Londinium piston removal - Page 4

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walt_in_hawaii

Postby walt_in_hawaii » Feb 10, 2019, 12:44 am

Yeah, the diameter of the eccentric pin means there must be a sleeve inside there, I didn't check. I've been thinkingi I should press the pin out in order to get the grease in there to the sleeve, but I don't think annual maintenance would emcompass that kind of work, so I'll just lube it well and see if it goes into the roller.

I'm measuring up the piston dimensions... the main seal lands are 1.613" diameter (bottom of lands)... I wrapped the ring lands with 2 or 3 wraps of teflon tape (a common fix for people with slipping seals) and it measured up at 1.630" ... cut the new lands at 1.635" ish? for a little extra outward pressure on the seals? If it gets too tight and too hard to move the lever, I can disassemble and recut them a touch lower. Maybe. how many wraps were people reporting? I didn't actually read the thread...

OldNuc

Postby OldNuc » Feb 10, 2019, 1:08 am

Not real sure on the amount of tape used but the proper cylinder fit is to have the solid side of the seal, either top or bottom depending, Just clear of the sleeve wall. Cut the land diameter to get this final dimension with a minimal clearance and that should be the end of slip but seal life will suffer as EPDM is not the material of choice for this hence the popularity of the silicone seals. I suspect the real issue here is the land diameter is not adequate to obtain proper sealing at low pressure so you get the bypass slip routine.

walt_in_hawaii

Postby walt_in_hawaii » Feb 10, 2019, 4:47 pm

Uh oh, my piece of brass measured 1.750" diameter... too small to make a piston! (I ordered it for a cylinder for the Europiccola) I thought it was larger :( I only have aluminum or stainless on hand in that diameter :(..... I think the stainless will take slightly longer to heat up, although not much longer. And it will be more heat stable, cuz it takes longer to cool down. Hmmm. I'll make a quick check of the *bay and see if there's any brass big enough...
OK, found a piece, 2" OD x 3" long... just long enough, $33; not too bad. Ordered it. Looks like my Londinium will be hung up till I can get this in; or maybe I should reassemble until it comes in? just to build familiarity with it...

OldNuc

Postby OldNuc » Feb 10, 2019, 6:08 pm

Depends on how fast that lump of brass will ship. Best to stay with brass so you don't get into the differential expansion and contraction trap.

My suspicion is the original piston diameter is too small for the sleeve and the seal lands are cut too deep so there is inadequate seal contact. Probably works fine out of the box but just as soon as the sharp edge wears off the seal it slips. Doing all of that undersizing does extend the service life of the seal though.

walt_in_hawaii

Postby walt_in_hawaii » Feb 10, 2019, 11:50 pm

Well, nuthin to do until the brass comes in; so I figured I would make one of those seal installation tools... stuck a chunk of aluminum on the lathe and got this far:
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Then thought: Oh damn! I'm stuck :( Cutting tapers like this always messes me up. Now there's no way to grab it and I have to spin it around to cut the other end (it's still a solid bar; it now needs to be rotated to work on the other end, and bored out to just slip over the piston)... cut it in the wrong order :( the taper should have been the very last cut.
Hmph. I can salvage it, but it'll mean working with a steady rest, which I hate.

OldNuc

Postby OldNuc » Feb 11, 2019, 1:55 am

I hate that when it happens. ... :?

I just use a sheet of polyethylene salvaged from a milk or antifreeze jug, works every time.

walt_in_hawaii

Postby walt_in_hawaii » Feb 11, 2019, 2:41 am

OK, the project is flipped end for end and the other end is gripped in the chuck... the right side is held in what's called a steady rest, which is basically 3 rollers or fingers which support the workpiece but still let it spin so it can be bored out...
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But, using a boring bar on this is painstakingly SLOW going. And it has to be bored out to almost 2 inches!

walt_in_hawaii

Postby walt_in_hawaii » Feb 11, 2019, 4:15 am

Finished... but its not even a good fit for the stock piston since I sized it to fit well on a slightly larger diameter piston (the one I have to build)... but appears to be ok otherwise...
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walt_in_hawaii

Postby walt_in_hawaii » Feb 17, 2019, 9:52 pm

Chunk of brass arrived!
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First order of business; thread that looooong hole M12x1.5....this is a VERY deep hole to thread. My tap got progressively harder and harder to turn until I was gritting my teeth and knew I was right on the edge of snapping the tap off and breaking it inside the hole (a very bad thing); this is as far as it would go in before it got too dangerous to turn anymore, about half of the tap length:
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The tap threads were causing friction as it dug deeper and deeper into the hole. So, I ground off the upper threads of the tap to lessen resistance of the cutting threads. Normally that's a bad idea, because the threads help to align the tap and keep it running straight. But since I was threading it on the lathe (by turning the chuck by hand), it was fixed on axis and wasn't in danger of going crooked. It was ground down from this:
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to this:
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piston taking shape. The seals are 7mm and the slots cut in the stock piston were about 8mm... (.312"). I decided to tighten up the slot to a touch over 7mm (.285"), this is the top slot being cut:
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The piston is then flipped over so I can cut the lower 2 slots:
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finished, almost, but couldn't resist the urge to do a quick side by side with a seal. Still have to drill some holes. Note that the 3rd (topmost) seal is in a different location. I decided not to install a 4th seal, so all that space on top seemed like it was going to waste... so I located the topmost seal a bit higher, so it would have a larger moment arm to act against the shaft. In rifle shooting we call this 'sight radius' and is why rifle shooting is inherently more accurate than handgun shooting. You can aim better because your sighting plane is longer. Also, the seal depth is shallower... they are closer to the walls of the group head. The stock seals lands in the piston have a diameter of 1.613", mine has a land diameter of 1.640". I'm not sure if this is too high... if it is, I will have to put it back in the lathe and adjust it slightly lower.
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walt_in_hawaii

Postby walt_in_hawaii » Feb 17, 2019, 10:01 pm

Now the seal tool slips on nicely!
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