Revisiting piston rings

Discussion in 'Piston & Spring' started by RobF, Jan 22, 2020.

  1. RobF

    RobF Administrator Staff Member

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    For a while I’ve been shooting o-rings and they’ve seemed stable but only in relatively mild conditions. So we decided to revisit piston rings, something we’d mulled over a few years back along with a seal/ring free design (which still made 725 FPS which we thought was pants) and another ring design we’ve yet to play with.

    The easy fix was to use the channel we had in the head for the o-ring and we duly dropped in a ring to fit and voila it worked. Except I didn’t like the feel. So we made one out of delrin which sealed tighter and felt better. All good, plenty of power, job done and I moved back to making the stock fit better.

    As we’ve had a cold snap and some of the stock work was complete we decided to test the rifle in the cold. Leaving it out last night and measuring the temp we had readings of -5 to -7 on the gun. Wearing the same gear as the previous day when I’d zero’d to the click at 25 yds we braved the elements and shot the same distance. We saw a definite rise in the POI from warm and the chrono was also indicating a significant increase from when we last chronod it. The cocking action also felt like it had a lot less friction.

    So we got the mic out for some measuring and selected one of our carbon rings that was similar in size and fitted that and left her to cool. Over the chrono the power was the same, and we were pleased to see that when the gun was warmed to close to 20 degrees the gun was the same speed.

    This would suggest that we might have found the tolerance for the fit and the material to do it with but it’s early days. What we are pretty convinced by is that any material softer than delrin is probably not going to work and you’re looking at tolerances of 1/100th mm if you want consistency.

    I think I’m going to have to leave feel aside for the moment and just rest consistency further for a bit. The hope is I’ll be able to bake the rifle during these cold snaps and get a wide range of testing done.

    If this doesn’t work then might just go to the seal less design and tune the feel other ways.
     
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  2. NeilM

    NeilM Well-Known Member

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    Sounds fascinating, but does this not require a considerable amount of measuring and fitting, for each part of each rifle?

    In other words, this is all great fun stuff for the experimenter, but so heavy on machining skills, material knowledge and time, that the average shooter could either not afford to go there, or does not have the skills to DIY.
     
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  3. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    Sounds good Rob, I wonder what the longevity of the delrin would be, sounds like you need to mod one for someone like Steve Privet who must get through a few rounds per month in varying conditions.
    Along the same lines but using more conventional means I've been testing normal o-rings but with graphite powder as a lube in both a ProSport and Sirocco. Both are still going well after two years and are extremely temperature consistent...;)
     
  4. hmangphilly

    hmangphilly warm ember

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    All part of making /fitting / balancing a piston seal Neil .

    Don't go fooling youself that there's any special skills or knowledge going on .
    It's always been monkeys and typewriters over here .It's a sad and pathetic sight :)

    Great fun though

    Especially the part when 'His Lordship ' ( who has the lurgy ) has been Half an hour sitting outside testing his poi in the dark ,(and minus God knows what temp) and comes back to the shed of love and announces ....

    'Yeah it's shooting a bit higher than yesterday ' (at the indoor club )

    Conversation follows ....
    Shed Chimp - " Yeah thought it might be "

    Chief Chimp - "Why ? "

    Shed Chimp - " Cos I put a couple through the Crony while you were putting the target out "

    Chief Chimp - " Ah what's it doing ? "

    Shed Chimp - " What was it doing at the club ? "

    Cheif Chimp -" 780's . What's it doing now ? "

    Shed Chimp - " More . Where is it ?"

    Chief Chimp - " In the car . Im cold and tired and I feel shite . I 'll try to get to the club and test again . I think I've found out what i need to know . "

    Shed Chimp -"Nah .We cant have that . Go and get it . "

    ................and a further 2 hours of stripping , measuring , guide machining , trigger block modifying ,heating and chronying follows .............

    I'm Rotten I am aint I ?

    And I love it :):)
     
  5. hmangphilly

    hmangphilly warm ember

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    Hi Colin .
    The Delrin was always a material for prototype rings . It's cheap and plentyful for experimenting with.

    Rob found some very exotic ( very expensive ) stuff and managed to get a foot or so offcut . I honestly cant remember what it is called so let's say 'good stuff'

    So the plan was , get a rough idea of sizing with a delrin ring and then make one out of the 'good stuff '.
    I think this is referred to in another thread but ........
    He had had a "good stuff ' piston ring fitted for a short while , but did'nt love the cycle . So wanted to try another idea .........( this experimental delrin size and shaped ring )

    It shot ok and we thought we had foun 'the one ' until last night .

    In fairness , I think there's no real reason why Delrin should be unsuitable as a piston ring , after all it's perfectly adequate to use as a bearing material on the piston .
    Time would tell as to it's longevity , but it's so cheap that it could be regarded as a consumable .

    Any way .
    Watch this space for more hilarius antics . I'm still keen to try an ancient ( new to us ) piston ring design that I've been on about for ages .

    Just gotta sort out a power issue now ............
    but we can do that
     
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  6. RobF

    RobF Administrator Staff Member

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    It's just that it changes size more than the metal around it rather than the special stuff which don't... it's got almost the same coefficient as ally or brass, forget which. Most plastics are in the order of 10-50x more variable. I think the bearing is more tolerant... but now you've got me thining about that as well. Drat.
     
  7. hmangphilly

    hmangphilly warm ember

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    Yeah knew there was a reason you spent the money .

    All seems a long time ago
    Where’s that other thread ? Can you remember ?
     
  8. Adam

    Adam Well-Known Member

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    The key thing with a ring, or o-ring, is the ring's ID should conform to the OD of the piston, assuming it's not too thick or rigid. So even though the material itself expands, the overall expansion is much less than in a traditional seal where the whole front surface of the seal can expand. This is why a parachute type seal with a thin lip and deep groove can be more tolerant of temp changes than a solid seal or one with a thick lip.
     
  9. RobF

    RobF Administrator Staff Member

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  10. hmangphilly

    hmangphilly warm ember

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    Hi Adam

    you mean the minor dia of the ring groove ?
    Not trying to be a twat .
    Just for clarity

    not that I’m not a twat , but that a different matter
     
  11. Adam

    Adam Well-Known Member

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    Yes I couldn't think of how to word it simply, so I couldn't be bothered. :)
     
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  12. hmangphilly

    hmangphilly warm ember

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    I feel a full Ti piston coming on .
     
  13. RobF

    RobF Administrator Staff Member

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    Think we need to... and some other bits. Call it the Titan. I like that.
     
  14. hmangphilly

    hmangphilly warm ember

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    Ti-tarse
     
  15. hmangphilly

    hmangphilly warm ember

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    Ti-tfit

    That's me in me vest >

     
  16. Nomads HFT

    Nomads HFT Well-Known Member

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    Rob - if you or Phil have occasion to visit the Midlands, I'd be more than happy to record the recoil cycle of the rifle, which might give you a handle on exactly what it is that you like or dislike about the 'feel'.
     
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  17. RobF

    RobF Administrator Staff Member

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    Super... yes... sounds like a plan! Thanks
     
  18. hmangphilly

    hmangphilly warm ember

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    nice one Jim

    is there any one up there who speaks southern ?
     
  19. Nomads HFT

    Nomads HFT Well-Known Member

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    Gor blimey Guvvna, nock it in the 'ed an give it summa dat.

    That fluent enough, Phil?
     
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  20. hmangphilly

    hmangphilly warm ember

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    By George . He's got it .
     
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