Thanks for the excellent reference to the Eriks technical document.

The answer to my question is found within:

"Spiral failure sometimes occurs on reciprocating O-rings.

The conditions which cause this type of failure are those which cause segments of the O- ring to slide and other segments to roll simultaneously. The twisted seal is forced by the pressure into the sharp corner at the clearance gap. Rapid stress-aging can cause a rupture of the O-ring to start adjacent to the clearance gap. Motion of the O-ring causes the rupture to penetrate about half way through the cross section. Thus, when the O-ring is removed from the gland, it returns to its original shape and the rupture appears as a tight spiral around the cross section. One of the primary causes of spiral failure is by reciprocating speeds of less than one foot (0,3 m) per minute and on low or balanced pressure compo- nents. At this low speed, the sliding, or running, seal friction is very high relative to the break-out friction.

Therefore O-ring seals are not recommended for speeds less than 1 foot (0,3 m) per minute when the pressure difference is less than 400 psi (27,5 bar)."

Of course, it's possible that o-rings will work just fine, but perhaps may fail earlier.

I have an abandoned two group Carimali lever project in my shed. It uses O rings. So it has been done in a commercial group before.

Interesting. Perhaps Carimali is to espresso machines what the Lada is to cars? I've seem plenty of bad engineering in commercial products . Kidding of course, I know nothing about Carimalis other than that it pains me to know that one is languishing in your shed.

Another plus for o-rings is that they are much easier to change than the W and V seals. Installing the latter is like trying to put a car tire on a rim made of butter using only a fork.

bidoowee wrote:Thanks for the excellent reference to the Eriks technical document.

....

Of course, it's possible that o-rings will work just fine, but perhaps may fail earlier.

I thought about this for a bit, especially taking into account my La Peppina which has served me quite well for a few thousand shots on a single o ring. It then occured to me that in an industrial setting, a few thousand cycles is nothing. At least, in my industry (packaging) a million cycles is usually a few days worth.

Also, rubber hardness, lubrication, and surface finish all play a part
https://www.marcorubber.com/o-ring-troubleshooting.htm

Thomas, I need my fix! Please give us a workshop update

Bluecold wrote:It then occured to me that in an industrial setting, a few thousand cycles is nothing.

I've looked into the piston design in some detail now but I'm undecided as to how to proceed. I suspect you are correct in assuming that a plain o-ring will work just fine for the thousand-cycle per year range of home use. After all, the V & W seals on my home machine have been replaced maybe twice in twenty years. Silicon o-rings would likely be more prone to spiral failure as the original piston is not designed with the softer material in mind. I had decided to try out a drop-in replacement piston design using off the shelf silicon quad rings but unfortunately, the 50mm bore of the classic Aurora group does not play well with AS568 (aka inch sized) o-rings. 50mm sits at an awkward spot between inch sizes which will mean either too much squeeze (compression of the ring's cross section) or too much stretch (elongation of the circumference). The correct metric size quad ring does exist, but I haven't found a supplier yet. This is compounded by the material requirement: silicon is rare, FDA approved silicon even more rare. In short: for now it's back to Faema V and W rings.

On the build front, I am anxiously waiting for another shipment of parts which has been held up because a few minor bits were back-ordered.
When it arrives, I should be in possession of every single part required to complete the very first machine (finally, yessssssssss)! Maybe in time for Christmas....

I have been busy with supplier negotiations for the last while and I don't have too much concrete to show.

I did put two wood blanks through the process for making the handles:



...quite a long time later:







These just need some oil.

With regard to Faema V and W gaskets, I order mine from Espresso Parts and they need to be changed approx. every 6 months. My machine is only used a few times a day on average but it is left on 24/7.

Disregard, I see you could not find quad ring in correct size.

I would suggest you look into quad rings if you want to try something other than conventional gaskets. They are a version of oring but have four lobes roughly looking like a rounded w gasket. I used them in an industrial food product pump and they worked very well. If you cut one apart through the cross section it would look like a four leaf clover minus the stem. When pressure builds on the top or bottom of the seal it tends to expand a bit due to the cavity between the lobes. They are available in food grade and high temperature compounds.

I am curious: What was the wall thickness of the 6" pipe you used to make the boiler?

The pipe is Schedule 40 in 316 stainless. The wall thickness is a little more than 1/8"; something like 0.135" if I remember correctly. Overkill for this application, but that is what is available.