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Airsporter Mixed Model??

Discussion in 'Technical' started by Wev, Nov 8, 2021.

  1. Wev

    Wev Member

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    Guys and gals, I'm new here. Have an Airsporter (bought 1970) and a Diana G36 (bought about 1968). After 45 years working abroad, when the guns lay in various attics, I'm back. Let's get the guns going and pretend its 1970 again. Decide to change the consumables, as engineers do. Airsporter serial No is GI 1640 so, (thanks to this forum) that's a MK IV, 69-71. Yup, date checks out. No auto-breech opening; non-tapered barrel; non-tapered breech trunnion; flat lever (not like the lovely curvy one on the Webley); one-piece slab-sided beech stock without the groove; visible line between chamber and breech. Check, its a Mk IV. So, breaking the First Rule of Engineering, I order a seal kit before I open it. To save delivery time. Bits arrive, look just like the pics on the exploded diags for the Mk IV. So clever I am!

    Opened it today. Piston has a conical nose and elastomer lip seal. No flat-faced piston head. No o-ring. No fool like an old fool. Poked inside with a welding rod - there is a conical recess at the breech end of the chamber. So the piston matches the chamber /breech. So, as I understand it, its pneumatically a Mk I or II. (Is that right?). Which is good news (a better pneumatic design). Apart from my redundant seal kit.

    I know people do mods and then years later forget they did them. In about '74 I bored my old Bonneville out to 750, then the next time I blew it up absent-mindedly bought the 650 pistons; very easily done with your mates over a few beers I'm pointing no fingers; and Ive seen it done with gas turbines (that wasn't me). But Im struggling to believe that my Mum took the gun from the attic while I was in Nigeria, bought and fitted a new barrel, chamber and piston, and forgot to tell me about it. She was a dark horse my Mum and a bit forgetful, but still ...

    So, is this common? Did the BSA factory keep less-than perfect records, making those neat model ID sheets somewhat approximate? Or did the fairies upgrade my gun for me? Or am I missing something?

    And what's my best move now - renovate it as a Mk II? I'm not doing any cosmetic work, but would like it mechanically nice. I'm an engineer but know nothing about airguns as such. Would very much appreciate to hear from people here that do.

    In a few weeks time I'll start on the Diana G36. In those days she was a light and fast 177, real beauty, I want to see her perform again. Anyone out there got one??
     
  2. Lightning22

    Lightning22 Busy Member

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    Looking at Chambers the Mk3 also had a conical piston, BSA is known for it’s parts bin approach so I’d be guessing at a ‘transitional’ model with a Mk3 action and Mk4 trigger block and serial.
     
  3. paj

    paj Posting Addict

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    There were at least 2 versions of the conical piston head. Nominally fitted to Mk's 1, 2 & 3.
     
  4. shergar

    shergar Big Poster

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    Some pencil marks, or similar indicator, on conical nose then rotation in chamber at transfer port should confirm if a suitable match.
     
  5. Mike Watkins

    Mike Watkins Woody Woodgate

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  6. Wev

    Wev Member

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    Aha, thanks. Clear. Looking closer the seal is actually leather. 50 years of marinading in heaven-knows-what my brother and I squirted in there have left it looking like a rubber or silicone compound. I’m assuming my shiny new O ring will do a better job? I’ve worked as an engineer for 44 years but have no experience of leather seals.
     
  7. paj

    paj Posting Addict

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    Hi Wev,:welcome:. What o-ring have you got? Is it the one to go on Nyloil piston head? Pics.
     
  8. Wev

    Wev Member

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    Here’s a pic. I take it the nylon buffer washer goes behind the O ring
     

    Attached Files:

  9. shergar

    shergar Big Poster

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    Check if there is a nut in the piston holding a latch rod with rolled threads.Maybe covered by a washer jammed in place by end of spring. If so there will be two grooves in nut for key to screw/unscrew. T R Robb's website gives good details of conical options.
     
  10. oilyfinger

    oilyfinger Newbie

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    J have an Airsporter SNo. GL 10447 which indicates a MkVI. It shares some parts with a MkV though especially the rear site.
     
  11. Wev

    Wev Member

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    It seems its a nylon head with a steel nose and a leather washer. I see on the T W Chambers website they sell a "Nyloil" piston head assembly with O-ring. I assume its a modified head, to take an O-ring instead of a leather washer. I also see T R Robb advises against that and offers a PTFE washer to replace the leather one on the original head. It does look a better engineered option to me than the O-ring replacement, and being flat-faced does not add to the dead cylinder volume (the volume in front of the curved O-ring front face that doesn't touch the cylinder end). Any experience out there on either of these?

    As an aside, Im surprised the later, flat-fronted floating head works at all. The pivot is well behind the pressure bearing face so it is unstable, it will cock in the bore as pressure bears on it. As the piston in a piston engine does, and is designed to cope with this by having sufficient skirt length and excellent lubrication, lacking in this case. Do people find the head cocks in the bore, and jams, or smears aluminium on the bore to damage the O-ring? What is the advantage of a pivoting piston head? Sorry, this is all new to me.
     
  12. Lightning22

    Lightning22 Busy Member

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    I don’t think it is intended to flex per-se, the buffer won’t really allow it (until it turns to cheese of course). The pin is more a retaining device.
     
  13. Wev

    Wev Member

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    Ah, of course. So why the separate head - to avoid replacing the whole cylinder if the head is damaged? Or to reduce piston weight?
     
    Lightning22 likes this.
  14. Lightning22

    Lightning22 Busy Member

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    There’s a variety of piston heads, export models had steel and I’ve got what appears to be a factory short-stroke alloy head. I’d guess at one piston with interchangeable heads to suit whatever market the rifle was being shipped to.
     
  15. paj

    paj Posting Addict

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    Re; post 11. Check Knibbs I think they have the original pattern leather seals for both conical piston designs.
     

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