TX200 piston seal lost volume

[T 20]

With holding the cylinder further off the barrel face you could have increased the lost volume in the system.



All the best Mick

[cooper_dan]

I know you know what you're doing, Dan. And that you can certainly shoot and you know what you want from the gun. But I think I'd be tempted to increase the stroke slightly, if do-able. Maybe 5mm. And let this slightly larger volume work slightly less hard than the reduced volume. There'd be hardly any difference in piston time. If by later fitting the parachute seal the efficiency was increased, you could then go even lighter on the spring?

You read my mind. I've got a 4mm lower stroke piston sat in front of me waiting to go The only thing stopping me is that the length of stroke does have a slight impact on my aim points, even if the muzzle energy is the same. I won't pretend to understand why it happens, but it does.

If tomorrow's test doesn't work, this piston will be going into the gun until the new seal arrives.
The shoot tomorrow doesn't matter too much, as you can drop a couple of bad scores over the course of the series. But Sunday there are some major bragging rights up for grabs so I won't take any risks for that one.

[cooper_dan]

With holding the cylinder further off the barrel face you could have increased the lost volume in the system.
All the best Mick

Completely agree. So I've shortened the seal as far as I dare to reduce this and measured it. The lost volume of the piston void is 24x larger than the breech seal void and barely affects the overall calculation

[TonyL]

Good luck with it and for the weekend, Dan.

And do be sure to take up Jim's very generous offer. It's not everyday that you can have an absolute guru look over your gun and actually be able to scientifically decipher what's going on in there.


Wish I could be there......

[cooper_dan]

Good luck with it and for the weekend, Dan.

And do be sure to take up Jim's very generous offer. It's not everyday that you can have an absolute guru look over your gun and actually be able to scientifically decipher what's going on in there.

Wish I could be there......

Chatting to Jim is a definite bonus to shooting at Nomads. I've got a few things to pick his brains about

[cooper_dan]

I have no idea what's going on with the different approaches to making the breech seal up (although I've always thought that two o-rings was an unusual idea/solution - therefore KT must have had a good reason for it).

But just a thought regarding the die 54 pellet skirts sitting proud of the barrel - I take it you can't 'thumb' them all the way in - so have you tried a pellet seating tool?

I did try a pellet seating tool but it didnt have any kind of depth stop. I ended up pushing half the pellets quite far into the bore. One that limited how far in the pellet could go is worth considering though...

[TonyL]

Balls!


I mean......"balls".


Now, I have never tried this but I've often wondered how it might work..

Assuming that access is restricted for loading due to a larger scope, making it harder to thumb them in.

How about a ball? About 10 to 12mm and hard. Maybe steel? Due to tight access, this could be welded onto a small rod. A bit like the "ball" on many bolt actions. Once this is pushed tight, the perfect spherical shape should mean that all pellets are seated flush and consistently.

Only a thought and feel free to call me stupid if it's a no-goer.

[Rickenbacker]

Balls!

Half a ball, Tone:

sizer Capture.JPG

[FPoole]

Maccari sells some over sized piston seals. I wonder, if you cut back the face of the seal, this would size it down since the sides taper inward, wouldn't this lessen the hollow spot? We used to have an old tuner that would do this and it just brings the piston dovetail forward to fill the void. The amount you could trim back would be limited by how much too over sized the seal is. Maccari sells several different seals ranging from .995" to something just over an inch. Next time I get an overly tight seal, I will try sanding the face flat with some wet and dry. This would give you a tiny bit more stroke, which was the reason the tuner I mentioned did this. He was always looking for a few fps with the old guns. This may not work on all MarkIII stroke guns because the piston might hit the rear guide flange before the rod latches in the trigger. Ask me how I know this.

[TonyL]

Half a ball, Tone:

sizer Capture.JPG

Cheers Phil.

The one I'm picturing, though, is much more than "half a ball" so that it might kinda "roll around" the breech face, especially with the more limited access on these "slidy guns"? If that makes any sense?

[PhatMan]

Dan,

I have found that opening the cocking lever a couple of inches or so during storage, extends the life of the O-ring breech seals in the TX - they take a set much less quickly.

I use a piece of stiff plastic, which sits between the end-groove of the cocking lever, and the barrel near the ball-catch.

In all my rifles, I store them with no pressure on the breech seals - break-barrels are broken, and sliding breech rifles have the lever wedged open slightly.

As I am still using the original breech seal in my 1974 Diana 35, it does work (for me)

I shim the O-rings in the TX200 breech with the little plastic binder reinforcing rings (from stationary shops) to obtain the 'best' pre-load (for me) on the seals (slight squeeze when cocking lever is closed).

Have fun & a good weekend

Best regards

Russ

[cooper_dan]

Fpoole - I was thinking about this actually. It would definitely achieve what I'm trying to do. But actually I've ended up ordering a gamo 25mm seal, which fills the void in as the seal doesn't have a hole all the way through it.

Russ - Very neat idea about storing with pressure taken off the breech seals. Sometimes the simple solutions like that escape me

I'm reluctant to go down the route of a pellet probe, mainly because I'm quite likely to lose it/forget it/drop it in the mud.

In the end I think I've done the sensible thing. The 80mm stroke piston has come out, and the 84mm stroke piston has gone in. The extra stroke and less pre-load seem to work OK in this setup and didn't give me any concerns at today's shoot. (Tony can feel free to say 'I told you so' here). This meant I could use the new breech seal and my favourite pellets, without any lead swarf being created which was the entire point of this exercise to begin with.
It didn't feel quite as soft as the lost volume test, but certainly not bad. More than good enough to knock tin chickens over if I do my bit right.

The 80mm piston is now waiting for the new seal I've ordered, and it will be going into my SPARE gun like it should have done from the start! I also have a spare comp tube and end plug for that. I think I'm going to fit a new 3mm transfer port. Then open it up slowly, measuring efficiency and trying to judge shot cycle. I guess in stages of 0.1 or 0.2mm. This will have to wait until my friend with a lathe has finished moving house because I don't have the tools to be this accurate.

I'm an impatient person, and find it hard to let an idea go once it's in my head. I just have to try it!

[TonyL]

Good news then, Dan.

Hope all goes well tomorrow.

[TonyL]

Hope all went well today, Dan.

How was the event? Brilliant I guess?

What rifles were most abundant?

And did you manage to spend some time with The Lord of The Springs?

[brassbanjo]

I did try a pellet seating tool but it didnt have any kind of depth stop. I ended up pushing half the pellets quite far into the bore. One that limited how far in the pellet could go is worth considering though...

Another possibility - maybe the lead-in to the barrel could be increased a tad, so that pellets seat flush (as in prevention is better than cure)

BB