Converting the Webley Senior from a piston ring to a leather seal?

[felix gunner]

Following:

http://www.airgunbbs.com/showthread....-down-on-power

I have tried more tinkering but the (IMHO) slightly 'too clever for its own good' design with a split PB ring is driving me crazy.

It's a fair idea in practise but there must be a reason 99.9% of air guns have never used it.

Unless it is sized absolutely 100% perfectly then it's never going to give the sealing of a more flexible synthetic or leather seal surely that would balloon out as it will be too tight and draggy or too loose and the split will allow leaks?

I thought of trying a nitrile O ring in the groove but the only ones to fit and actually enter the piston and seal were a pair of thin ones but after 100 shots or so they are not long for the bin...

Anyway, has anyone converted a PB Senior to a cup washer a la Junior, MK I etc?

The piston has a dimple at the seal end presumably from lathing so is it simply a matter of drilling/tapping here and fitting a MK I cup and support washer?

If anyone has done this or has ideas please share.

It would be handy if there was a more common seal/support (SMK??) that could be fitted rather than trying to locate and paying the premium for vintage parts.

[wild boar]

i just streched a o-ring on to one of mine , had to be a little too small so it was not too thick but if it was a thinner diameten than i had in the box then i need not to have streched it as much - seems to work perfectly , having said that i dont think the metal ring is a bad thing as i dont think much if any air will get past in the split second of firing

[felix gunner]

I have one of those boxes of multiples sizes and the only way to get something to fit (and actually be insertable) was to sit two thin ones side by side in the groove - it worked OKish but as I say they are pretty nicked after not many shots.

IIRC Bruce states that the Senior showed an increase in power over the MK I partly due to the PB ring (as over time the leather washer was compressed by impact) but my MK I puts out 3.2 ft lbs and runs beautifully with a very old cup washer.

Bruce also states that the piston on the Senior was 1/10" shorter for slightly more cylinder volume but more importantly was internally 0.4" less deep to give more spring compression. The MK II Target used two piston rings so maybe this would have worked better.

I know adding a leather washer will reduce cylinder volume slightly but would rather try this first than mill the piston and if I could get my Senior to match my MK I and (reliably and consistently) make over 3 ft lbs that would be desirable.

In the pic below you can see the piston has been milled back.

I would have thought that rather than using a leather inner washer (as on MK I) - that the delrin/polythene inner as on a Junior II would be better and stop the cup washer compressing if it impacted the end of the cylinder? (Interesting stuff on all this and how Webley tried alternatives to the PB ring etc on p88 of Bruce.)

[ggggr]

If you want to drill and tap the end when the dimple is then any washer you fit will shorten the stroke by that amount.
If you drilled and tapped for maybe 2BA or 5mm then you could use the washer (screw on not the mushrrom fitting) off a Premier.
If you think that the current ring is not giving a good seal, maybe try a winding of ptfe tape under it , which should last for a few shots to chrono.
I'm sure asking on here that someone will tell you the O ring you want to replace that ring.
If you have put 2 rings into the groove ( I think you said you did) then one ring would pressurise the other and get hot and power would fall off (tried it on an Osprey).
I have short stroked a Tempest in the past and a Webley Junior. The Junior seems to work quite well, being easy to cock and not seeming to have lost much power

[felix gunner]

Thanks Guy. After I posted above I've been flicking through Bruce some more and all along the way it seems the seal caused them headaches through the range, lots especially on different things they tried on the various Premier models and also considering and experimenting with leather on the Senior following sone impact issues but returning to the ring. I understand there must be a small volume loss but with a thin cup this should be reasonably minimal. I would not want to mill the original piston as a threaded hole is one thing but altering it irreversibly is another...

[felix gunner]

Well, this kind of ties into the Senior down on power topic but did some experimenting with various breech seals I had.

Transfer port is free of debris.

TBH the Senior is all over the place with spreads (50+) but various seals made no reliable difference.

I dismantled the MK I and Senior to compare/swap:

My Senior spring is 10mm longer but putting in the Senior made it harsh, twangy and did nothing to improve power of consistency.

MK I piston will fit in Senior but will not cock, BUT, it seals beautifully with finger over port and half cocked there is a spring created and woosh of air when releasing finger. Repeating this with Senior piston and ring - nothing - so this definitely seems to point the finger at the piston ring (and that the breech seal is not the main issue) - tried two rings and both same result.

As a contrast the MK I puts out 3.33 ft lbs with a 4 fps spread over 5 shots.