What to expect, TX short port

Discussion in 'Piston & Spring' started by FPoole, Jun 9, 2018.

  1. FPoole

    FPoole Member

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    I have a TX comp. tube with 3 mm off the inside of the nut. With no other changes, what would this do to a MarkI, MarkII stroke gun? I know it only takes a few minutes to find out for myself, but just wondering what to expect from a shot cycle standpoint.
     
  2. hmangphilly

    hmangphilly The Doctor

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    With no other changes you are going to have problems .
    I assume you have taken the 3 mm from the compression chamber side of the end So you have lost The O ring seal you will have to re-establish an airtight joint .

    If you are successful you will have created some extra volume and it is possible that your Power will have increased which may allow you to lighten the piston or decrease some spring preload

    The so-called static compression ratio should also have increased

    This is an interesting modification
     
  3. Nomads HFT

    Nomads HFT Active Member

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    I carried out experiments some years ago into varying the transfer port length of my old HW77 between 6mm and about 26mm (altering only the TP length and nothing else with it), and proved a general relationship between port length and energy efficiency (shorter tending to be more efficient - up to a point), but with huge caveats, depending on compression stroke piston displacement, in turn dependent on spring, preload, and pellet start point.

    The only way to know the effect is to try it and see.
     
  4. FPoole

    FPoole Member

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    The tube is sealed and I've been using it for a couple of years with an LGU piston at 195 gr. It's a little snappy for my taste, so I'm going to try the extra stroke of the standard MarkI piston. I know this piston is heavier and the seal is supposedly not as good as the LGU seal, but I'll try it. I also don't like the sloppy feel of the way the LGU seal attaches. It has movement on the piston and I feel it in the return stroke after cocking. All the HW type seals I've seen are loose, so I know it works, I just like the TX set up.
     
  5. FPoole

    FPoole Member

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    I finally got around to swapping out the pistons, but it was dark when I started, so no chrono results. All I can say, is that it has less preload and the shot cycle is super soft and calm even with the action out of the stock. Will have a chance to try it out Saturday. Feels like it's doing 600 fps with 8.44 Exacts. It's been so hot and humid here I just haven't had the urge to do the swap. I come in from a long day in the heat (construction) and it's, get something to eat, and chill out on the couch.
     
  6. FPoole

    FPoole Member

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    Got the gun over the chrono yesterday and found it was doing 615 fps with the 8.44 Exacts, so it's way low on power. The seal on the factory piston was pretty tight, so I sized it down a bit and added .25" of preload at the same time. Shooting right at 800 fps now and fairly calm. I don't have the snappy feeling anymore, but it does seem a fraction less quick in the firing cycle. Still very accurate and, when it was at 615 fps, it was amazingly easy to shoot out to 25 M. Very quiet and dead feeling, almost like a pcp.
     
  7. bigtoe

    bigtoe Member

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    You will need a rod 3mm longer as the mk3 stroke will exceed what the action will allow. or go with a mk3 piston and try one of my -5mm snap in seals, in your case it would lose only 2mm as the comp tube added 3mm.
     
  8. FPoole

    FPoole Member

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    Using a MarkI piston, so I think I have around 87 mm of stroke now. Definitely the comp. tube I used would not cock with the MarkIII piston. Some of those will barely cock in stock form with the piston skirt all but touching the rear flange washer. I was amazed how much power I got from sizing the seal down and adding a little more preload.
     

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