Swept volume.

Discussion in 'Piston & Spring' started by Tinks, Oct 24, 2018.

  1. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Unfortunately not, the added 40fps came with the softest spring as previously it gave me 690 jsb and 670 superdome, other springs give very similar results to previous tests with maybe a +/-10fps, so nothing conclusive.
    I put the standard internals back in the gun yesterday to check all was ok at the breach and have actually gained 10fps from when they were removed, so am pretty sure no leaks at breach.
    So in short I still have to prove/disprove the piston sealing, excessive friction or something wrong with comp tube. Comp tube will be the biggest unknown as I only have one to play with.
    Thanks again.
     
  2. hmangphilly

    hmangphilly The Doctor

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    How are you achieving ths 23 mm tube ?

    Some kind of removable sleeve ?
     
  3. Tinks

    Tinks Another tricky target at Lincs.....!!

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    The sleeve is made from 23mm ID CDS precision tube with the OD turned down to fit the original comp tube and fixed in palce with loctite.
    I have pressurised the tube with the piston in place by pushing the piston almost to the end of the tube with a sash cramp and leaving it for 5 mins after which the cramp was released and the piston returned to the start position within a mm or so.
     
  4. hmangphilly

    hmangphilly The Doctor

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    Sounds quite a faff to remove it to revert to original internals .

    Cleaning the loctite off would be great fun


    Any way

    Assuming you now have a free moving piston with no dragging
    Decent piston and breech sealing
    Cyl liner sealing well
    Enough piston weight that it s. It bouncing hideously

    I'd look toward your sooper sooper barrel leade as a possible culprit for the low power

    Giving low peak pressure

    You can Sometimes get a clue by flaring the skirt out with the head of another pellet.

    It makes a big difference with reduced oring tunes

    Might not show with a parachute seal
     
    Last edited: Nov 1, 2018
  5. hmangphilly

    hmangphilly The Doctor

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    Check out the 'walther look see' thread around pages 5,6,7
     
  6. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Sorry if it was unclear, I have a the original internals from this rifle (comp tube, piston, spring and guides) which I put back in to make sure I hadn't knackered the lead in with my removal of visible light around a thumbed in pellet. Only change was +10fps so if anything, it seems better. Do you have any recommendations for barrel lead in geometry, angles, depth etc.

    Piston seems free enough as it now falls under it's own weight (currently 160g) and testing for leaks has not shown any, have tried various weight of piston from 130g - 205g, no real improvement.

    The modification of the TP was to remove the void created by the above (as I was concerned this would create a peak pressure drop) I have tested with this in and out and cannot see a noticeable improvement. Also I made a range of TP sizes, coned (as in my photo) and parallel (to replicate standard) between 3.2mm and 3.8mm, again no significant change apart from pronounced piston bounce with the smaller diameters.

    I still have a suspicion that the o ring gives up the seal somewhere near peak pressure but have no way to find out.

    I think I will have to strip my competition gun and try this 23mm set up in a different action and barrel, I have been putting off doing this as a last resort, but think now might be the time.

    What is the reason it might not show with a parachute seal, only asking as the standard internals use one from Protek (softer than the HW original and thinner lip).
     
  7. hmangphilly

    hmangphilly The Doctor

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    We've been pretty happy with a 45 degree leade on a springer .
    Gives a nice pop foa long stroke reduced bore .

    Yours looks like a 2 stage leade . assume the entry is 45 and the secondry is shallower .
    Noticed big power iussues with shallow leade

    Not too bothered about eliminating light around the pellet so long as the leade is concentric .
    Quite happy with a nice big tp chamfer it still worked 5 or 6mm tp at 3.0 mm
    This 21mm lgu would smash 12ft lb easy with 90 or so stroke . You do the maths for cc's. Next to no crush ( sub 0.2total ) on the 2mm section o ring ,cant remember the numbers , it's probably in the thread

    Not a fan of the tp contacting the barrel . I know Nick likes it .
    But heres what happened to an ali end plug after 2 dry misfires on an lgu 21mm.
    The lost volume here will deffo cost power lgu plug 2.JPG lgu plug.JPG

    Don't know why pellet flring didn't do much with the para set up but didn't pursue it much as that tune had so many other issues any way .
     
