Londinium piston modification?

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walt_in_hawaii

Postby walt_in_hawaii » Aug 08, 2018, 4:12 pm

I have read on a different thread that Bosco now makes their pistons (and have been for a few years now) with 4 piston seals, as opposed to the 3 currently on the londinium piston. Is there anyone that can post dimensions for modifying the current 3 seal piston to go to 4th seal piston? I heard it will use the washer above the piston as the retaining wall for the new top seal, so I guess copying the 3rd seal's width would be in order..

when installing the 4th seal, would it go with the lips upward? or downward?

I just installed cafelat seals in my early londinium and the lever catches quite high, I was hoping for improvement. I will probably have to try finding a set of the bosco seals.... I saw a reference to winding Teflon tape under the piston? Is this under the existing seal's seat? inside the groove/under the seal? Sounds dangerous... maybe a thin delrin split ring might be better? if it can be cut that thin and not destroy itself... I'm pretty sure I could manage it, but don't know the tolerances involved... sounds like it'll be 10 or 20 thousandths thickness?

aloha,
walt

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JohnB.

Postby JohnB. » Aug 08, 2018, 4:54 pm

Just measure the other seal openings & remove that much from the top. The bottom spring seat holds it in place. The new seal faces up like seal 1 & 3. If you use the Bosco silicone seals there is no need for any tape unless your sleeve is out of spec. Cafe Lusso Coffee Roasters usually has the seals in stock. I find that the Bosco seals only require lubing once a year, if that. No idea how many years it takes to wear them out. :lol:
LMWDP 267

walt_in_hawaii

Postby walt_in_hawaii » Aug 08, 2018, 6:15 pm

Thanks JB!! I just called Café Lusso and ordered a set of 4 of the Sorrento seals. I'll modify my piston when they come in.
Does anyone need a set of cafelat silicon seals? they have 3 or 4 pulls on them... all of 20 minutes' use! Works fine, just my lever grabs a little too high... might as well throw in the Sorrento seals and see what happens. If it still grabs too high, I'll look into shimming out the seals by making the delrin spacer rings to go under the seals, I guess. Maybe I should have just made the delrin without ordering the Sorrento seals... but I want to keep the machine as stock as possible, as I don't know if I'll keep it or the Strega... 2 levers are a bit much, and they are pretty close.

mivanitsky

Postby mivanitsky » Aug 08, 2018, 6:22 pm

Paul Pratt has also teased replacement pistons for these machines on his Instagram page. No idea when they will be available.

HoldTheOnions

Postby HoldTheOnions » Aug 08, 2018, 7:05 pm

Is it trying to catch and slipping, or just not catching very high? If the latter, then thinking extra seals won't help, but I could be wrong.

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Paul_Pratt

Postby Paul_Pratt » Aug 08, 2018, 8:51 pm

mivanitsky wrote:Paul Pratt has also teased replacement pistons for these machines on his Instagram page. No idea when they will be available.


Not really teasing, we made some to test the theory that the current piston design is wrong and it proved to be correct with the testers so far. Replacing the actual piston is quite a tricky process and will require 2 special tools, one of which will only ever be used once and is then useless.

If there is demand I could look into it, but really it is something a manufacturer should be doing.

walt_in_hawaii

Postby walt_in_hawaii » Aug 08, 2018, 9:29 pm

Paul, I EXCEL at making single use tooling! Seems I've been doing it all my life :(
When I walk though my garage... er, shop, I pass by all the little things that will never see use again. Such a sad thing, for a machine, to have no use. I once spent a few days rummaging through a large metal chest that I acquired in a clearing out sale from a business that had been there for longer than I've been alive, and handling all the strange tools (some of which have contemporary equivalents, but the forms of these tools have evolved slightly over time), and wondering about their prior owners and wishing they could talk... so many scars and so much experience in them.
Would your prototype pistons have slightly larger diameters on the ring grooves, to push them closer to the walls? or did you select entirely different sealing rings entirely? Would you mind sending me or posting the dimensions here? I belatedly remember that I purchased a big hunk of brass that I was going to turn into a cylinder for my millennium europiccola... which I then sold... so now I have this big piece of brass staring wistfully at me :)
It COULD be a piston in disquise ...

walt_in_hawaii

Postby walt_in_hawaii » Aug 08, 2018, 9:32 pm

[quote="Paul_Pratt"] Replacing the actual piston is quite a tricky process and will require 2 special tools, one of which will only ever be used once and is then useless. quote]

Hahahaha, as a matter of fact, I was designing a spring compressor in my head during my lunchtime walk (I don't eat lunch, I just go out for exercise). trying not to blow my head off (in my mind) when the spring would let go....

walt_in_hawaii

Postby walt_in_hawaii » Aug 08, 2018, 9:36 pm

HoldTheOnions wrote:Is it trying to catch and slipping, or just not catching very high? If the latter, then thinking extra seals won't help, but I could be wrong.


Joe, it doesn't slip and catch, it goes straight from the preinfusion lowered state up to around 11 o'clock, pretty high... maybe 10:30 and has a soft landing there. Nothing in between, no slight catch or nothing...

chappcc

Postby chappcc » Aug 08, 2018, 10:05 pm

I've found I can overcome this behavior on my SCL (CMA group dipper fed through the thermosyphon loop) by bleeding some steam off the boiler just before I pull the lever down to start pre-infusion. My theory is at a higher boiler pressure, I get steam in the chamber which condenses and reduces the overall volume of liquid water for extraction - the lever engages higher (~11).
For a Londinium with a heat exchanger, you might get the same result by lowering the boiler pressure - which reduces the temperature in the heat exchanger.