walt_in_hawaii wrote:For what its worth, I'd love to have a Bosco piston here to measure up; but I don't unfortunately. I've never been convinced the 4th seal was the only modification they made, but they don't seem willing to post measurements. Not a big deal, its not difficult to get the info, just a pain in the butt

Having had their old 3 seal & a new 4 seal piston side by side I can tell you there are no obvious differences aside from the top cut away for the new seal. Next time I have the group apart I can make some measurements & compare it to my old 2011 3 seal piston. Probably won't happen before next summer when I do the annual clean & relube.

I've been using loctite only for all these years, but I know its only cyanoacrylate, so I decided to try another no name brand which was touted as a 'gel' ...its interesting because it really IS a gel, like toothpaste and stays in place... but so far it doesn't hold worth a damn Well, gotta try new things!
Piston before cutting... 1.640" diameter lands:


Setting up piston in lathe... you can't really stick it back into a chuck so it runs absolutely true, but I got within half a thousandth and called it good:


Piston lands after cutting... now they are at 1.627" or close to it...


This time, I'm going to wait the full 24 hours before trying to install things, and give the new CA gel time to setup and see if it still fails.

Loctite 620: https://www.mcmaster.com/loctite-620 This will attach the piston to the rod and more importantly survive the operating temperature.

I would drill a pocket hole in that piston rod shaft for a Delron plug and be done with thread locker as there is minimal to no rotating torque on the piston assembly.

Haha I am not sure what it is about this lever group that makes people do strange things...

I would keep the groove OD (lands I think you called it) the same as the stock. That is what I do. The only dimension I adjust is the height of the groove, I think IIRC it may be 7mm and I drop that down to 6.5mm.

Silicone is a strange beast and getting the durometer is tricky. Too hard (say above 75-80 shore) and it no longer has an elastic quality that you would expect since there is less silicone in the mix, it actually becomes quite brittle. Too soft (say under 50) and you get that silicone pliability, but it can be very gummy and not that desirable. Something inbetween is where you want to be. I have also found cheap silicone tends top be gummy as well.

Paul!
The seal height in the piston was 8mm (.315") and the samples of Bosco and Cafelat seals I got were both about 7mm (.276"), so I reduced the height of the groove as well so my piston would be a tighter fit top to bottom. On the stock piston its .315", and on mine I made it initially a touch over 7mm, about .280" which I thought would accommodate a 7mm silicon seal just right... but when I stuck the Bosco seal in there, it seemed like a rather tight fit. So I adjusted the fit slightly, and shaved a tiny bit off the top of each ring land to make it slightly taller and got to .295", where it is now. Still about .020" tighter than stock, but I don't think it should realistically make that much difference. Although, what I think is actually happening is, because the slot dimensions are now tighter, it doesn't allow the seal to deform as much and holds it upright more, and therefore stiffer. The amount of forgiveness in this system seems not to be a whole lot, so I think I'll be cutting and testing for a bit ...but, once I get it, it should work great for a long time and at least others won't have to do the workup again. I'll post the dimensions if anyone needs them.
Following this train of thought, though, I think you're right; I can make the lands shorter top to bottom, but should probably stay close to stock dimensions in land depth (i.e., about 1.613" diameter to bottom of seals)... but i'm at 1.627" now... I'll reassemble and test. If its still too tight, I'll reduce again to about 1.620" I think (half the dimension each time).

When you compress those seals into a tighter groove that results in the seal lips flaring outward. The more you stretch the inner at rest diameter the more the seal lips tend to pull in. There is an entire book of FM relating to these seals just as there is for o-rings.

Haven't read that one, only the O ring book, because everything I had to work on before only used O rings and not square section rings. These V section rings are behaving in an intuitive fashion, so far... but then, I'm not pushing them really really hard except when mounting and unmounting them. Digging them out without damaging them is a neat challenge and I'm wincing when I do it. Hope I'm in the ballpark with this iteration! I guess this will be trial #2... Will know tomorrow night, when the CA has cured. Hopefully.

JohnB. wrote:The Bosco silicone seals are much softer then the hard black rubber original seals & under normal circumstances seem to work better then the even softer Cafelat version. Bosco solved the seal related issues associated with this group by 2013 with a simple piston mod, a better seal & some QC improvements from the supplier that manufacturers their version of this group. The soft Cafelat seals may be a better choice with Walt's oversize piston lands but they may also wear much faster when pushed hard against the sleeve wall. Like Reiss's machined seals fix this solution may just create other issues.

I've been using Reiss's custom seals on my L1 for over a year and I've had zero issues with them.

Thanks for that, Matt! Nice machine you've got there. With any luck, in a year I'll be able to say that I've been using Walt's custom piston for a year now and have had no problems!
Its not rocket science or voodoo art; as Rich and JB have pointed out, the work on silicon seal clearance values has already been done, if I had just gone to the correct resource and studied up, I could just calculate the correct clearance values and not guess. But laziness rears it ugly head, its simply easier for me to keep cutting a few thousandths at a time and testing since I'm not going into production or anything; its more of a curiosity for me since I like tinkering and I'm not in any rush. While the Londinium is down I'm using the Strega, so there's no down side to working slowly and methodically toward an eventual empirical solution instead of a calculated one. Not an elegant solution, but its workable... for me. Not to mention its a cheap solution. I paid about $33 for the solid block of brass which became the new piston... which is less than a set of seals; so if anyone else has any issues with their old Londinium I, it wouldn't be difficult or costly to replace the piston with a modified one (brass was sourced on ebay), I don't mind helping out a fellow HB member now and then since I don't do this commercially. I fly a desk in an office by day; the caffeine junky work is my night time hobby since I don't have any crime fighting super powers.

Shore A 50 is a bit harder than the usual orange eraser on a #2 wood pencil when it is new. Shore A 70 is a shoe heal, and Shore A 60 is a road tire tread. The usual black U or V cup seal is a Shore 70 or 80. This is why you have all those holes in the piston face as other than that it may never seal at low pressures. Silicone is normally found as a Shore A 50 or 70 but custom formulations are available. A Shore A 50 would be about ideal for a properly fitted piston. If there is excessive clearance then a Shore 70 would be the choice to try and keep everything centered.