  8. Nick G

    Nick G Active Member

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    My point with the port contacting the barrel, is that once the piston starts moving it will contact anyway, if its in contact to begin with it stops it hammering into the barrel, but each to his own :). I don't worry about a lead in as such, I put a small polished chamfer in the end of the barrel, this ensures no burrs , and once the pellet has been pushed flush with the barrel the skirt is forced into said chamfer sealing it . Its worth a lot of fps in the 21mm set up. Like you i have seen 12 fpe and more in 21mm @ 32cc.
     
  9. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Hi Phil, thanks for that, read the walther post again and see what you found about the lead.
    You did mention your build was stuck at 700fps and then started working as you wanted, can you remember what it was that was holding it back? (stp and moly powder??)

    I stripped my competition gun :eek: and swapped the internals into the action with the new barrel lead, 775fps superdomes and 780 fps jsb 8.44 (30fps above where it was minute before removal), removed 4mm preload and now back near where I like it. For info this is a 26mm piston, short stroked to 77mm and running an o ring seal.

    The downside is, with the 23mm set up installed in my other action it still stalls at 700ish fps, this makes me think it's not the barrel/breach.........

    Nick, the positive lock up on the breach with coned TP is basically a reverse of the FWB300s breach arrangement and I found with mine that removing the seal only cost 10fps (admittedly mine is low on power) which was a pleasant surprise having had a breach seal go mid competition and finding out later I lost 100fps!
    I wouldn't mind altering the standard comp tube to have this threaded TP for experimenting with, it would confirm the arrangement works/or not.

    Looks like another busy day again tomorrow...
     
  10. hmangphilly

    hmangphilly The Doctor

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    Reading back through the walther thread it looks like I had a combination of piston skirt mushrooming at the head .

    Pellet pop made a big difference .
    And the t p pushing through into the comp tube after a couple of accidental dry fires ( easy on a lgu)

    Check your piston goes flat to the end with some blue or plastigauge Or summat


    So what s your setup ?
    23 mm x82 @160 g ?

    What spring ?
     
  11. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Do you mean check piston with spring in and under preload to ensure it is aquare to the end of the comp tube?

    23mm piston with threaded latch rod so can adjust stroke from 78mm to 85mm, currently set at 82mm (could go out to 88mm but this means woodwork alteration on the stock, testing at 85mm makes no difference to output, but shot cycle is a little smoother)
    Bare piston weight is 150g, this can be changed with the addition of bronze washers to replace preload washers and go to 205g, I have found that 160-180 seems .
    I tried several springs but the ones below gave the best results so far:
    1. 3.1 wire, 20.4 OD, 25 active coils, 225mm long, 21mm preload,180g piston = 700 superdome, 700 jsb (standard 97 minus a couple of coils)
    2. 3.0 wire, 19.0 OD, 23 active coils, 205mm long, 18mm preload, 160g piston = 720 superdome, 710 jsb (this spring will not last long at that even if it did make power!)
    3. 2.9 wire, 20.9 OD, 22 active coils, 216mm long, 30mm preload, 160g piston = 700 superdome, 700 jsb (noticeable piston bounce with this one)
    All above with 3.5mmTP, I have to make some new TP insert to suit this other action if I want to change this.
     
  12. hmangphilly

    hmangphilly The Doctor

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    Yep check the piston issquare and actually goes all the way to the face of the end plug

    I managed to form a pip that held tbe piston away from the face

    Prob 0.1 or 2. But lost stacks of power
     
  13. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Checked the piston with a thin film of grease on the front face and saw a horseshoe shaped contact area around the outside of the piston, cleaned it off, rotated piston 180 degrees and repeated. Same horseshoe shaped contact area, it is opposite the TP and did not rotoate with the piston so must be in comp tube. That's going to be fun to remove at the botom of a blind hole!
     

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  14. Nick G

    Nick G Active Member

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    I have done a few 97's now 22mm and as much stroke as the under lever will allow , full ( non rotating) o ring pistons at around 140 gram with a shortened tube rear bearing and a rotation stop in the slot,I use the standard 97 spring and 20mm pre load , only had trouble with power when I got the stroke a mm too short, makes a big difference , but that's at 22mm you should be ok at 23mm. I still suspect your port, testing with the 26mm piston wont show it up , the reduced bores are susceptible to port sealing/pellet seating.
     
