Rotating piston & fluctuating power (LGU/LGV, TX?)

Discussion in 'Piston & Spring' started by Adam, Feb 28, 2019.

  1. Cooper_dan

    Cooper_dan Well-Known Member

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    The only help i need is finding more competitions to shoot! Can't get enough of my boingers :p
     
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  2. Adam

    Adam Well-Known Member

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    @Nomads HFT Jim, that is very, very interesting. Was that with standard transfer port or enlarged, and did that make a difference?
     
  3. Nomads HFT

    Nomads HFT Well-Known Member

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    Both standard and 3mm port, Adam.

    I think it may be connected with the compression stroke length, the relatively short barrel, and the mainspring stiffness and preload probably also contribute. The nearest I've found is the TX200 HC.

    .
     
    Last edited: Mar 1, 2019
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  4. hmangphilly

    hmangphilly Can't Re Member

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    20 shots in quick succession should deffo get the heat into a seal

    The std lgu seal seemed relatively tolerant of heat
    But pretty tight to start with

    The oring Pistons we built will drop under there own weight unless the tp is closed off
    Tight para seals is just asking for trouble

    Been there got the tee shirt :(
     
  5. jamy

    jamy Member

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    Hi Adam,
    Have you actually chrono graphed the rifle on the range when this is happening? I've had the same issue with my Walthers and when I get the Combro out, the velocity is where it should be.
    The more I ponder this, and I have done for years, the more I think it's just me and not the gun.
    Food for thought I hope.
    JA.
     
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  6. FPoole

    FPoole Active Member

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    Rotating piston in an oval comp tube wouldn't make any difference because the tight spot is there for every shot. This is assuming the piston is pretty much round as well. A little bit of cant can make a big difference in poi at 55 yards when talking about the elevation. It's very hard to give an exact measurement, but with a half a bubble of cant, I see around 1" drop in elevation at 55 yards. Again, it's hard to be exact with a springer, outdoors, off the knee. I can purposely cant the gun a small amount and see the hits, on paper, go down. I keep my scope mounted bubble level set to the point the bubble rests against the reference line rather than in the middle of both lines. Probably no need to be that perfect, but it makes me feel more confident. Forgot to add, check that the spring isn't bent as I've seen this cause my guns to act a bit off.
     
  7. tillygti6

    tillygti6 Tilly's gun stocks

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    Maybe not, but it can sure make them hold sensitive....
     
  8. Adam

    Adam Well-Known Member

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    Yes if the piston is round that's true. I'm assuming however it isn't 100% round. Or more precisely it's not the piston itself but the seal and/or any bearings. Assuming a piston starts out perfectly round but the comp tube is oval, it may start to wear the bearing to its own shape, i.e. a little more wear on the area moving over the tighter surfaces of the tube. Now you may say if the piston is rotating a little with each shot it should even this out? Yes it should, IF the piston rotates at a fairly constant rate. I'm not sure that is always the case.

    Consider Lancaster oval bore rifling. The bullet is forced into the oval shape of the bore and then follows the spin of that shape down the barrel. That would be one extreme where the piston is worn in to the comp tube so much that it is now effectively no longer free to rotate. The other extreme is the constant rotation. Could it not be somewhere in the middle?

    Imagine that the piston wants to stay in the loosest orientation where it has worn more, but there's still enough rotational impulse to move it. It would rotate a little less each shot when the rotational impulse is trying to move it out of the better fit, and a little more when it has passed the tightest point and is now moving back towards the loosest point.

    Considering this further. Perhaps for factory fresh guns it isn't an issue. The seals are tight and will rapidly wear into the shape of the comp tube. A more tuned gun with a sized seal will have less of a tendency to do that.

    Don't know for sure but it's food for thought. My action is a skirtless piston and it doesn't quite drop under its own weight, but it just does if you add the spring with top hat to it. I tried doing this at quarter turn intervals and there did seem to be one where it was slightly less willing to slide down.

    I'm convinced a non-rotating piston like the HW is the best solution overall.
     
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  9. Adam

    Adam Well-Known Member

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    I think I did once once on the practice range. Very cold day, had been shooting 10.5 initally. Started dropping out of kill on 2 or 3 shots. I had my Combro in my pocket so checked and it was down to 9.5. A few shots later it went back up to 10.3.
    Since then I haven't managed to "catch it in the act", especially as my Combro has thrown one of the sensor covers/filters and it's now a little tricksy if the sun's out.

    On occasion I've had an issue where I think it's been me. If it was just the odd shot randomly I'd blame myself or possibly dodgy pellet. With this there did seem to be cyclical thing and accuracy was still there but zero had moved.
    Sometimes I've had up/down issues and it's been heat on the seal but velocity wasn't affected, particularly last summer in the hot weather. As Rob has said, it can change the pellet exit timing without necessarily changing muzzle velocity.

