Londinium piston removal - Page 3

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OldNuc

Postby OldNuc » Feb 07, 2019, 9:25 pm

The threaded portion looked like it is cut down and then threaded into a recessed end on the shaft from the angle of the pix. Just thinking that is just compounding the chance for axial misalignment.

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Paul_Pratt

Postby Paul_Pratt » Feb 08, 2019, 12:37 am

The shaft is just 1 piece of steel. The lower thread is so long so that you can assemble the piston onto the shaft with the spring in place. I do not think they thought about removal! Otherwise they would have permanently locked the top threaded section to the knuckle.

walt_in_hawaii

Postby walt_in_hawaii » Feb 09, 2019, 12:42 am

When removing it, I had to stop and reheat. Twice. The thread was so long, and my pin wrench is so clumsy to use, that it took too long to remove it and it cooled off enough for some of the threadlocker to start working again :( I kept thinking damn, this thing is going to twist off any second and its BRASS (soft) and I don't want it to hit the floor... but its also HOT and I can't grab it! I think when they made this one guy held it in one room and the guy doing the threading was in the next county.
What makes it problematic is that I wanted a piston on it that wobbles a bit... and with the threaded shank going nearly all the way to the bottom of the piston, it makes that more difficult because it means having to remove more material from deep in the piston... I wanted that material there for heat stability of the piston crown. I think I will end up having to cut the threaded part off a little bit so the shaft does not go as deep into the piston :(

walt_in_hawaii

Postby walt_in_hawaii » Feb 09, 2019, 1:15 am

OK, initial measurements:

piston diameter: 1.928"
group head internal sleeve diameter: 2.017"

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So...that means clearance of .044" in between the piston skirt and wall. I know it will expand when hot and some of that will go away, but that's still quite a bit more clearance than I thought I'd find. I will assume that most of that clearance is to let the piston 'rock' slightly in alignment so that you don't have to build it into the piston-shaft mount. But since that's what I want to try anyway, I'm going to cut a new piston and take some of that clearance out. Usually you start a little too big and trim some off if its too tight... but I don't want to scar the sleeve wall. I was thinking 1/2 the clearance? .022"? hmm, that still sounds like too much. maybe round down to .020"? If I had to guess, I would have said something like .010" would be plenty... that's about what a racing engine would use, with aluminum pistons, which expand slightly more than brass, I think. Or does anyone think that's still too loose even with heat expansion? Paul?

OldNuc

Postby OldNuc » Feb 09, 2019, 9:53 am

The piston, sleeve and group body are brass so that clearance should be near constant from cold to hot. Think cast iron pistons in a cast iron block. Max piston + seal diameter should not compress the narrow side of the seal. The land depth is how it is adjusted

walt_in_hawaii

Postby walt_in_hawaii » Feb 09, 2019, 4:57 pm

Now that the piston is out, I can see the eccentric knuckle... there is a big slot in it! I thought the shaft moved forward/back and the piston would rock a tiny bit as the pivot point moved front/back... but it doesn't. Its sort of like the Europiccola, it has a big slot in the eccentric knuckle so the shaft can stay centered, more or less. But its buried inside the lever housing so is difficult to see until you take it apart. But everything is dry, and in need of lube. But this means all I really need to do is tighten up the tolerances a little; I don't think I really have to make a wobbly mount for the piston... well, at least for the first try I think I'll see if just a tighter piston will make any difference.

OldNuc

Postby OldNuc » Feb 09, 2019, 7:38 pm

Be sure to use a full synthetic high pressure automotive type of lube on all of that linkage. The contact pressure will fail most lubes in a mater of a few cycles. There should be some type of piston rod guide in that top housing and ideally that should have minimum clearance to the rod and have a replaceable bushing. All of that should maintain proper piston alignment. The other little issue is that the upper spring seat horizontal plane and the piston top spring seat horizontal plane must be parallel and 90 degrees to the axis of the piston rod. A failure of this being the case is the likely cause of the spring rubs. The centerline of the piston rod must be concentric with the sleeve axis. Likewise the piston axis concentric with the piston rod axis.

walt_in_hawaii

Postby walt_in_hawaii » Feb 09, 2019, 8:54 pm

Thanks Rich :)
Yes, there is what appears to be a brass rod guide inside the housing with tight tolerances and barely enough lubrication... and no O rings on em to keep the lubricants in place. Glad I took this thing apart. That will have to be lubed with 111 or similar since it gets exposed to the piston/brew water directly. Strange I don't hear about people servicing these more regularly, there is no zerk fitting or anything topside that obviously needs regular attention,yet I would consider this an annual maintenance item, at least. The eccentric is outside, so a synthetic high pressure lubricant is a good idea. I have a few cartridges lying around, need to go check their rating.

walt_in_hawaii

Postby walt_in_hawaii » Feb 09, 2019, 9:38 pm

Its hard to envision, so I took a couple pics of the eccentric knuckle with the sliding boss at the extremes of motion. The first pic is probably closer to its normal position. The top of the piston shaft screws into the bottom of the boss that is sliding. You are looking at it sideways, and the picture left side is toward the front of the machine where the operator is standing. The lever screws into the top. This is as-removed, no cleaning. You can see traces of old grease, baked on but everything is pretty dry. No galling, though, so looks like I got to it in time.
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OldNuc

Postby OldNuc » Feb 10, 2019, 12:28 am

There should be a roller or ball bearing on that pin and located in the eccentric slot. I built one for my La Pavoni and it made a world of difference in the operation of the machine. It may take a pair of bearings depending on the pin diameter, width of the slot and height to get it to work but it would most likely be worth the effort. It may just be a roller on that pin and once it decides to slide the slot will wear rapidly. This is a relatively common design and what is inside is of interest as it looks like there is an antifriction sealed or shielded bearing on each side so I would expect to find a good bearing in the eccentric loop as well.

Definitely time for a proper lube job. If those are just shielded bearings they can be washed and relubed with any 5 or 10 weight synthetic lube. A plain R&O industrial synthetic would work such as DTE797 in the synthetic formulation. This is usually sold as turbine oil to the public in small containers.