Hawk MK1 mainspring and fun

[T 20]

If you are not an engineer, I suppose the easiest way of sleeving is to us some Ptfe rod or similar to make a breech seal and open up the hole a bit at a time.
I also had the leaking cylinder thing with a hawk. I had to unscrew the cylinder end and jaws and use loctite to sort it. When chronoed for me, that had gone up from about 6.5 to 8.8ft lb.

Hi Guy

I doubt that PTFE would last very long as the hot air would erode it.

I have been told that you can buy 4mm OD microbore brass pipe on the bay, this could be cut to length and worked in an electric drill to size it.

I normally form the chamfered lead in by simply spinning the brass sleeve up and putting a Stanly knife blade up it --- works well.


I wonder if the leaking Hawks could be sorted the same way as I did my HW35, with a little loctite 271 forced into the joint from inside the cylinder ?




All the best Mick

[ggggr]

Hi Guy

I doubt that PTFE would last very long as the hot air would erode it.

I have been told that you can buy 4mm OD microbore brass pipe on the bay, this could be cut to length and worked in an electric drill to size it.

I normally form the chamfered lead in by simply spinning the brass sleeve up and putting a Stanly knife blade up it --- works well.


I wonder if the leaking Hawks could be sorted the same way as I did my HW35, with a little loctite 271 forced into the joint from inside the cylinder ?




All the best Mick

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.

[BTDT]

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.

I think the breech seal on the Hawk needs to be elastic, to deform as the breech is closed, then to return to its original shape to seal the breech. PTFE is not elastic; squash it and it stays squashed.

The other point is that Mick is reducing transfer port size to raise the compression ratio, which you would not achieve by restricting just the hole in the seal, I'm afraid.

[ggggr]

I think the breech seal on the Hawk needs to be elastic, to deform as the breech is closed, then to return to its original shape to seal the breech. PTFE is not elastic; squash it and it stays squashed.

The other point is that Mick is reducing transfer port size to raise the compression ratio, which you would not achieve by restricting just the hole in the seal, I'm afraid.

Re the Ptfe washer not returning to shape---fair enough. I assume they are ok on the cadet and Major becasue the block slopes? I would have thought that any restriction of air flow would up compression ratio? made leather washers for small Webley and Diana rifles, where the hole in the washer closes up to the point where the gun wont fire (although clearing the hole a few times until the seal settles works). assume that the compression had increased to the point that the piston could not force the air out? I think (offhand) that compression ratio (on an engine) is the volume of the cylinder/volume of the head so a 500cc cylinder with a 50cc head = 10 to 1 compression? On a webley, as you could consider that the seal is about 1/4 inch thick, any reduction in the size of the hole in it must reduce volume and therefore up compression? Obviously with a thinner washer, less of an effect.

[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.