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.

[felix gunner]

Anyone else got any thoughts or knows someone who has done this conversion?

Or has any ideas for alternative cup washers or can help with an O ring that might fit?

I'm also still slightly suspect of the Breech washer, ones I've got from dealers don't seem that great - anyone got a supply of better ones or tubing that can be cut to fit?

[Troubledshooter]

I've still got a few original Webley 'Nitrile' breech washers @ £3.80 posted to UK address.

They can be a pig to fit, but a touch of lubricant can help and they sometimes need fine trimming for the barrel to close properly.
Even the original tool used for the fitting of the leather and brass breech seals doesn't help much with the 'Nitrile' seals, but it's better than nothing.

[felix gunner]

Cheers David, I'll take a couple and PM you.

You got any other thoughts on the whole power/PB ring/conversion to cup etc? You know more about these pistols than most!

What I might do is find an older scrappy/bitsa MK I or Senior to break up and mod the piston from that to try.

[Troubledshooter]

Cheers David, I'll take a couple and PM you.

You got any other thoughts on the whole power/PB ring/conversion to cup etc? You know more about these pistols than most!

What I might do is find an older scrappy/bitsa MK I or Senior to break up and mod the piston from that to try.

There are others on the BBS who know a lot more about the workings of Webley air guns than I do, as I seldom strip them. I occasionally get the inclination to have a play if I come across a problem, but that inclination doesn't occur often nowadays as I'm winding down my collection.
I've never attempted a piston conversion such as you are considering, as I don't have the facilities to carry it out. This lack of power has been discussed previously on the BBS and I think you have covered all the main reasons for it.
The old leather and brass breech seals can be a problem if they've been allowed to dry out, as can a tired mainspring or worn piston ring, so the only thing to try after thoroughly cleaning the cylinder etc., is replacements for all, and if that doesn't improve matters much, I don't know what to suggest.
I've probably got a used Senior piston amongst my spares, so PM if you want one.

[Modski66]

Anyone else got any thoughts or knows someone who has done this conversion?

Or has any ideas for alternative cup washers or can help with an O ring that might fit?

I'm also still slightly suspect of the Breech washer, ones I've got from dealers don't seem that great - anyone got a supply of better ones or tubing that can be cut to fit?

I fitted a new rubber breech washer (from Knibbs I think), I had to trim about 1mm off it to get the barrel to latch, but it was a fantastic improvement in power over the leather and brass original which had dried out and started to crumble - no amount of oil would restore it!

My power measuring is based on bean-can bashing; when I first got it, 0.22 pellets would bounce off, but after the new seal, they would punch hole in the can, sometimes though both sides.

However, my 0.22 Tempest will easily punch through both sides, and is a darn-sight easier to cock. But I still love shooting my Senior!

[abellringer]

Felix hi, I can understand your frustration with the piston ring system and if you want to tinker a little more, I have genuine original Webley piston ring material for sale. It does have to be pretty accurate to be fair, but with the 45 degree angled cut and care in fitting, air loss is fairly minimal and the gun was never considered under-powered for its era.
I do agree a piston seal of leather is probably better, but MKI leather piston washers aren't that easy to find. However I have successfully fitted a leather conversion to the senior in several guns. I actually had to resort to using Junior outer washers with an oversize custom made inner, made from nylon, this expands the outer washer enough to fit the senior cylinder. The pistons do seem to vary in hardness and I have had to resort to grinding the surface away a little to allow machining with carbon tip tools.
I measure the piston length overall, drill and tap the front [size can be to suit yourself, but I use M5 countersunk allen screws]; cut the front off and machine to the back face of the piston ring groove.
I build up the new inner and outer; then screw this unit to the piston tightly to expand the outer enough; then measure the length, determine the shortfall and make up a nylon backing washer. This actually simplifies things a bit and I have to accept you have enough engineering knowledge to attempt it. It does make for a nicer shooting pistol, but in general terms not that much more powerful than the average of the model. I fully understand the reasoning behind not wanting to modify an original piston, it's a big decision and I've only done it as a last resort after explaining to owners the risks and benefits and how difficult it can be to fit a new ring to factory standards, those guys were very talented for sure.
Hope this is useful and If I can be of any more help, please PM me for an e mail address.
abellringer

[felix gunner]

Thanks very much for that info, fantastic - I now have a spare Senior piston and a selection of leather washers to try so I have some experimenting to do.

Before I go further though I will try this piston as it has it's original PB ring and see what that does.

I have contacts at an engineering firm but will they have issues then trying to mill down the end of the piston?

[Bruno27]

Hi

I am very interested by this thread as I will have to refurb my junior. Junior has a leather piston seal but the breech seal is my main concern.

For my curiosity, I have a few questions for you guys:

What does 'PB ring' stands for?
What is the material of this ring?
Beryllium copper?

Many thanks

[DCL_dave]

Hi

I am very interested by this thread as I will have to refurb my junior. Junior has a leather piston seal but the breech seal is my main concern.

For my curiosity, I have a few questions for you guys:

What does 'PB ring' stands for?
What is the material of this ring?
Beryllium copper?

Many thanks

PB stands for phosphor bronze.