Springer transfer ports

Discussion in 'General Airgun Chat' started by Knockedover, Sep 12, 2015.

  1. Knockedover

    Knockedover CSFTA Champ

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    As a fisherman I spend many hours lakeside with not a massive amount to do other than ponder and I got to thinking about the black art of spring gun tuning and all and sundry that goes with it and in particular transfer ports. Now there's a lot of balancing going on with all the factors within a spring gun tune of which the transfer port is one of these and wondered if anyone had played around with a venturi screw, as per a pcp, within the port? Could it settle a gun that has potential but suffers with piston slam? Would it be able to fine tune power without tinkering internally? Could it be another variable to confuse matters further???? :D Or am I barking up the wrong tree completely? :D
     
  2. Tench

    Tench WHFTA World Champion 2016.

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    I think its a great idea, on a rifle that has a basically sound set up it could offer a method to fine tune the velocity. But then I am a PCP guy so there may be other factors I have not considered?
     
  3. Scooby

    Scooby Pete Dutton

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    John Whiscombe used a system on the JW50 where the transfer ports were screw in with different sized holes, they were made from grub screws with holes drilled down the centre. You could then easily change the grub screw for one of a slightly different size, or try tapering or whatever you fancied.
     
  4. Ratinator

    Ratinator Well-Known Member

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    Steyr port

    Could you set it up with a Steyr port with the o ring as the barrel seal?
     
  5. SteveC200

    SteveC200 Active Member

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    I remember seeing a rifle for sale somewhere with a hole drilled and tapped from the top of the action down to the transfer port to take a grub screw which partially blocked the port. So it has been done before but how successful it is, who knows.
     
  6. fireblade

    fireblade Team AirArms

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    Going back a lot of years, i seem to remember this being done but there was an issue with air flow being ruined, so causing large a fps spread.
    Beyone that i cannot remember ever it getting done again.
     
  7. springboy

    springboy Member

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    I saw a post the other day and they were talking about opening up the port of the walther lgu to 3.7 mm I think and it greatly reduced the recoil. But it's one of those areas where the slightest mistake and the fun is scrap.
     
  8. skires

    skires Well-Known Member

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    Firstly ... you need to take more beer with you when you go fishing. Fishing is for drinking and sleeping and getting to wee in bushes ... you really need to switch your brain off and relax.

    The Walther LGU has a small port as standard. They probably use the same port diameter as they use for the German 6fp rifles. So when they put in a longer spring ( for UK 12fp ) or an even longer spring for unlimited markets, too much compressed air is trying to be rammed through too small a port. Some folk say this has overstressed new springs.

    Opening up the port for UK 12fp improves the shot cycle.

    This is probably a similar story with the Weihrauch 77/97. They used a port of 2.9mm. That's fine for low power leves and especially if using very light pellets ( sub 7 gr ) for paper punching at 10m. Using a pellet around the 8gr or above weight and the rifles improves by opening up the port ( up to @ 3.8 - 4mm ).

    Lots of people have tried experimenting with this. Like Scooby said there was the Whiscombe experiment. Jim Tyler and others have tried it using the drilled out grubscrew method. Some have drilled a hole at the top of the action so that a grubscrew can go through and hold in non threaded inserts. Others have drilled out and tapped the port area and then screwed in drilled out grubs.

    I think you mean using a proper Venturi screw ( that always remind me of trying to set up the Weber carb on my old Mini Clubman ) where the screw could be moved in and out to adjust air flow.

    This is fine in a PCP where the air flow is pretty stable and constant ( ish ). In the piston/transfer port rifles there's a massive blast of air going through that port. So I don't know how something in the line of flow would affect the consistency of the output to the pellet.

    I opened my 77 port from 2.9mm to 3.9mm and using 8.4gr pellets the power went up by over 1fp and the shot cycle improved.

    If you read some of the articles by Bigtoe and Wonky Donkey they have pretty much got the piston weight, stroke, spring size and port size worked out for different power levels and pellet weights ( pellet fit and breech design also plays a part ).

    Your idea sounds great, and has been done by physically changing the size of the hole ( grubs /inserts etc ), but I don't know if a Venturi screw would work.
     

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