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Thread: Original 75

  1. #1
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    Original 75

    Been asked to do a bit of work on one.

    Its only putting out 2.8ftlbs. The guy who has it reckons its the mainspring, but I've never had one of these apart. Not entirely sure that this is the problem. The breech washer is a bit knarley to say the least.

    Anyone with some experience with one of these?
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  2. #2
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    It is most probably the compression washer that has disintegrated into dust, a common fault on these with the Giss system. They are not the easiest guns to work on and a spring compressor is highly recommended as there is a lot of pre-load on the three spring system. Make sure that you mark the position of the pinions so that they go back in exactly the same position so that the two piston timing is correct.


    Neil
    Current airguns:- Steyr LG110: Steyr LP10: Air Arms HFT500: Weihrauch97 fully customised.

  3. #3
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    http://www.airgunbbs.com/showthread....piston-systems

    This thread may help, inc links to resources.

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  5. #5
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    I've been repairing these since the early 70s. As has been said the early type piston washers disintegrate and the breech washer can be squashed so as to be inefficient. They will come apart with not too much difficulty but the mainsprings are almost 50% compressed so be aware. Re assembly is critical there has to be a 0.30 mm gap between the cylinder end and the stepped face on the front disc inside the end cap. If you get this even the tiniest fraction wrong you will destroy the innards as soon as you fire it. If you want to have a go I can give you factory instructions and special assembly tool data; to make life easier.
    Not for the faint hearted, but I was importer trained, so got first hand instruction, fortunately. Loss of power is rarely due to mainspring problems, even well collapsed ones will do better than the gun you're referring to.
    PM me if you want info, instructions or help.

  6. #6
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    Just like to add that the critical setting of end cap shims mentioned is easily verified after assembly.
    In truth we always check before dismantling 'cos may have been set up incorrect previously.
    When correctly set the rack & pinion teeth will have back lash with gun not cocked.
    In this state the threaded trunnions can be turned with finger grip after initial slackening.
    If the teeth are under load the shim state can be checked by turning the end cap in increments.
    If unscrewing produces backlash the shim is too thick. If tightening does it the shim is too thin.
    Handy if no accurate measuring device to hand.

  7. #7
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    Cheers Guys
    I'm leaving this well alone....

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  8. #8
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    Wimp! Joking aside they really aren't difficult.

  9. #9
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    Antoni is offline There's nothing cushy about life in the Women's Auxiliary Balloon Corps!
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    Quote Originally Posted by deejayuu View Post
    Wimp! Joking aside they really aren't difficult.
    Replaced the disintegrated seals on my 75 with nothing more than an exploded diagram and said new seals. Had bought the gun for £25 and shortly afterward it could no longer eject a pellet from its barrel. I did not know what I was doing, just took some time over the job.

    Afterward shot a tin of crap pellets through it and was disappointed with the/our accuracy. Then put some decent German lead through it and was very surprised at the improvement in its accuracy. It did about 4 or 5 FtLb IIRC.

    Point is I did not know what I was doing, yet the gun survived for years afterward. Take your time and have a go if you dare!
    P1V1overT1=P2V2overT2

  10. #10
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    Quote Originally Posted by deejayuu View Post
    Wimp! Joking aside they really aren't difficult.
    True as long as one knows what is going on and what it takes to make the masses(pistons) stop simultaneously.
    A spring compressor with a spring guide goes a long way to free your hands to find the sweet spot between loaded and unloaded gears.

  11. #11
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    Well
    Its up for sale £250. Marked up as low power.

    Any idea how much it would cost to repair it?
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