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Mercury Problem

Discussion in 'Technical' started by springerfan, Dec 31, 2019.

  1. springerfan

    springerfan Well-Known Member

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    Having stripped my Mk3 BSA Mercury and replaced the main spring (Welsh Willy kit) I am having a problem with the rebuild.

    The piston, with new piston washer is really (and I mean REALLY) tight. Had to hammer and grunt the thing back in the compression chamber and it will move when cocking but is terribly stiff.

    I have stopped here because I know there is far too much friction for it to shoot properly if at all.

    I cleaned the comp chamber of all the 40 year old crud and it is now has a a lovely satin sheen to it - not too shiny so that it will glide (theoretically) rather than stick. But it is sticking and feels like the piston seal O ring is too big. It is a BSA O ring for that rifle not just any old ring.

    Does it need lube? if so what? I thought all lube should be behind the piston seal on the spring side with just a smear of molly on the rim of the seal at rebuild.

    Thoughts greatly appreciated.
     
  2. hillbilly

    hillbilly Well-Known Member

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    Hi, You using the nylon piston head ? These usually need sizing a bit to be a sliding fit. If only a bit can be done by hand by wrapping around some sandpaper (I use 150 to 220 grit, then something finer to finish if needed). If very tight, fit to a bolt and spin in a drill using sandpaper -- check frequently for fit. Cheers.
     
  3. shergar

    shergar Keyboard Hero

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    O rings are cheap and readily replaceable, I would try an undersize ring knowing when stretched diameter will reduce. Thread on @bigtoe lubricant will help.
     
  4. shauny

    shauny Top Poster

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    at the point where you needed to hammer it home,common sense would dictate that something not quite right ,surely?
    lube on the o ring for first insertion always a good idea. sizing o rings not such a good idea,as with piston seals.either lube em,or go for a
    smaller o ring:thumb:
     
  5. hillbilly

    hillbilly Well-Known Member

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    Hi, Should have mentioned that my comment above referred only to the nylon head (if not replaced with the alloy head) WITHOUT the o-ring installed as the nylon head can swell to an over tight fit. Good comments above regarding the o-ring if that is causing the tightness - i.e. don’t resize the o-ring. Cheers.
     
    shauny likes this.
  6. PhatMan

    PhatMan Posting Addict

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    springerfan,

    When I fettled my friends 0.22" Airsporter, the 'correct' O-ring was so bleeding tight it wasn't funny - the rifle coughed out ca. 7 ft.lb :(

    The Meteor O-ring is of slightly thinner stock ( 1 vs 1.5 mm ? ), and I put one in my friends Airsporter, and all was less bad :)

    Have fun :)

    Best regards

    Russ
     
    shauny likes this.
  7. jaisalmere

    jaisalmere It’s their hormones

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    I'm having the same problem, I bought a new piston head from Knibbs and it's far too tight, however much i sod around with o ring sizes, I too have a ww kit, but that's not the problem.

    The knibbs piston heads seem to be over size.

    A kind chap on here sent me a nylon piston head , this was a tad smaller but the O ring groove, i found a bit tight, widthways and a a bit deep depthwise, this could possibly due to the fact that, maybe, back when these things were made BSA we're using imperial size o rings and we are trying to cram in metric o rings

    I haven't resolved this yet.
     
  8. shergar

    shergar Keyboard Hero

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    When used in a dynamic situation, the width of the groove requires equal consideration to the depth. Any modification of the groove should take this into account. If machining groove to new dimension, substituting D ring for O ring could be option.
     
    jaisalmere likes this.
  9. Patrick

    Patrick Donator

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    If it's got the nylon head dump it and get the alloy head. The nylon head used to swell after much use that's why BSA went over to the alloy head
    I had a Mercury, Airsporter and Airsporter 'S' back in the mid/late 70s and early 80s but never found the pistons in any of the to be that tight,they would go in with hand pressure. The thing you had to be careful with was that you didn't damage the 'o'ring betting it past the threads on the rear inside of the cylinder that secured the trigger block and the cocking slot never damaged i.e cut or nicked the 'o'ring when pushing the piston down the cylinder especially where the cocking slot ends.
     
