston back in the sleeve without damaging the top seal though.

I've had both issues with my Caravel.
April 2013: OE lower seal failed.

Starting in September 2013: o-ring repeatedly shreds (OE, Francesco, Trukaffe, I've tried them all...).

This is a disturbing thread. Here's another similar one:

Caravel Group Gasket Blowout! Why?

I'm afraid to look in mine, but so far it's performing well. Also I don't have the time because I'm moving.

How finely are you grinding and how forceful are your pulls? I'm wondering if stressing the machine to its limits gets the piston gasket distorted and bulging out where it doesn't belong and gets damaged?

Some photos inside the cylinders of your machines could help isolate this, and comparing piston versions too, if you're willing.

It is a disturbing thread and a confounding one. I am the OP of the Group Gasket Blowout thread the Gary referenced. I never did solve why that happened. It has happened a few times since then, but always on restrictive pulls. I have learned to ease off when the pull doesn't fully cooperate. However, I used to be able to really lean into a pull when I wanted to.

So I second Gary's question: has this happened during a very tight pull? George, you theorized that it was the result of damage over a period of time. I am curious if that is true and the U cup weakens over a period of time or if it all goes down in one event.

One other clue is that my blowout always happens in about the same spot of the pull. Going by memory, it is towards the bottom of the pull, like maybe in the last quarter of the length of the pull. I guess that makes sense if the pressure is building until something has to give.

I have no answers, only more questions.

Pete

I agree this is a puzzling one. It seems we are (maybe) looking on three separate issues here.

Firstly, the group seal. I forgot to mention that in the incident described above it failed in two places, on opposite sides of the piston, so I figured it had nothing to do with the water inlets. I've had no problems with those since installing Francesco's u-cups. I have a spare OE set, so it would be interesting to try and see if there might be an issue between those particular models and my machine (they seems to present no problem to most people).

Secondly, the small o-ring. I've looked closely for asperities that might grind down on the material during heavyish pulls, but It's all nice and smooth down there. Could it be the compression that shreds the material like that?

Lastly, as for the group gasket, I recently changed it from the OE replacement I put in there roughly two years ago. I noticed it was lightly "twisted", or bulging in one place, but I never had any of the issues described in the other thread. It did end up letting out some droplets of water during high resistance shots, though.

I'd say I tend to grind moderately fine, seldom having to lean very heavily on the lever. If I feel to much resistance, as in choked, I rather wait it out, or abandon the shot. I do one brief pull until I get a drop or two, then fill up again allowing 5-10 sec pre infusion. I never do proper double pulls.
Now, maybe it's all a question of too heavy use anyway? Maybe there is too much resistance in my shots. I can't help asking myself wether we are trying to make these machines do things they were not designed for, i.e. extracting thick concentrated shots like modern pump machines? What kind of espresso did Italians expect from a domestic machine in the fifties? I just wonder...

My Type 2 Arrarex and my Zerowatt Caravel Ursula (both with o-rings in the cylinder walls, no seals on the pistor) have never had this problem, and the little piston end o-ring has gong occasionally, but not unduly so, on those, despite my pulling fairly tight shots on those. When I get replacement seals for the type 1 Arrarex piston, I'll do some experiments on loosening up the grind a bit.

Ideally, a fiberoptic camera in the boiler to record the seals passing the water inlets would be informative (plus probably cool to watch). Hmmmmm. I see that one can get such a thing with VGA resolution and a USB connection for under $40. I foresee a YouTube video in the future.

I now see more of the reason in this thread with regard to buying OE seals. They are now sold as a set for $22.

I picked up a later version with the u cup seal and the o ring on the piston. No small one up top.

Would be great if they can be sourced more easily elsewhere. I like supporting OE - but if these are going to be routinely replaced, it will get costly really fast.

I do suspect we are using these machines differently than the old days and the intense pressure may be a culprit. I'm sure the kid and the nun in the brochure weren't sweating like I do!