TX200 piston seal lost volume

[cooper_dan]

Just to add more to the current TX thread's....

I've trawled through the search function but haven't been able to find what I'm looking for, which is....Has anyone successfully filled in the lost volume at the front of the TX piston seal, or is the only option to buy something like the Aussie version?
Here's why:

- This week I've swapped out my o-ring breech seals for a 1 piece version made of harder rubber
- The shot cycle now feels a bit more 'slammy'
- My current theory is that the softer o-ring seals absorbed some of the force of the piston when it comes back the second time with no pellet in the barrel. The new harder seal doesn't absorb as much force, so the piston 'slams' a little bit harder which is enough to be noticeable
- Measuring compression ratio gives about 365:1 if you just consider the TP
- This drops to 210:1 if you add in the empty space in the middle of the standard seal

Question 1: am I correct in thinking that increasing the compression ratio up to 350 ish will help cushion the piston on it's second go forward?
Question 2: Has anyone filled in the standard piston to achieve this, and what did you use? Was is a reliable solution?

I've got an aussie seal on order but want to have a play in the mean time. Going to leave the TP and piston standard for now as I know I can go back to how I had it before (with the o-rings)

[pjinoz]

Just to add more to the current TX thread's....

I've trawled through the search function but haven't been able to find what I'm looking for, which is....Has anyone successfully filled in the lost volume at the front of the TX piston seal, or is the only option to buy something like the Aussie version?
Here's why:

- This week I've swapped out my o-ring breech seals for a 1 piece version made of harder rubber
- The shot cycle now feels a bit more 'slammy'
- My current theory is that the softer o-ring seals absorbed some of the force of the piston when it comes back the second time with no pellet in the barrel. The new harder seal doesn't absorb as much force, so the piston 'slams' a little bit harder which is enough to be noticeable
- Measuring compression ratio gives about 365:1 if you just consider the TP
- This drops to 210:1 if you add in the empty space in the middle of the standard seal

Question 1: am I correct in thinking that increasing the compression ratio up to 350 ish will help cushion the piston on it's second go forward?
Question 2: Has anyone filled in the standard piston to achieve this, and what did you use? Was is a reliable solution?

I've got an aussie seal on order but want to have a play in the mean time. Going to leave the TP and piston standard for now as I know I can go back to how I had it before (with the o-rings)

Hi
If you increase the SCR ratio there is a good chance that velocity will increase taking you over the 12 fpe limit. Whilst one might think there may be an extra cushioning effect due to the extra air compression you have to consider that the air will be heated much more and simply "calculating" the increase SCR 210 to 365 may be completely 'out of wack" due to the very much hotter air and thus much higher pressure. Hotter air will flow through the transfer port much faster than colder air. So if need be check your energy level.

Cheers
pjinoz

[cooper_dan]

Hi
If you increase the SCR ratio there is a good chance that velocity will increase taking you over the 12 fpe limit. Whilst one might think there may be an extra cushioning effect due to the extra air compression you have to consider that the air will be heated much more and simply "calculating" the increase SCR 210 to 365 may be completely 'out of wack" due to the very much hotter air and thus much higher pressure. Hotter air will flow through the transfer port much faster than colder air. So if need be check your energy level.

Cheers
pjinoz

My chrono is always on hand when I make any changes. I'm expecting the power will rise slightly, and I've got 4mm of pre-load washers I can remove if needed.
I'm more interested in the feel of the shot cycle though, and if it will 'soften' the piston landing after the pellet has left the barrel

[BTDT]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

[cooper_dan]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

I'll be no rush to get away but it's too late, I've already taken the plunge and made a modification.

The best I can describe it is that before, I could shoot the thing all day no problem. After changing the breech seal, the shot cycle felt more 'sudden' through my leading hand. And where my face touches the cheek piece my teeth felt like they were rattling and skin was tingling. It was actually fairly uncomfortable, hence making this change so quickly.

[cooper_dan]

Actually Jim, the modification is reversible. So maybe we could measure it post modification, then strip it down and undo the modification to measure what I thought was 'slammy'. Could be an interesting exercise and I won't be in any rush

[Hawkeye20]

good afternoon cooperdan, if i was you i would put it back to factory as it seems all your "mod" has done is make the gun more bouncy

[cooper_dan]

good afternoon cooperdan, if i was you i would put it back to factory as it seems all your "mod" has done is make the gun more bouncy

Its hard to go back to standard, and I guess I didn't properly explain my reasoning. There are 2 main points:

1- Why did I change the breech seal?
My favourite pellets in this gun are by a clear margin, die 54 JSB exact. I'm talking 12mm groups (10 shots) at 45 yards. They are basically magic in this barrel. However die 54 have a slightly longer skirt than other dies, so they ever so slightly stick out of the barrel (just a fraction)
With the standard o-ring seals, they compress when firing and allow the compression tube face to touch the face of the barrel. Unfortunately this means that fraction of pellet sticking out gets clipped off and leaves lead swarfe behind. This then gets behind the seals/round the comp tube etc and is just a mess.
The one piece Weihrauch seal I've put in there is harder than the o-rings, so the compression tube is always held slightly away from the barrel. I've put about 200 shots through with this new seal and not had any clipping or lead swarfe. So one problem solved.

