...split from DIY-Olympia Cremina piston rod seal "tool" and Fix by moderator...


Hello all,
I am new to this site, and while I am not blessed with a Cremina, I am repairing a machine similar (and very dis-similar) and I hope you folks might help me out. I have read this thread beginning to end - very informative! Thanks to all contributers.

Nobody (at another site) knows my machine - it seems. It is a CMC or C.M.C. from Napoli. The piston head seems similar to yours and I need gaskets, so thorough my search I find myself here. I am not sure if I should re-publish photo links or mention the other thread, but if you have the time you will find my thread if you google 'cmc napoli' - or - gummybun - I'm the second hit. You will see my oddball machine there.

So...I need to re-gasket my piston head.
May I ask you the exact dimensions of your Piston gaskets? I know they have a 'V" section. If they are the same as mine I will chance an order. Also I have the '4 hole' thing going on. I thought it was for a specialized wrench to remove the gasket head! Whoops. ... So do you stretch the new gaskets over the piston head??? Mine are so hard in their old age they have no elasticity left whatsoever.

Thanks again - I hope I am not appearing to crash your thread but I am grasping at straws here and I would be greatfull for any thoughts.

Simon

I have been following your thread over at coffeegeek, it is a very interesting machine, perhaps you would like to start your own thread in the lever forum here?

Hi Simon, can you give the measurements of the gaskets on the machine, or better yet, the diameter of the cylinder, and the diameter of the piston, and the groove that the gaskets go in? It will be easier to provide you with help if you can post those.

Whoops.. I got bumped (sorry Dan, wasn't sure of the protocol)

Thank you all for your responses. I am certainly amused by the machine here, with its' questionable pedigree.

There are two gaskets. Both are 6.5mm x 40mm OD. While they are still on the cylinder head they appear to have a complex sectional profile. The bottom unit appears to have had a centre ridge - I will draw a sketch of what I imagine the profile might have been. Per the sketch I can get a pick under both top and bottom flanges of the ring.

Piston OD = 39.5mm or 1.54"
Cylinder ID = 40mm or 1.57"

I must admit that I am shy/worried/scared to remove them - I was hoping for a Non-destructive method of removal so that in a worst case scenaro I could re-fit. I will just have to swallow hard and do it. Then I will have the ID that I suppose is needed.

Simon

Hi Simon, alas, there is no other way to get the measurement when the seals are toast... If the seals were even marginally usable, you could stretch them enough to remove them, but if there is no stretch, they aren't any good. Once you have that last measurement we'll see what we can find!

OK Doug,

I done the deed... They split off easily to reveal the sketches are correct in principal.

The OD of the piston seat is 28mm or 1.104"

I have sliced a couple of sections of the rings and will post them when my camera re-charges.

Simon

OK, here is the sad photo of the piston with its' old rings. Note the section cut-offs.



I understand that I may be looking for some good 'O' ring suppliers now.

Progress is indeed being made...

The top seal is not a problem due to the square dimensional cross section, and we sourced a standard U-cup for this one (# 6226-21), but the bottom is a bugger and from what I can tell will require some experimentation.

My logic for the possibility of substituting an o-ring for the fuggetaboutit seal goes like this--- the seals on the piston are to achieve two purposes in the two different machine states, first is to seal the cylinder while the machine is cruising, so there are no leaks at pressure when hot, and the second is sealing while the piston is in motion during the shot phase. Two surfaces on that lower seal make sense but would one surface work as well? Seems like it, as there are other machines which use a single o-ring by design on the lower part of the piston.

Of course, there is no such standard o-ring, particularly as the rings get fatter and fatter the choices begin to drop. All the seal companies do and have done custom jobs over the years after they make a seal from the special mold (at your cost) they keep the mold, and in most cases make extra rings or seals from the mold and hold as a small inventory.

So, what you need is a 40 x 28 x 6.5, and our main source does not have that size exactly and the closest I see in the data sheets is 39.45 x 26.80 x 6.3. There are a few others close but this is the best fit from at least this one company. Bear with me here, my thoughts are that when applied, the ID will become 28mm, as the new rings are real stretchy and will go on the piston with no problem. this will seal the inside surface of the ring to piston and stop any leaks from there. Once the ID is forced to 28, the question is what will happen to the OD? The ring will be thinner (you have the same amount of material stretched to a larger profile), but will it be enough to seal? If you work out the geometry just by simple math stretching the ring will likely kick out the OD to something over 40mm, but not so much as to not fit into the cylinder since anything over that 40mm mark makes the fit tighter and tighter (enter Dow 111). Once you have the ring on the piston, you can measure it to see if it is even close enough to consider putting it into the cylinder for testing.

Maybe you can find the exact o ring by calling the seal and o ring companies, which would be ideal, but with that bottom seal change to an 0-ring is going to take putting it all together to see what happens and if it does not leak when it is hot and running then it will likely make espresso, or at least be a lot closer to full function. But then again, it is an experiment and you could be back to the drawing board.

Doug

OK...lots to think about there

One thought... have you heard of anyone water-jetting out of solid sheet stock?

Simon