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Short-stroking shenannigans!

Discussion in 'Anything Airgun Related' started by Tinbum, May 23, 2014.

  1. Tinbum

    Tinbum Delusions of adequacy

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    I had another afternoon to myself today, so I turned my attention back to short stroking Mk3 TX's and prosports. Their 99mm stroke is lazy and although swapping the heavy steel tophat for one of my lightweight ones speeds things up considerably, its still not that efficient.

    I had been thinking that an extended latch rod is the way to go. But with my main priorities being an item that anyone with the most basic toolkit can fit at home for the cost of a couple of tins of pellets, added to the risk of a "hardened" rod failing in the future and surgically removing someone's thumb, it seemed a piston extension was the way to go! I decided on 13mm as a starting point, reducing the stroke to 86mm with a swept volume of 42.2cc. Slightly over the ideal, but a length which would still engage the anti bear trap mech.

    The next question was which material to use? As keeping the moving mass at a minimum I stuck with aluminium and Delrin as test pieces. They are identical in size, but the Delrin has half the mass of the aluminium, so that's what I'm testing!
    Prior to fitting, my Prosport was putting 7.9grn express out at 11.3fpe. With the Delrin extension it dropped to 9.9!! Its inevitable that power will be lost when the volume is reduced, so that was expected. I also tried the aluminium nose to see what effect the extra weight would have, and it went to 10.1fpe. I put the Delrin one back in and added a power washer that brought it up to 10.5 which is where its staying.

    I'm struggling to see a downside to using Delrin in this application. The stroke isn't short enough for any slamming to take place, and used in conjunction with my TbT Drop-in set, the firing cycle can only be described as very sweet! Its a club meet on Sunday, so I'm gonna do my best to pass it around on the zero range and try and get two tins through it before stripping and checking for signs of wear or damage.
    It may be that I will have to start experimenting with stiffer springs to get the power up for the shorter barrelled models, or maybe reduce the extension to 10mm, but its been an interesting afternoon!:)
     
  2. doodsy

    doodsy Donator

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    nice job tinbum, I often wonder if my Diana 52 would benefit from short stroking
    as I think they have a large volume ????

    be nice to hear and update on how things go.

    atb

    doodsy
     
  3. Tinbum

    Tinbum Delusions of adequacy

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    I have a 52 here that I'm gonna make one for tomorrow, also an hw80. Its a dead easy job to fit one, and it makes a big difference to the way the guns shoots. I just need to do some serious testing with this particular one before I can let other people try them.
     
  4. Tinbum

    Tinbum Delusions of adequacy

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    I tossed and turned all night and just couldn't settle. By 6am I was up and playing with the vernier, by 7am I had the Diana 52 and HW80 in peices across my workbench, and all morning I've been testing. The 52 stroke kit is spot on and ready to go as soon as my Prosport testing period is up. I'm not so happy with the way the HW attaches, I may have to stick to aluminium for that one.....

    While I was at it, I made a couple of 10mm noses for the TX range. I reckoned that getting that small amount of volume back would make it work better with the factory spring. The down side would be that the recoil would creep back in.

    WRONG!!:D

    Its shooting better than it was with the 13mm nose. It must be the better balance between spring, volume and preload but a quick nudge and its done, with improved FPS to boot!
     
  5. JD

    JD Donator

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    Tinbum, are you going to be offering these short stroke nose cones for sale ?
     
  6. Tinbum

    Tinbum Delusions of adequacy

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    Yes. As soon as the tests are done I'll start offering them for sale. As yet I'm unsure whether to sell them on their own, or with a modded top hat to increase preload, with power washers or a full internals kit. Any ideas welcome!
     
  7. ste

    ste Posting Addict

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    Looking good mate,
     
  8. Samobaggins

    Samobaggins I own to many fx rifles

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    It would be nice as a full internals kits! I've been thinking of short stroking my TX HC for some time.
     
  9. saddler1980

    saddler1980 Busy Member

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    I'd give people the option mate, you may have customers already having your guide/tophat kit that want the extra's ;)
     
  10. Tinbum

    Tinbum Delusions of adequacy

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    Right folks, I've answered my own question this weekend. The extension has worked flawlessly and is now unmarked after several hundred shots. My only worry is that the over-exuberant public would over-tighten the grub screws and strip the threads.
    It is for that reason alone that I am going to stick with aluminium as a standard material and just use delrin in special cases. Ie, me!:D

    I'll put an ad in sales later.......
     
  11. hairyarms

    hairyarms .

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    Hi Tinbun,
    Could you make the extension from mild steel and hollow it out to reduce the weight,you wouldn't have to worry about the threads stripping so much then?And also with mild steel you might be able to make the extension a friction fit and heat it up to contract it so it never comes off?
     
  12. Tinbum

    Tinbum Delusions of adequacy

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    I've stuck with aluminium as its very light, cheap and easy to work with. Also, its a lot softer than the metal around it, so there's zero chance of scoring the comp tube if it was incorrectly fitted!
     
  13. Tinbum

    Tinbum Delusions of adequacy

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    I did some testing today. I started with my prosport in .177 with standard internals, it was running at 11.4fpe and felt quite nice to shoot. Well, they do don't they!:)

    I fitted an aluminium 10mm extension and two 4mm preload washers on the rear spring guide. This resulted in an average 11.1fpe which is nigh on perfect, and a firing cycle which can only be described as "CLONK!" One movement that's short and to the point. Its a truly amazing difference for such a cheap and easy modification, and I'm sure that some fettling with the TP will allow me to lighten the piston and reduce it even further.

    I'm loving this gun all over again!:D
     
    Last edited: May 29, 2014
  14. hairyarms

    hairyarms .

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    Tinbums Short stroke kit arrived today.It came well packed with some good instructions,I fitted it this afternoon and first impressions are great. The anti beartrap still engages on the last notch so that's a good thing.I will test it the weekend but I have fired a couple of shots and it is a different gun, it now shoots a lot faster and recoil is a lot less.I was happy with my TX before but this mod has improved the firing cycle no end so I am well chuffed,thanks Nick.
     

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