    I'm working from home today so I'm going to put the rifle in the garage now to acclimatise, then when I finish I'll shoot a string from cold over my Chrony F1. It always seems to read 10-12fps over my Combro but seems to be consistent and my Combro always used to agree with the club's R2A within a couple fps.
     
    Last edited: Mar 5, 2019
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  10. Nick G

    Nick G Active Member

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    If its a skirtless design, is the piston long enough to prevent it from cocking over when fired , ie is it longer than the diameter, and is the latch rod supported in the guide , both can cause friction , but not always consistently.
     
  11. Cooper_dan

    Cooper_dan Well-Known Member

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    I put 40 shots over the chrono today with my LGU.
    Un-weighed pellets, shooting and writing as fast as possible.
    Highest = 764, lowest = 752

    I've done longer strings before with both my TX200's and my Prosport. I've never encountered random peaks or troughs of power like your original post speculated. If there is any chance of rotating pistons being an issue, I don't think I've encountered it.

    LGU JSB Exact 8.4.png

    This is probably going to be an unpopular suggestion, but maybe look a bit closer at the skirtless piston. I don't rate them.
    My LGU was second hand and came with a skirtless piston. It seemed to make power OK at first. I stripped it down and the head of the piston (including the front face of the seal) had quite a bit of lube on. When cleaned off and lubed like I normally would, power dropped to a very consistent 9.3 ft-lb.
    I tried to add some pre-load, but just 2mm extra made the spring coil bound and it wouldn't cock. The only way that setup could make power was by dieseling. Now maybe it was setup wrong from the start, but I just don't trust them. I've seen plenty of people shooting HFT with this type of tune and there seems to be a lot of issues.
     
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  12. Adam

    Adam Well-Known Member

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    Thanks Dan and Nick. I'm open minded about the skirtless piston. I tried it as an experiment, and it is lovely to shoot. However I'm fairly sure I experienced something similar on the original standard piston last summer. As above I'm wondering if it's a potential side effect of using low friction sized down seals on a rotating piston, if the tube happens to be slightly oval.
     
  13. Adam

    Adam Well-Known Member

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    Oh boy! Do you guys like data? We got data! :D
    Just need to mess around a bit with Excel first
     
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  14. Adam

    Adam Well-Known Member

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    Chrono string. Temp was 9C, gun acclimatised in garage for 2.5 hours. 89 shots from cold over the chrono. No warmup shots, every shot was recorded.

    The action was stripped last night, comp tube, piston, bearings and seals wiped clean then re-lubed sparingly and about six shots taken, so the first shots are quite erratic and probably not representative.

    This is better data than I thought. I hoped I'd get a full cycle of dropping and recovery. I got 4!

    upload_2019-3-1_19-9-15.png

    The pattern stands out very clear here. You can see the two power levels of around 745 and 720 and the sudden transitions between them. It gets less clear cut in the middle but then there's a 22 fps drop between shots 59 and 60 and the next 3 in the 720s before back up to the mid 740s.
     
  15. RobF

    RobF Administrator Staff Member

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    Yeh that's not a happy bunny is it. Was that 89 shots no stopping, consistentish shot to shot timing?
     
  16. Adam

    Adam Well-Known Member

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    Yep, from cold, approx 30 seconds between each. Time to type number into laptop, reload at a leisurely pace and fire. Apart from between shot 9 and 10 where there's 2 min gap as an LED light fell off my front skyscreen (elastic band broke)
     
  17. Adam

    Adam Well-Known Member

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    Conclusions? The std action is going back in tonight! :D
     
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  18. Adam

    Adam Well-Known Member

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    Interesting that the lower power is super consistent too. And I've just noticed a slight upward trend of both lower and higher powers towards the end. Could be warming up
     
  19. RobF

    RobF Administrator Staff Member

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    I know you say it's super consistent, and I've no idea what my TX is doing over the chrono, but my Vmach 97 was like 1-2 fps not 10 between shots.

    Although we know that ballistically 10 fps isn't that much we tend to see guns shoot better with consistent MV... the caveat is that if the barrel or gun is doing something horrid to the pellet the MV can be consistent but the groups bad, but they don't tend to be consistent in the direction, they shotgun and throw fliers. If you're drawing lines up and down in paper then in my experience that's power... the cause might be anything but i'd be looking at things that cause power fluctuations first.
     
  20. Adam

    Adam Well-Known Member

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    What I meant is they are consistently around 720 rather than shot to shot, though my eye was drawn to the 5 shots from 35-39 which were within 1fps.
    Should have said noticeably more consistent than the upper power level.
     
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