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Old 24th August 2016, 01:25 PM
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Default Effect on temperature on springers...

Removed wobbly bit from equation to see how the shift from 785 uk at 20 to 711 at 42 degrees could have occurred.

Stacked some 300g weights on end of latch rod sitting on top of comp tube until it moved under the weight them and gravity.

20 degrees required 1500 to move it freely

30 degrees required 1800 to move it about the same

40 degrees required over 2400 to move it the same. Ran out of weights.

We could get more accurate with this but the correlation between friction of the piston and muzzle velocity and temperature concurs with what I've seen.

Measured comp tube at 30 and 40 degrees and piston at 8 points to 0.01mm. No perceptible change.
Measured piston seal which was awkward and highly error prone. Seemed to indicate it shrank in cold. Really need a durometer as well.
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Old 24th August 2016, 04:38 PM
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I have read a lot of the posts in the past on this subject , but not experienced it myself,
On a similar note I have been struggling with a 21mm conversion, I can get it making the power but am dangerously close to detonation, It would shoot around 50 shots then the piston would tighten slightly and the power would go down, ( I am using an o ring skirt less design made from bronze running a tight tolerance to avoid detonation).
I decided to line the rear 75% of the piston diameter with derlin and run that % of the piston with more clearance , I lubed with very light oil (sm50), the power jumped to 12.4 ft lb, I had to remove 5mm of preload( which brought it below the point where the lube detonated),
The point is friction is very bad for springers .
All my guns bar one run small section o rings could this be the reason I don't see the temp shift problem, or have I just not experienced a high enough temperature .
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Old 24th August 2016, 04:46 PM
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What about the opinion that piston should falls under it's own weight down the cylinder. Movement can be stopped by putting finger on transferport.
See http://www.airgunforum.co.uk/forums/...m-light-weight

Sorry could not find better example
If so the influence of temperature on will be mutch less

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Old 24th August 2016, 05:13 PM
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The free falling piston, is not indicative of what the piston or the seal is doing when under pressure, I find a little more resistance is better as it inhibits bounce
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Old 24th August 2016, 05:43 PM
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A subject close to my heart living down here with typically 5-40C winter-summer temp range.
Having chosen the TX platform for both my FT & HFT springer rigs I have spent more than two years experimenting and testing numerous "tunes" / components to measure basically two key criteria:
1. Reduced sight picture movement (i.e. the ability to view the pellet flight and strike point)
2. Manageable consistency (fps) through cold / hot conditions
Any other subjective factors (feel, noise, twang, etc.) have largely been ignored, with the exception of durability (e.g. service, seal replacement, lube intervals).
I have for now settled on Nick G's lightweight, short stroke, o-ring pistons.
With the right lube regime (another journey akin to the quest for the Grail!) they deliver a winter-summer variance of around 10fps (conveniently 1 skinny washer above the top hat, or with weighed pellets no issue!).
In my testing, everything with a standard type piston seal (LGU, HW, AA, V-Mach, Maccari, Vortek, Aus Custom) seems to expand / shrink considerably, necessitating recalibration. As do the Vortek o-ring noses (which seem to have lousy crush calibration in the first place!).
Happy to share as I could have saved a lot of frustration, time (better spent practising) and indeed money if I had found this sooner and enough math with the scope behavior to keep the brain cells exercised!.
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Old 24th August 2016, 10:09 PM
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a nice straight and round comp tube is a good place to start.

look at the cross hatching on a std tube and think ...' good effort AA , this has been nicely honed ' .......then put a bore gauge down it ........

cross hatching means nowt .........i can do that with wet and dry on a stick .

i had 2 tubes properly honed .....1 of them is loads straighter than a std tube,..... the other is even better !!!
of course both are a little bit bigger in the bore now

robs std tube , tight at the entrance and looser at the end plug end ......got the numbers somewhere ....

oh ........not so round either


these para seals deffo expanding with temp and causing increased drag.........bit of a sizing learning curve in progress

Last edited by hmangphilly; 24th August 2016 at 10:13 PM.
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Old 25th August 2016, 02:00 AM
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Funnily enough I'm just on with the same problem.

After fitting a short stroke kit with a hw piston seal and using the" prescribed" lube, I was getting around a 100 fps variation after around 50 shots.

The piston had the perfect resistance whereby with just the added weight of the spring it would slide slowly down the comp tube. I thought it was a breech seal problem but after replacing everything came to the conclusion that it was something to do with the piston seal and friction.Turned out the super duper"prescribed" grease ( 5 for half a teaspoon ) is garbage.

