Piston orings

[NickG]

Had a little bit of progress today 😎

I fitted a 2mm oring and cut 1mm groove and was planning to modify from there but no need as we had decent fps and far better consistency than before, still feels harsh to me but some might call it fast.

I ran a test with the same piston and original spring and guide and simply fitted four different seals via a m6 thread, a few shots to settle down and then ten through chrono and took average of the same 8.44 JSB.

Hw 638
Oring 666
Cometa 650
TWChambers 680


This .177 220 has near 50mm of preload and my .22 has 15mm, the triggers are basic and this .177 is firmer and I’m guessing it’s the extra spring tension so I may drop fps down a little and see if it improves break

2 mm section o ring, would need 2.2 wide groove, depending on the amount of crush your running.
The cushion effect comes from the seal , whether it's an o ring or parachute seal, creating a cushion of air at the end of it's stroke.
Just spotted your picture on the other forum , you have too much lost volume in the Allen socket head.

[Shed tuner]

Just spotted your picture on the other forum , you have too much lost volume in the Allen socket head.

fill it with solder

[32:1]

This is all “Heath Robinson” engineering ��

I think the Oring setup has promise but needs to be carried out by someone with some knowledge rather than guesswork and fluke, I think I had combinations of to little and to much crush, all grooves were cut with exact same width tools as each Oring but with various depths, so thank you Nick for the info ��, I left the Allen head lower in case it would/could strike plug ��

For a tinkerer parachute seals are a wee bit less technical and an Oring conversion is best left to someone experienced or someone with lots of time on their hands ��

[32:1]

I’ve only ever used O rings in static applications so I did a google, as you do.

Are folks on here using these types or something else?

That’s good info 😎

[TonyL]

This is all “Heath Robinson” engineering ��

I think the Oring setup has promise but needs to be carried out by someone with some knowledge rather than guesswork and fluke, I think I had combinations of to little and to much crush, all grooves were cut with exact same width tools as each Oring but with various depths, so thank you Nick for the info ��, I left the Allen head lower in case it would/could strike plug ��

For a tinkerer parachute seals are a wee bit less technical and an Oring conversion is best left to someone experienced or someone with lots of time on their hands ��

If you were to persevere and get it to your liking, it's a superb sealing system. I've shot a good few rifles with 'O' ring piston seals by Nick and Mick and they've been utterly sublime. And the '35 that Nick did for me recently is excellent. Very efficient (close to 12ft.lbs with a short stroke and minimal preload), quick but not harsh. And I have shot a good few standard 'O' ring equipped BSAs and Air Arms sidelevers in the past that performed admirably, with power and consistency. And many of these delivered admirably and reliably for years.

Factor in the ultra-low cost of seal replacement, consistency and apparent less temperature sensitivity vs a parachute seal, and there's lots to like.

[32:1]

If you were to persevere and get it to your liking, it's a superb sealing system. I've shot a good few rifles with 'O' ring piston seals by Nick and Mick and they've been utterly sublime. And the '35 that Nick did for me recently is excellent. Very efficient (close to 12ft.lbs with a short stroke and minimal preload), quick but not harsh. And I have shot a good few standard 'O' ring equipped BSAs and Air Arms sidelevers in the past that performed admirably, with power and consistency. And many of these delivered admirably and reliably for years.

Factor in the ultra-low cost of seal replacement, consistency and apparent less temperature sensitivity vs a parachute seal, and there's lots to like.

I agree there’s some promise and advantages about an Oring seal, and the info above about material is helpful too.

This little .177 Cometa is feeling stressed with that 50mm preload and Id like to reduce that to help trigger pressure too, not sure what the 26x67 b&s should deliver but at 8.5ftlb it’s near the redline as they say, weirdly my .22 is mid 9’s and is mild mannered with a nice trigger and just 15mm preload

I thought the original .177 spring was too much and over sprung but going 25mm lrss preload on a spare spring see 70fps drop. All the Cometa’s I’ve bought so far, all .22 have very little preload and why this .177 has thrown me with all that spring

Anyone tuned a .177 220 ?


Chris

[maximus]

I'd say that generally a 177 is 2 ft lbs down on a 22 with the same power plant.

[32:1]

I'd say that generally a 177 is 2 ft lbs down on a 22 with the same power plant.

No wonder this is feeling stressed then 😂😂😂

[T 20]

This is all “Heath Robinson” engineering ��

I think the Oring setup has promise but needs to be carried out by someone with some knowledge rather than guesswork and fluke, I think I had combinations of to little and to much crush, all grooves were cut with exact same width tools as each Oring but with various depths, so thank you Nick for the info ��, I left the Allen head lower in case it would/could strike plug ��

For a tinkerer parachute seals are a wee bit less technical and an Oring conversion is best left to someone experienced or someone with lots of time on their hands ��

As I said in an earlier post, I normally do final sizing by feel as you would a lip seal, the only difference being you have to reduce the O ring groove ID rather than reducing the lip seal OD to reduce the crush.

I always make my O ring heads from Aluminium or Brass now as it's easier than plastic to polish the groove ID in the lathe to reduce the O ring crush.

I also test fit O rings on the head and measure for crush while the head is still in the lathe --- I lock my verniers at cylinder bore size and see if they slide nicely over the O ring.

I'm happy to admit I still cock up and sometimes resort to using the next sized O ring down and stretching onto the head to decrease crush when I've left the groove too big an ID to polish out.

When machining an O ring head I drill and tap the centre of the head to the same thread as the mounting hole in the piston, I then mount the head to the piston with a Loctited stud, so there's absolutely no lost volume in the head.

Keep trying and the force will be with you.

[Shed tuner]

yeah, 2fp down is about right.

26 x 67 compares with an hw99, 26 x 71, but it's generally agreed that 10.5 ish is the max for a nice shooting .177 99. I had one that was still sweet at just over 11, but that's pushing it. (yeah I know with a bunch of tuning you can push that a little, I'm talking generally)

you are 4mm stroke down, so it feels like 9 - 9.5 FP is about right for that powerplant.

Things you can do to help a shorter stroke setup make more power:
- lighter piston, to accelerate faster ( but I'd not go less than say 180g)
- Stiffer spring to get faster initial piston acceleration
- try and wring a bit more stroke out of it

Also, what are the transfer port dimensions ?

[32:1]

yeah, 2fp down is about right.

26 x 67 compares with an hw99, 26 x 71, but it's generally agreed that 10.5 ish is the max for a nice shooting .177 99. I had one that was still sweet at just over 11, but that's pushing it. (yeah I know with a bunch of tuning you can push that a little, I'm talking generally)

you are 4mm stroke down, so it feels like 9 - 9.5 FP is about right for that powerplant.

Things you can do to help a shorter stroke setup make more power:
- lighter piston, to accelerate faster ( but I'd not go less than say 180g)
- Stiffer spring to get faster initial piston acceleration
- try and wring a bit more stroke out of it

Also, what are the transfer port dimensions ?

Excellent ��

The Cometa piston is only 150g with the seal so quite light

So with volume that critical I think it’s why I see the HW seal making a bit less power as it’s a few mm taller.
Not sure I can get more as cocking slot shows very top of piston, I’ll get a couple of different springs and make the nose from brass, I did have a small piece but forgot the measure twice cut once rule ������

[32:1]

Has anyone any knowledge of a DT fitting seal which has a solid front face like the HW

Custom seal show one for a Gamo but it’s 25mm

[xbow]

There are lots of sources on the net for cutting the correct recess for O Rings.
Just one at random:

https://www.sealanddesign.com/techni...design-metric/