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There's a hole in my piston!

Discussion in 'Anything Airgun Related' started by Darren Petts, Dec 30, 2013.

  1. Darren Petts

    Darren Petts Temporarily Alive

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    I'm stripping a Benjamin Trail Nitro Piston and the piston has what I can only describe as a part drilled hole in the business end. There's nothing in the tube that this hole seats on to, it's not quite central, not threaded for anything and seems to serve no purpose. I can only assume some "home tuner" has drilled this for reasons that escape me.
    Before I weld the hole up can anyone shed any light as to why it's there?
    [​IMG]
     
  2. SteveO

    SteveO Top Poster

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    I suspect that's been done to lighten the piston and reduce recoil!
     
  3. Darren Petts

    Darren Petts Temporarily Alive

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    Surely though anyone with half an ounce wanting to do that would remove metal from somewhere non-critical. By making the hole there they've decreased the available compression.
     
  4. secretagentmole

    secretagentmole Low down, dirty and quiet...

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    Perhaps that was why it was done, to lower compression and power and keep under 12 ft lb!
     
  5. Darren Petts

    Darren Petts Temporarily Alive

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    Ahh - think I've found the answer on GTA.

    "Some Canadian models just have a hole in the piston to bleed out air, it's quicker and cheap for MFG's vs special parts"

    Looks like I have a factory bodged detuned piston. Time to break out the welder. Will use more traditional methods to keep it under.
     
    Last edited: Dec 30, 2013
  6. Dag

    Dag Pro Poster

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    If it performed ok before you found the hole, or recess, why weld it up and run the risk of a damaged piston? Surely the old adage of "If it 'ain't broke don't fix it " applies?
    Dag
     
  7. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    That does look bodgetastic.. from the image there doesn't appear to be a through hole anywhere though - just looks blind so it can't bleed air out of the cylinder.

    Looks a bit crap, but then you do now have a ready-made rig for testing lost volume in the piston - would be interesting to chrono it as is, then fill the void with something lightweight (epoxy or similar) and see if anything changes..
     
  8. Darren Petts

    Darren Petts Temporarily Alive

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    Was only running at 10 and a bit ft/lb so there's room to come up.
    The hole effectively lowers the compression. The air doesn't bleed off anywhere it just doesn't build up full compression.
     
  9. Dag

    Dag Pro Poster

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    Cloverleaf's suggestion seems spot-on. Fill it with epoxy or some other heat resistant material.........or did you get a new welding kit for Christmas and you're just looking for the next thing to have a go at?
    Dag;)
     
  10. SteveO

    SteveO Top Poster

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    Does the piston seal get drawn into the hole slightly on the compression stroke, effectively lowering the swept volume of air and decreasing the power output?
     
  11. Darren Petts

    Darren Petts Temporarily Alive

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    Had the welder years.
    The seal doesn't go into the hole. The hole simply provides a place for air to be compressed into rather than down the transfer port.
     
  12. Darren Petts

    Darren Petts Temporarily Alive

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    Curiouser and curiouser.
    Filling the hole in has lost me half a ft/lb! I may have to put it back in!
     
  13. Dag

    Dag Pro Poster

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    Did you weld it or just fill it in with another material?
    Dag
     
  14. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    That's very interesting.. everything else the same (spring, lube, pellets etc)? What did you fill the hole with? Was it nicely flushed with the surface of the piston?

    This result seem to somewhat question the "compression ratio" argument with regard to transfer port size and volume..
     
  15. Darren Petts

    Darren Petts Temporarily Alive

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    Hole was welded with mild wire and finished flush returning the piston to it's normal state. The loss could simply have been the disturbing of the piston seal having been off and back on the piston in almost certainly another orientation. If this is the case then I am at a loss as to why the factory put the hole in as re-bedding in isn't going to recover more than I've lost!
     
  16. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Ahh.. in that case then there are probably too many variables to put the difference in energy down to the lack of hole alone. You'll have increased piston mass (by maybe 10-15g) and as you say you've disturbed the seal. In addition there are the possible effects of heat to consider on the shape of the piston which could well have an effect..
     
  17. Darren Petts

    Darren Petts Temporarily Alive

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    I think I'll experiment with putting holes back in though not in the front of the piston.
     
  18. Akita177

    Akita177 The Absolute State of Britian podcast

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    Is it a gas ram the nitro piston?
    Maybe it needs slightly more pressure with the new slightly heavier piston and higher compression?
     
  19. Dunkman

    Dunkman Posting Addict

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    The hole must be there for a reason but for what exactly??. My suspicion is to provide a small cushion of air at the end of the stroke in order to smooth the firing cycle. From the before and after results it doesn't seem to affect the performance in terms of kinetic energy and my guess would be that on this rifle, the acceleration of the pellet in the barrel is all over and done with before the piston reaches the end of the stroke (as is the case with most spring/ram guns).
     
  20. Darren Petts

    Darren Petts Temporarily Alive

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    It is. There's a M4 hole in the base of the ram as though for charging. I've yet to see any info confirming this though.
     

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