2 - Why mess with the static compression ratio
Unfortunately the fix above came with the downside of a harsher shot cycle, which to me feels 'slammy' (although I could be dead wrong about this, I just know there was a definite change and I don't like it). I think because I no longer have the squishy o-rings in there to soak up some of the perceived force.
My TX has been short stroked to 80mm. So with the 25mm diameter piston and new breech seal, should be almost identical to an old Weihrauch HW77 or HW97. However its a lot harsher (side by side comparison).
Looking through the rifles, the clear difference to me is that the Weihrauchs have a much shorter and smaller diameter transfer port, and a flat faced piston seal.
Looking at the static compression ratios (simplified calculations), the Weihrauch is approaching 1000:1. My TX200 (including the lost volume in the seal is nearer 200:1. By filling in the seal volume, it rises to 400:1, therefore should get me a bit closer to the HW77/97 numbers.

Basically I'm trying to make small adjustments so that I can keep using these pellets.

[Rickenbacker]

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?

[nick-nic]

Would a very thin shim between the standard o rings help Dan.

[TonyL]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

As usual from The Gentleman of Springer Deity, what a lovely, generous offer.

[cooper_dan]

Would a very thin shim between the standard o rings help Dan.

Tried this a couple of times. It kind of helped, but after another tin of pellets the seals just degraded even faster.
It seems to be that with the o-rings you want the minimum possible sealing to help o-ring life. But this doesn't work with my chosen pellets.

Update on the situation. I tried a temporary 'piston seal plug' today using some instant gasket type rubber. It worked wonderfully for about 300 shots. Definite improvement in firing cycle and it had gone back to being pleasant to use. I had a knock down target with 15mm kill out at 45 yards and was knocking it over 3/4 times. Great stuff. Placebo effect? Maybe. But it was working.

However after 300 shots I took the piston out and the 'plug' was starting to come loose round the edges. I think this material was too soft. I took it out and carried on shooting which lasted about 20 shots before I hated the gun again and came home.

Not one to be deterred, I am trying another 'plug' material tonight. Will let it set overnight and build the gun up first thing in the morning. Then chrono check and straight to Furnace Mill for a round of HFT (nothing like trial by fire). If today's effort lasted 300 shots, I'm fairly confident I can make it round a 30 shot course.

If it doesn't work I'll reluctantly go back to the o-ring breech seals until my new piston seal arrives from Australia (no idea how long that will be!)

[cooper_dan]

Also I'm fairly sure the point in the piston stroke that the pellet starts moving changed with the plug in place. Zero had moved 15 clicks high at 20 yards

[TonyL]

Its hard to go back to standard, and I guess I didn't properly explain my reasoning. There are 2 main points:

1- Why did I change the breech seal?
My favourite pellets in this gun are by a clear margin, die 54 JSB exact. I'm talking 12mm groups (10 shots) at 45 yards. They are basically magic in this barrel. However die 54 have a slightly longer skirt than other dies, so they ever so slightly stick out of the barrel (just a fraction)
With the standard o-ring seals, they compress when firing and allow the compression tube face to touch the face of the barrel. Unfortunately this means that fraction of pellet sticking out gets clipped off and leaves lead swarfe behind. This then gets behind the seals/round the comp tube etc and is just a mess.
The one piece Weihrauch seal I've put in there is harder than the o-rings, so the compression tube is always held slightly away from the barrel. I've put about 200 shots through with this new seal and not had any clipping or lead swarfe. So one problem solved.

2 - Why mess with the static compression ratio
Unfortunately the fix above came with the downside of a harsher shot cycle, which to me feels 'slammy' (although I could be dead wrong about this, I just know there was a definite change and I don't like it). I think because I no longer have the squishy o-rings in there to soak up some of the perceived force.
My TX has been short stroked to 80mm. So with the 25mm diameter piston and new breech seal, should be almost identical to an old Weihrauch HW77 or HW97. However its a lot harsher (side by side comparison).
Looking through the rifles, the clear difference to me is that the Weihrauchs have a much shorter and smaller diameter transfer port, and a flat faced piston seal.
Looking at the static compression ratios (simplified calculations), the Weihrauch is approaching 1000:1. My TX200 (including the lost volume in the seal is nearer 200:1. By filling in the seal volume, it rises to 400:1, therefore should get me a bit closer to the HW77/97 numbers.

Basically I'm trying to make small adjustments so that I can keep using these pellets.

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?

[TonyL]

Also I'm fairly sure the point in the piston stroke that the pellet starts moving changed with the plug in place. Zero had moved 15 clicks high at 20 yards

Sounds plausible as pellet release pressure would be achieved ever so slightly earlier in the compression stroke.