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Early Theoben Sirocco - . awful bang !

Discussion in 'Anything Airgun Related' started by 177, Jan 29, 2014.

  1. 177

    177 Donator

    Likes Received:
    Berwick upon Tweed
    OK, before I tear this gun down and get to grips with it, I thought I throw this open to you lot.

    I suspect a worn piston seal/crown (thought I'd get that in early).

    This gun went to America for a new ram to be made by the wonderful Martin Rutterford at Rapid Air Weapons :up: :up: :up:

    It's had a very rough life prior to me rescuing it, but after replacing the breech seal and barrel pivot bolt it settled into a very nice and consistent performer.

    I plan on rubbing the Walnut stock down which is as dry as a bone and shows some pleasant, if unspectacular, grain.

    Using it at Stuart's range a couple of weeks ago I was giving the spinners hell when all of a sudden the gun gave a shocking bang when I shot it. There was an unmistakeable smell of burning lube and smoke in the barrel. The next shot and maybe twenty or so more were all fine, but it's been preying on my mind so I'm going to tear the gun down, degrease everything and check every last bit.

    I suspect the piston seal/crown might have seen better days.

    It remains pretty consistent and then, randomly, you get the odd shot that is a good 30 - 40 FPS faster, and then it returns to single figure consistency again before spiking once more.

    Could be a little lube creeping forward once in a while and burning...

    Anyone encountered this sort of behaviour from a gas ram gun ?
  2. Accuspell

    Accuspell Pro Poster

    Likes Received:
    Uttoxeter, Staffordshire.
    I shall be interested to hear replies, because I have never had any problems with mine (SLR and Taunus) - seals can be made by a firm in Sheffield, if you send them your old seal they will make you a new PAIR and post them for about £20. I will find out where I have written it down.....if I can find my notebook...:D
  3. 177

    177 Donator

    Likes Received:
    Berwick upon Tweed
    A pair of new seals sounds cool :)

    This is unusual behaviour but not unsurprising given the gun's pretty awful history.

    I suspect there's an abundance of lube in there which probably isn't helping.

    Just going to source a Schrader adapter for my pump before I tear the gun down.

    It's been a while since I had one of these in bits - from memory there's a big o-ring at the back. I'll more than likely replace all the o-rings and reassemble with sparingly applied lube and see where things stand.

    Maybe the seal is in good shape - maybe it's over-lubed...

    My Fenman went 19 years straight and never missed a beat until I was stupid enough to sell it :(

    For all I know it's still (very probably) going strong - what was I thinking :rolleyes:
    Last edited: Jan 29, 2014
  4. paul finlay

    paul finlay Banned

    Likes Received:
    Hi , I'm really interested in this , re my thread , my new evo makes a loud bang on firing , sounds like a springer fired dry , no pellet , atb paul
  5. andy draper

    andy draper Posting Addict

    Likes Received:
    It could be the seal worn,and every so often oil/grease gets in front of it causing it to diesel and ignite,this would explain the extra fps you get every so often.If it has the sticky tar stuff in it,when it gets hot it tends to melt apparently and could be seeping past the worn seal as it liquifies.Does it only do it after you have been shooting it for a while? if so this could be the reason.
  6. 177

    177 Donator

    Likes Received:
    Berwick upon Tweed

    Just to be clear, mine does this very infrequently.

    The 'big bang' has only happened twice so far - both times after a few dozen shots.

    Intermittently though, the FPS spikes for one shot and then settles back to boringly consistent.

    I've said from the off that I think the piston seal is at fault and lube is getting to where it ought not to be - if I get the time I will tear it down next week and see what I can do with it. My schedule just went nuts though :rolleyes: so it may get shelved for a week or three...

    If mine was as loud as hell every shot I'd stop shooting it to prevent terminal damage until I figure out what's going on with it.

    At 10 - 11 FPE they shoot beautifully, provided they are mechanically sound and the seals are in good shape.

    Usually they will go for years and years. Almost all of the problems I have ever seen with rammers is where some nugget overinflated one. This tends to burn the seal very quickly due to heat (FAC rammers have a different piston seal/crown and, in some cases, an entirely different piston) and when the seal gets burned, power tends to drop. Most people's reaction at this point is to simply (over) inflate the gun again, so you end up with a gun inflated to ridiculous levels, way into FAC territory, that generates far more heat than it ought to, which damages the seal/crown still further, and when power drops again the stock response will be to once more inflate the gun, and on it goes.

    So in an ideal world X ram pressure = 11 FPE

    Worn seal requires greater than X pressure to = 11 FPE

    An even more worn seal requires still more pressure to = 11 FPE

    Each subsequent (over) inflation simply exacerbates the problem by generating more heat and the ram has to work harder and harder for a steadily lower return.

    Quickest way to kill a gas rum gun IMHO...

    Time will tell with this one - truth be told, it's been almost twenty years since I tore one of these down and I can't even remember whether I used the same lubes as a springer, so I'm not touching it until I research it a little and until I get a wide window of opportunity.

    This is not a job to rush.

    Luckily for me this is not my only gun :rolleyes: ;) :D

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