At Devon it chrono'd at 635 fps after about 20 lanes,( Maybe cos it was a tad warm eh? ) I'd set the gun up at 760 fps. Iv'e only had the gun a year so I'm only just learning the in's n out's.

Now after reading all the forums and gleaning all the information I can , I decided to do exactly the opposite.

I decided on a V-Mach washer. I didn't "size" it, I just bunged it in, and my, is it tight. I have to pries it out with the Allen key and it dint's my hand when I shove it in.

Now I haven't really started with lubes as yet but this seems to be, in my opinion, the be all and end all of spring guns.

At the moment I'm using Copper slip. WHAAAAA I hear you say. But iv'e always used it and I personally think it's one of the best all round lubes. ( It's all Iv'e got ).

After 3-4 shots at 790 fps it settles to a constant 780 fps for around 100 shots ( counted ) then slowly starts to drop off much like a PCP but not as pronounced, and over the next 100 shots ( counted ) will not lose more than 20 fps which doesn't seem to effect my POI, then I have to repeat the lubrication process much like recharging a PCP.

You can actually feel a slight but subtle difference in the gun when it requires re lubing, mainly when the cocking lever is pushed forward after cocking.

Iv'e only had to do this twice so far in the last week or so. So the V-Mach washer has only shot around 400 shots so far and isn't bedded in yet, so it's early days.

It's a bit of a ball ache, but the gun shoots snappy, smooth as silk and virtually recoil free, and more important a constant velocity.

I refitted the hw piston seal and it shot God awful compared to the really tight V-Mach seal. (As a matter of interest. ) It seemed a lot more "mechanical" if you get what I mean? And the V-Mach developed more kinetic energy.

So is a really loose, friction free seal which everyone seems to strive for the answer?

Iv'e ordered some Silicon oil and Moly slip to try and Iv'e also got some Silicon grease to try.

Meanwhile I'll just keep re greasing with copper slip every 100 counted shots, after all, on a TX it's not much more complicated than recharging a PCP.

And because of the massive resistance of the piston seal it certainly won't be as susceptible to extreme temperature shift as a loose piston! ( I hope ).

Wot wi temperature shift of guns and temperature shift of scopes, no wonder I can't bleed'n hit owt!!!!!!!!!
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Old 25th August 2016, 07:26 AM
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The idea behind low frictional losses is to use as little spring as is necessary to make the power, even an over tight guide whether in the spring or on the piston rod will rob the gun of power.
I agree with the "true" comp tube , though I haven't come across a gun yet that has suffered from low power when installing my set up, I don't bother measuring them as I haven't the kit to sort it out, my bore comparator won't go down that small. I have telescopic gauges but its a bit off a faff.
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Old 25th August 2016, 10:02 AM
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Nice to see some consensus...

Phil skimmed a seal yesterday and we developed the TTT. Two Tin Test. We found that was consistent over the chrono from 38 degrees down to 22.5 degrees. Same seal, just not so tight as original. Equates to about 600g deadweight to get it moving. We're not sure if less would be better... we didn't try it... at some point the losses will be due to the seal not sealing... but we now think the seal can seal too well and friction becomes the enemy. With the apparent swelling of a seal, the friction has more effect than the seal and the power drops.

We've got a couple of comp tubes which have had some treatment which have bore consistency better than 0.005mm down them. Which is better than some of the standard ones which have had variations of more than 0.2mm.

So the gun is back together, and locked down at 770 with Exacts.

The idea is that i'll just be shooting this when I see a temp change and noting any POI and speed changes. We're running it fairly dry, so there's no lube consumption to deal with.

Odd thing was after this, the batch that shot well shotgunned, and the one i rejected was much tighter. It was that dramatic that I thought i'd taken the wrong batch out to Portugal, but then I remembered putting in some fairly tight groups. Sat beside Calps one day for windage (because that seemed to move with the POI shift)

Shot Jon Budd's 22mm on day 3, which has super small sight picture movement (like a walther PCP) and was consistent all across the day. Only catch was it ran at 8.9ft-lb/689 so my windage estimation went a bit do-lally, and i had twice the clicks out to 55. So like shooting a .22 springer. But if Brian and Jon can get it up to 750 without affecting either then they'll be onto something.

Back to PCP for comps now. Honest!
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  #10  
Old 25th August 2016, 11:18 AM
hmangphilly hmangphilly is offline
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the TTT is about to become the OTT




going .............




.......going........



......gone.......





errrr......... sorry the pics are sideways .........dunno how to fix that ...........boffin required , if you know one .............

forgot to say .........that tin is empty

Last edited by hmangphilly; 25th August 2016 at 12:22 PM.
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