  15. hmangphilly

    hmangphilly The Doctor

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    Hmm
    Something's not flat.

    I thought you were on a 23mm o ring piston .

    Cant really make out what that head is
     
  16. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Thanks Nick, I haven't done a critacal measure with this action to see how much crush the underlever gives me on the breach seal or where the comp tube touches the end of the barrel, also need to look at the 'pop' as per Phil's info.
    This action will allow approx. 88mm of stroke before the underlever binds in the action (really, really didn't want to change the woodwork to allow this).
    I had wondered if, with the 82mm stroke, I was chasing the impossible. Since the power always peaked at 10 ish ft/lb. I know smaller volume tunes will give the power but from my understanding the majority are in TX with a long stroke 90+ mm, so it's possible that the peak pressure pulse is too short to achieve this.


    That's the spare piston with a head fitted ready for sizing to the desired stroke, front face was faced off and then the test was done.
    That head running 80mm stroke and no seal made 7ft/lbs (couldn't resist trying it....)

    IMAG0603.jpg
    Left is the stroke adjustable piston, middle is too light, right is the 'spare'.
     
    Last edited: Nov 2, 2018
  17. Nick G

    Nick G Active Member

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    Something else to consider, as if you don't have enough to think about:confused:, In the tx a small recess in the centre of the piston to create a little lost volume ( I back the rod out a little ) is worth some power and softens the shot cycle a little .
     
  18. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Thank you Nick, will add this to my list and update once I have tried it.
     
  19. Tinks

    Tinks Another tricky target at Lincs.....!!

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    Several weeks since an update so here goes......
    A few days after my last post I managed to fracture a vertebrae, which rendered me useless for 4 weeks.
    Firstly managed to lap the inside end of the compression tube so the piston now goes flat to the end of the tube, have confirmed no leaks in comp tube and a good piston seal with the 23mm poly u cup seal resized in a hot tube and allowed to cool naturally overnight, so no machining required and a very nice finish on the sealing edge along with low force required to get the piston moving.
    Having returned to my problem project in the last day or two, I focussed on the breech seal and barrel lead, after some very careful measurements of all parts of the compression tube to barrel positions when locked up with the underlever and also when rested with no lock up, I discovered that my compresion tube is hard up against the barrel end of the action with only 0.3mm crush on the breech seal, this leads to a gap of 1.2mm between the TP and the end of the barrel (without TP insert). My thought is I need more pressure on the breech seal to function properly (my other 97 has 0.9mm breech seal crush at lock up), so currently reworking the breech end of the barrel to remove the excessive 45 degree lead with a 6.35mm insert to allow the head of the pellet to engage the rifling and the skirt to seal in a tiny 45 degree lead on a 4.65mm smooth bore insert. Tested with delrin parts and got 730fps with JSB exacts and 705fps with Superdomes (numbers which are more normal at last!). Running a standard spring on 9mm preload at 86mm stroke, so very promising at this point. Next step is to make permanent steel parts as I'm certain delrin will wear out quick, pictures will follow. Basically returning the breech end of the barrel to standard with smaller than factory lead, if this doesn't work I will be reverting to a 26mm set up in this action and moving on to the other one.
     
  20. bigtoe

    bigtoe Member

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    23mm will work, built a few 23's and they all made power very easy, 22mm works awesome also, my 77k Mick Fern is shooting runs 89mm stroke skirtless with a Mk3 TX spring (a clone of mine) and a CD trigger, effortless power.

    Piston seals, HW45 seals are shyte, if you want a good 22mm talk to me, I will be ordering 23mm seals soon along with 21mm also, i run a dual seal set up, on the 22mm no crush on the O ring at all and size the front seal down pretty close to 22mm.
    Consider power is NOT generated by swept volume per se, its generated by stroke, you can get 30CC to work BUT you need stroke to get that 30CC working, so max long stroke that HW and use a soft spring such as the Mk3 TX. What happens with the shorter stroke is you need a higher rate spring and the pressure is built to fast for JSB's, this is where superdomes start to shine and i see a a few are using domes to check for over sprung now ;)
     
    Tinks and MickyFinn like this.

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