  10. trebike0

    trebike0 Busy Member

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    Had a similar issue on a Meteor and so turned down the inner groove that the o ring sits in , this sorted the problem ok .
     
    foxtrott likes this.
  11. springerfan

    springerfan Well-Known Member

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    Thanks very much for all the replies.

    It is an alloy head and one thing that struck me after a rest and a think was that the O ring must be too thick. Rooted around in the bin and found the original and on visual inspection it is a match - Damn!. Will get the Vernier out and do a more precise check though because when in comes to friction fits etc I suppose a couple of thousandths of an inch can make a difference.

    Getting the O ring past the threads was a cow and then the end of the cocking slot even worse. They are tough though so I am guessing it make it through a bit bruised but not catastrophically damaged (wishfull thinking?)

    Letting it settle for 24 hrs has resulted in little change (why would it?) I can move the piston under normal cocking action but it aint right and pushing the piston forward in the comp chamber requires my weight on it pushing the top hat against the floor.

    I am going to have to try and source some thinner O rings I think and do some trial and error.

    Any ideas on a source of O rings? The one I bought was BSA Mercury specific (and no good) so I am going to have to go 'off piste' somewhere.
     
  12. jaisalmere

    jaisalmere It’s their hormones

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    Have you measured the original piston head? measure the o ring slot, depth/ width, you may find the problem there. My new ally piston head measures, 27.60 mm dia,and the o ring groove is 2.89 width and 2.77 deep, if it measures the same as mine, we have the same problem.

    The bay is a good source of o rings, I have been advised, in the past to use Vitron o rings.
     
  13. springerfan

    springerfan Well-Known Member

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    Not done any measuring yet but will do at next sitting. Its time to get some precision going.

    If O ring trial don't work, may go for new head and seal combo.
     
  14. El Caro

    El Caro Keyboard Hero

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    It is surprising the amount of friction a dry O ring can cause. Try a little suitable quality lube. KY jelly is not suitable for this job but Abbey sell a liquid called SM50 which is ideal:)
     
  15. trebike0

    trebike0 Busy Member

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    9E868980-D695-46D4-9431-92608F13E73F.jpeg
    ive tried these in both steel and delrin and the steel gives a better option power wise.
     
  16. springerfan

    springerfan Well-Known Member

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    My measurements are alloy piston head 27.4mm O ring groove width 3.68mm, depth 2.5mm

    Replacement O ring is 3.6mm thick and although the original is a bit tired and distorted, I reckon it is to within 0.1 to 0.2mm of it.

    Current working hypothesis (with thanks to El Caro above): O ring too dry by the time it reaches the compression chamber. The arduous and trying journey past the threads and the cocking slot removes the thin coat of bumslide. SM50 will be bought next and try that. May add a drop into the comp chamber ahead of the piston as a sort of slippery welcome (ooh er missus).

    Not a fan of lube in the compression tube (Deiseling) but it is looking like the lesser of evils at the mo.
     
  17. PhatMan

    PhatMan Posting Addict

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  18. norman

    norman Donator

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    The bay of E can be a good resource...shore70 is the right quality/strength I think...or if you have a hydraulic Co. near you they can help...:)
     
  19. Patrick

    Patrick Donator

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    You can use a piece of thin pvc sheet or celluloid to put inside the end of the cylinder where the thread is to get the 'o'ring in past the threads without damaging it.
     
    trebike0 likes this.
  20. hmangphilly

    hmangphilly Keyboard Hero

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    Your measurements suggest that your oring is under a great deal of crush .


    My guess is it is being squeezed /trapped between the body of the piston head and the cylinder
     

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