Hawk MK1 mainspring and fun

[T 20]

Hi Mick---I meant making a full breech seal out of ptfe and then putting a hole in it, rather than actually trying to sleeve the transfer port itself. assumed that this would be ok as some people use ptfe to make breech seals for Cadets and Majors? Would hot air open up the hole in the middle if you more of less copied the Webley seal out of Ptfe but just put a smaller hole in it? The 4mm pipe would be handy for the centre of Webley pistol seals made of leather.
The trouble with the leaking hawks is that there are only about 2 threads on the plug, beyond the stock screw hole.At first I thought the stock screw hole was too deep but the air was actually coming around the thread. I think I used 271 loctite to sort it out.

Hi Guy

As Jim says the idea of reducing the transfer port is to raise the static compression ratio of the gun.

A standard MK2/3 Hawk has a swept volume of about 37 cc and the standard transfer port has a volume of about 0.201cc giving a compression ratio of 184/1.

If you sleeve the transfer port down to 2.5mm over its full length the transfer port volume reduces to 0.079 cc giving a compression ratio of 468/1.

2.5mm is the standard size of a Vulcan transfer port by the way --- so no flow problems with that size port.

If you just replaced the Transfer port seal with a PTFE seal/reducer I think there would be problems with the double reduction in transfer port diameter. The PTFE would probably erode as the air went through it and as Jim says PTFE has no memory so if you squash it it stays squashed.

I'm still experimenting with all this at the moment but I'm finding synthetic seals and O rings seem to like a compression ratio of in between 300/1 - 600/1 whereas leather seals prefer to run below 200/1.

This could explain why folks who change HW55s from a leather to synthetic piston seal don't like the firing cycle ?

On my favourite HW77 I have opened the transfer port up to reduce the compression ratio from its standard 1055/1 down to 490/1.







All the best Mick

[ggggr]

Part of the thing trying to get across here Mick is that because of the design and thickness of the hawk seal, is that the volume of the hole in the washer should also be taken into consideration? (the volume of the seal must be about 1/3 of the volume of the transfer port?) Ie that the hole should be considered a part of the transfer port in just the same way as if you fitted a sleeve to the transfer port that extended as far as the front edge of the seal and then for example fitted an O ring for a seal instead of the Webley job. I can see you trying that soon to save on Webley seals except I bet you turn your own.
On something like a meteor, with a very short port and a seal on the barrel it would be a different case.
I wonder why the small Webley and Diana rifles had a washer on the cylinder side? I think on the Diana 23 it was a very shallow recess as well?

[T 20]

Part of the thing trying to get across here Mick is that because of the design and thickness of the hawk seal, is that the volume of the hole in the washer should also be taken into consideration? (the volume of the seal must be about 1/3 of the volume of the transfer port?) Ie that the hole should be considered a part of the transfer port in just the same way as if you fitted a sleeve to the transfer port that extended as far as the front edge of the seal and then for example fitted an O ring for a seal instead of the Webley job. I can see you trying that soon to save on Webley seals except I bet you turn your own.
On something like a meteor, with a very short port and a seal on the barrel it would be a different case.
I wonder why the small Webley and Diana rifles had a washer on the cylinder side? I think on the Diana 23 it was a very shallow recess as well?

Hi Guy

I know what you're saying in that the original Hawk seal has a smaller hole in it than the Vulcan, the seal is however shown as the same part number on Chambers for both guns --- so does the Hawk seal collapse and closed up over time?

My calcs above are based on using a new Hawk/Vulcan breach seal which has a 4mm hole through it so with a new seal the Hawk has a 4mm transfer port straight through.

Yes I do make the transfer port sleeve so that it goes from breach face to cylinder face, this also holds the seal better than the standard setup.

For the transfer port seal I just cut some hard rubber tube to length, I mount the rubber tube on a rod in the lathe and cut using a Stanley knife blade in the toolpost.

I think the Hawk has the seal in the breach so that you only need one breach seal for the two barrels --- the design of the Hawk/Vulcan locking pin also leaves little room for the seal in the end of the barrel.

I think I have a Diana 23 kicking about somewhere and if I remember correctly the breach plunger is in the end of the cylinder so there's plenty of room in there for the seal. The other problem is that the barrel is sleeved and not solid so it's not ideal to put a seal in the end of it.


Oh and don't think that I've forgotten that I owe you a Vulcan cylinder Guy, I still have it safe and sound for when you're settled and you want it.





All the best Mick

[ggggr]

Cheers Mick-- give you a shout when I get sorted with a place.
Part of the thing I was on about was that the width and volume of the Webley breech seal should be taken into consideration in a calculation, but it looks like you have done that.
As a bit of an experiment Mick, could you try a Ptfe seal with a smaller hole in it and let me know the difference in fps or ftlbs, compared to a standard seal? I know you say that it wont last I would just be interested as to what it does, because I found that it moved POI on the 2 guns I tried it on and it SEEMED to be hitting the backstop harder (No chrono obviously)
I think the reason why Webley,s have the breech seal on the cylinder is something to do with on old airguns that had this set up, you could take out the breech seal and fit a firing pin to fire live rounds. Everybody went over to a seal on the barrel but Webley stuck with the seal on cylinder set up but choked the barrels to stop you doing this. pretty sure I read this in an article in AGW about 1980 It may have been something to do with an air shotgun?
Re the Diana 22/23 small Webley rifles, I think on the Diana 23 version I had, the breech seal was very thin (about 1/2 th thickness of the 22). I think I made it up out of the tongue of an old boot
Ps the 23 is the proper barrel version of the 22, like a Webley Ranger.