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Early BSA Scorpion fill pressure please

Discussion in 'Anything Airgun Related' started by cloverleaf, Jun 5, 2014.

  1. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    I'm currently sorting out an early .22 Scorpion. When the gun came to me it was doing 14ftlb; peaking at a fill pressure of around 170bar. After some tinkering I have it down to within the legal limit but the muzzle energy is now peaking at around 120-130bar - suggesting a particularly low ideal fill pressure of around 150bar, a pronounced velocity curve and not a lot of shots per charge.

    I'm far from a BSA afficianado but my experience of later models has shown me that they usually like a fill pressure of around 180-220bar, depending on calibre and model.

    Contrary to this I'm also aware that the gun is probably pushing 10yrs old and that fill pressures have generally increased as guns have become more modern.

    The gun is a pretty early example (same stock as the Hornet it replaced) and has a particularly long barrel and cylinder compared to later models. This, coupled with the fact that the transfer port appears untouched (at around 1.9mm dia) and the calibre makes me think they were all like it from the off.

    Any thoughts, experience (or even a chrono string!) would be much appreciated, though :)
     
    Last edited: Jun 5, 2014
  2. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    For anyone who might give a toss, I've just found this post from a while ago on the other side - looks like this chap's (.177) Scorpion is peaking at around 110-115bar; suggesting that this sort of behaviour might be normal for this type/age of rifle.

    The only way I know of to push the peak efficiency pressure up would be to restrict the port - something that can't be done without a replacement for / mods to the transfer port assy on this gun..
     
  3. Billboy

    Billboy Active Member

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    Hi, I dont think the TP needs restricting, all the new models have a 4mm bore in the TP along with a lighter hammer weight. My old t10 which was 7 years old had a bore that was closer to 3mm, I never measured it and have recently sold it so I cant check. You could always get a replacement from knibbs and try openening the bore?

    Hth, billy
     
  4. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Thanks :)

    That's interesting - in my experience restricting the port has always pushed up the ideal fill pressure, with larger ports preferring lower fill pressures. Daystate work around this by restricting valve lift while still using a fairly large port; I wonder if the newer BSAs use a similar approach.

    I could try playing about with the ports but tbh my remit is only to get the rifle working as was originally intended. Were it mine I'd be a bit more inclined to tinker,but it ain't and I don't think the owner is worried about the finer points as long as it goes bang :p
     
  5. Akita177

    Akita177 The Absolute State of Britian podcast

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    Check the exhuast valve spring and knock open valve stem?
     
  6. El Caro

    El Caro Keyboard Hero

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    If you ring JB I am sure he will will help you with all the information you require. He designed it after all and arrived at the settings for different versions. I had one of the rifle length versions when they first came out and before BSA decided to do carbine only ones for the UK. From your description of its present performance it sounds as if the regulating sleeve is either missing or has been "modified":D
     
  7. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Thanks - being fairly unfamiliar with the design I didn't notice that the regulating sleeve / restrictor was indeed missing. It was replaced which maybe upped the fill pressure a little but not massively.. I hear there are two diameters of exhaust valve head used in these rifles - I used the newer, smaller one as I thought that would be the spec to which the restrictor was made; while using the larger, older spec valve would have given very little clearance (maybe 0.05mm on the diameter) between the two parts.

    In other news I was shocked to see that the exhaust valve stem runs inside a solitary PTFE O-ring (rather than within a long, well-fitting bore as is the case on most other guns); which seemed like a bit of a lash up!

    Anyway, ta for your input :)
     
  8. reaper1064

    reaper1064 Engaging Member

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    Have you tried ringing XTX he's a sound guy and I'm sure he will be able to point you in the right direction.
     
  9. El Caro

    El Caro Keyboard Hero

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    Like I mentioned before. If you ring JB I am sure he will will help you with all the information you require. By your own admission you do not understand how the system works. I am sure he would give you the dimensions if you asked so you could use the correct parts, otherwise you are p*sing in the wind.
     
  10. El Caro

    El Caro Keyboard Hero

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    As you seem too shy I just rang JB for some answers for you:D. I was lucky and he picked up. This is what he said about the settings he arrived at for factory production of the 12 ft/lbs UK model Scorpion carbine.
    "Transfer port size .177" carbine 2.3mm, .22" carbine 1.9mm.
    Valve diameter .255" working in a regulating sleeve with a throat of .275".
    BSA used a ptfe ring but that is not ideal. A simple nitrile ring works best. You want something that is flexible. The ring acts both as seal and bearing and allows the valve head to be self centering to seal properly from zero pressure. Unless it floats it will leak. A well fitting bore for the stem will not allow it to work correctly. The design was first used in one-offs during the 70s/80s by me and then mass produced in the Titan Manitou.
    Set up correctly the .177" carbine will produce 50 shots in the sweet spot and the .22" 75. There were a few rifle length, 18" barrel, versions made. These used a smaller port and gave another 20-25 shots.
    Hope this helps get."


    I scribbled down the dimensions as he spoke but think I got the gist of his other advice.:up:
     
  11. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Thanks - tis very much appreciated :)

    The valve and sleeve dims sound very much like those fitted - 0.02" clearance is about 0.5mm which sounds familiar. Interesting and valid point about the valve being able to self-centre; however I'd counter that by saying that using a single ring of any flexible material (esp. nitrile) is likely to cause velocity inconsistancy, while sealing shouldn't be a problem if the valve has been made concentrically :p

    For the record the rifle in question is one of the longer ones - can't remember what diameter the transfer port was, though..

    Thanks again for taking the time to investigate for me :up:
     

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