Londinium piston modification? - Page 18

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Postby mathof » Sep 02, 2018, 9:21 am

walt_in_hawaii wrote:Matt, probably for a 2 pin spanner wrench to turn piston for mounting/unmounting.

Thanks very much.



Postby HoldTheOnions » Sep 03, 2018, 12:42 am

HoldTheOnions wrote:So drilled 4 holes in the piston today. I filed off the burrs after I took the pics, but shows the idea of it anyway. Applied a very light coat of lube as recommended to the seals (bottom one is Cafelat and top two are Reis superduper) and none to the sleeve. Pulled two shots and no slipping, which is encouraging, but obviously too early to reach conclusion. Will report back regardless.

The shots yesterday seemed normal, but today had really bad channeling off the bat, with a big hole same place in the pucks, so opened it back up and by chance the sleeve holes were closely aligned to the piston holes and clearly jetting through one of them and tunneling down through the puck. Rotating the sleeve 45 degrees to bisect the piston holes sorted the problem. Just FYI if anyone tries it.


Postby OldNuc » Sep 03, 2018, 9:35 am

That is the obvious reason OEM does not drill seal energizing ports.

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Postby JohnB. » Sep 03, 2018, 10:28 am

Those were awfully big holes. I drilled 4 1/8" holes in my Sorrento's piston a few weeks ago. The holes were closer to the edge & I haven't had any issues. Didn't make any difference one way or the other but I wasn't having any seal issues.


Postby HoldTheOnions » replying to JohnB. » Sep 03, 2018, 7:05 pm

I used 1/8 bit, but freehand so holes ended up bigger. That said thinking bigger holes better to minimize jetting though to the puck and next step was to try 3/16 holes if rotating the sleeve didn't help, but never got that far. If you have bad slipping, then IMO nothing to lose by trying it, can always fill the holes if not happy with the results. That said, I'm very happy with how it is working at this point, but thought I had it solved several times previously, so cautiously optimistic.


Postby pcdawson » Sep 04, 2018, 10:20 pm

Paul Pratt has mentioned several times that it is important not to mark or scratch the groove on the Piston where the gasket sits. I'm wondering if someone can tell me why? I'm guessing the scratch would have to be pretty deep to have any imapct on the seal made between the gasket and the sleeve? Given how difficult it can be to remove older gaskets I would think the odd scratch would be a pretty common occurrence.

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Postby Paul_Pratt » replying to pcdawson » Sep 05, 2018, 5:57 am

Yes light scratches would not be a big deal and are, as you say, hard to avoid. I've had to skim many an old piston on the lathe to get it to seal as a result of pitting or deep divots. So prevention is better than a cure.

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Postby civ » Sep 05, 2018, 8:55 am


WRT to holes in lever pistons (undoubtedly wide generalisation) I can offer the case of a very early (ca. 1969) Pavoni.

As you can see in the cross section of the piston I drew at that time, the depth of the gooves that hold the seals would be 3.0 mm ie: (41.8-34.0)/2.


If you look at these photos of the piston, you can see that both at the top and at the bottom, there are three holes near the edge at 180 degrees to each other, the ones at the top being displaced 60 degrees with respect to the ones at the bottom.


I believe few of these early Pavonis had these piston holes as I have not seen examples with them in machines made in later years.

The point I want to make is that they appear to be (sorry, I did not measure them) less than 1.0 mm in diameter.


This would make the hole roughly 1/3 of the working space of the seal, ie: the space where the seal moves in and that there most probably is (by design) a relationship between the size of these holes and the the space where the seal moves in.

I believe that both this relationship and the placement of the holes with respect to the edge of the piston are of importance if the holes are to work as intended.




Postby OldNuc » Sep 05, 2018, 9:38 am

Installing seal energizing ports is a band-aid for manufacturing, seal tolerances and material. The quality of seal material existent in the late 60s compared to what is available today is another issue. The seals tended to bake to a rock hard unyielding condition in short order.


Postby walt_in_hawaii » Sep 05, 2018, 1:19 pm

Pete, to remove the old seals I stick a wooden chopstick under it so I can lever it up; once clear of the piston a little, I use a razor and just snip it in half so its not necessary to work so hard removing that old seal (assuming you won't be saving it to re-use it).

Carlos, interesting pic! that would be about the size I was thinking of, if I needed it. New seals have not slipped at all yet, so I'm pushing that decision off into the future. I don't think you need the holes on the top, though; that's just a water seal and does not really have the same pressure on the other side of the seal that the bottom one has...