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Thread: BSA Mercury - Worst strip ever! + what replacement piston seal?

  1. #1
    Join Date
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    BSA Mercury - Worst strip ever! + what replacement piston seal?

    Just pulled apart a BSA Mercury, early Mk2, worst strip ever, its not been apart since 1975 or whenever it was built!

    First off, I struggled to unscrew the rear block. Put some heat on, then some more heat, then some internet to make sure there is no hidden pin or screw, then more heat and a big adjustable wrench and a copper face mallet.... there she goes.

    Cocked the piston back with the linkage, as its sticky, the main reason for the strip, and with the rod out of the rear of the action an inch, thats it!
    Finally realised that I could jack the piston out by clamping the piston rod in the vise, and using the barrel against a long piece of wood, bit by bit!

    nearly 2 hours!

    And how many seals does it really need? An O ring and a leather washer and a circlip? (leather washer crumbled) Is there a better solution for a replacement?


    Thanks!
    Thanks - Geoff.

  2. #2
    Join Date
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    sounds like you have the white nylon piston head which has the steel ring then an O ring then the buffer washer behind it which believe it or not was once upon a time rubber they always look like old nasty oily leather when they are stripped due to age the nylon heads always seem to be tighter than the later alloy ones, some say the nylon heads expand over time causing a tighter fit, I have in the past just wet and dry sanded them until they slip in nicely, keeps the piston head smooth too

  3. #3
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    Blimey! The rubber has totally failed!
    Yes, think it was the nylon head that had swollen to be a tight fit. After doing a bit more searching, I have found that replacing the entire head with ally at a slightly shorter length, the slight increase in stroke is not an issue to help air cushion the piston at end of travel.
    Thanks - Geoff.

  4. #4
    Join Date
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    Hi Geoff,

    This is a common problem with the Airsporter and Mercury. POD at the club can fix them though - he did with mine.

    Kind regards,

    John

  5. #5
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    I do these every month. The nylon heads expand because they absorb moisture. BSA should have conditioned the nylon before getting final O.D. I take a skim off them in the lathe, sometimes you need to deepen the O ring groove because that also becomes shallower. I use a thick polyurethane washer for the buffer washer, but you can use a standard rubber tap washer that is available if you open up the centre hole to 10mm.

    Baz
    BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD

  6. #6
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    Hi Geoff,
    I knew you would enjoy her (I think?) :-)
    Rich
    WANTED: Next weeks winning lottery numbers :-)

  7. #7
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    Hey mrgeoff,

    Yesterday (Sunday), I picked up a cheap, but very tidy Mercury Mk2. It had the usual symptoms of being very stiff to cock - where have I heard that before? Anyway, went through the same difficulties apart from the initial releasing of the trigger block, a sharp blow with with hide mallet did the trick fortunately. But, getting that piston out was frustrating and time consuming. The stock and metals are in good condition and looks like it has never been apart as the stock screw heads look as new. Probably, it has not been used for years due to the problem.

    All the best.....

  8. #8
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    Geoff, 28mm diana seals with a suitable piston nose adapter threaded onto the piston spigot make for a nice, refined firing cycle, and give you the option to increase stroke - well they do on an airsporter, as there is about 7mm of clearance from the cocking slot in the piston to the cocking shoe, but you'd need to measure the merc's clearance first. But even if you keep the stroke std, it'll still be a nicer firing cycle and less hassle than the overly complex BSA arrangement.
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

  9. #9
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    Quote Originally Posted by mrgeoff View Post
    Blimey! The rubber has totally failed!
    Yes, think it was the nylon head that had swollen to be a tight fit. After doing a bit more searching, I have found that replacing the entire head with ally at a slightly shorter length, the slight increase in stroke is not an issue to help air cushion the piston at end of travel.
    Quote Originally Posted by averageplinker View Post
    Hi Geoff,
    I knew you would enjoy her (I think?) :-)
    Rich

    Some fat bloke owned this rifle for about 5 days and instead of servicing it, he past it on at cost.

    Clever them fat blokes

  10. #10
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    its all good fun, and a nice learning experience, will be a lovely thing for sure when finished :-)

    I want to drop the nylon head for sure, still un-decided on what piston seal I will run.
    Thanks - Geoff.

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