AirGunTech: Re Engineering Spring Powered Air Rifles for smoothness, consistency, and to better suit UK power levels.

The 80/LGV is still a work in progress but wanted to talk a little of transfer port tuning.

When I decided to pass over the new LGV 2012 for a while and have a go at building my own part of the criteria was that I would design the gun based around known values, swept volume and static pressure etc. Now over the summer I have tinkered with 2x HW35’s and a HW55, all needed something different although the 55 was the most exciting…this is where transfer port tuning came in.

The 55 had a leather seal, its a 1980 ish gun so was probably on the boarder line of having a leather seal or having a synthetic parachute style seal….having a leather seal it had a transfer port of 4mm diamater.

Now this is where i made my first mistake, I bought (was feeling lazy) one of the conversion kits from Vortek, now don’t get me wrong there was nothing wrong with what Vortek supplied me, the mistake was i left the transfer port at 4mm, and here is why.

When you tune a spring air rifle (I am going to talk about for UK use so sub12FPE) the goal is to to make around 11FPE and make the gun as smooth as you can to shoot. Each and every gun you tinker with has its own set of variables to deal with, in the case of the 55 this was a 25mm piston with a swept volume ofย  33.8CC from 69mm stroke. The transfer port is 26mm long (same as a HW35) and was 4mm diameter, this gives a swept volume of 0.3268571cc.

So…we can now work out the static compression ratio, which is simply piston swept volume/ transfer port volume.

Now, I built the 55 with the 25mm Vortek kit, with the standard spring it was not to bad, it delivered 610ish FPS with 8gr pellets BUT it felt a little gas rammy, with Hobby pellets it actually felt harsh to shoot. The reason why? the static compression ratio was too low.

So lets look at what we have.

33.8/0.3268571=103.4 which translates to 103.4:1

This is fine for leather seals that create their own braking effect but a fast flying synthetic seal….nope not cutting it.

Fellow tuner T20 (Mick) has been advising 300:1, my concern with 300:1 was I felt this could be to high and restrict flow through the transfer port so sap power, and seeing that I had planned to long stroke the 55 i was looking for every ounce of power i could get. Anyway I hinted back in the 80 build posts how to reverse engineer to work out the transfer port diameter from a given static compression, im going to do it the lazy way here so I will jump right to 2.7mm.

26mm x2.7mm transfer port volume =0.14892428571cc

so with standard stroke 33.8cc/0.14892428571cc = 227:1

So double what we had before (near enough) and this is what I tuned the transfer port to (for now)

To tune the transfer port smaller I tapped the transfer port M5 and bored some 26mm long M5 grub screws to 2.7mm then thread locked it in (simples ;))

The rifle now has 75mm stroke so 36.8CC, so the static compression has risen a little due to the increase in swept volume:

36.8/0.14892428571 = 247:1.


The 55 is delivering around 10.4FPE with 7gr Hobby Pellets (this is a .177 remember) and is a joy to shoot although I do want to push the transfer port down to 2.5mm as it does feel a tiny bit gas rammy with light weight pellets as obviously I have used a stiffer spring also. 2.5mm will give me 288:1 which is around as close to 300:1 I think I will get, my fear though is port flow will drop and i will lose power, we shall see.

If you are wondering the swept volume is large enough to make 11+FPE in .177, the issue is there is just not enough room to get a spring in large enough to do it and the piston is probably a little to light weight also. I now run a phosphor bronze piston head and O ring set up on the 55 to long stroke it so the piston is heavier than it was but not heavy enough. I settled on 10FPE with 8gr pellets and it shoots at this power level beautifully, not harsh at all and deadly accurate…just super light weight pellets could have a better shot cycle.

So…to get back on track and focus, tuning the transfer port on the 55 effectively brought up the static pressure ratio, which in turn increased the peak pressure created within the compression cylinder on the firing cycle and lead to enough air cushion being developed to dampen the piston at the end of its compression stroke so that the shot cycle was sweet with no slam.

I have found 300:1 is good for long transfer port guns such as the HW35, the 55 etc, mid length transfer ports like we see on the HW80 I pushed this to 350:1 and this is what I aimed for in my LGV build. For short transfer port guns such as the HW77 its looking like 550:1 with standard weight pistons and 500:1 with light weight alloy pistons is the way to go..this means a 25mm piston HW77 will have a transfer port of around 3.8 to 4mm in diamater.

So now we need to ask the question what if the static compression ratio is higher than what I have quoted?

Generally a small transfer port is used to restrict power, a prime example of this is the Diana 52 Ultra carbine that is for sale in the UK, these guns have massive swept volume (28mm piston and 100mm stroke) This is plainly aimed at the magnum airgun market so Diana restrict the transfer port to 2mm. I will not go into the power the gun makes without the restrictor in place BUT I can tell you with it in place the gun will shake all your teeth out of your jaw bone when you shoot it…I can’t say I have shot anything quite so bad..lets do the math to see why its so bad.

100mm stroke 28mm piston 61.6cc swept volume

The transfer port is only around 4mm high on the D52 and is 4mm BUT has a 2mm restrictor in place, this makes it awkward to work out the true volume, my best guess would be to use 2mm dia x 4mm long restricted.



Static compression 61.6/0.0125714285 =ย  4900:1


What this meant is Diana fitted a monster spring which is trying to force a lot of air through a tiny hole, the piston bounces like crazy off a solid wall of air around half way through its firing cycle and literately shook the gun violently. The piston is bouncing back and forth many times with an end result the gun is almost un shootable….and now we know why ๐Ÿ˜‰

In an earlier blog entry I short stroke the D52 and remove the transfer port restrictor, bringing the static compression ratio way way lower, so moving peak pressure much closer to the end of the firing cycle which means the piston is much more efficient with minimal bounce and the gun becomes a joy to shoot. The Diana 52ย  is just one example, many manufacturers compromise guns internal shot cycle efficiency to meet given power outputs for given sales regions…IE the UK is 12FPE, Germany is around 6FPE etc.

Really what they should be doing is supplying different pistons with different swept volumes for the different regions so that they can optimise the shot cycle of the air rifle for that region and the buying public have a much better experience with the gun..but that would probably cost to much.

Its what I would do though ๐Ÿ˜‰


Back to the 80/LGV next, will update with some cocking shoe detail and if the info comes thru the stroke and swept volume for the German spec LGV piston which just may be even better for the 80 conversion for UK use ๐Ÿ˜‰












  • I love reading of your exploits, In the blog you hit on the HW55 with long transfer port.
    I own an original HW50 which shares the same mechanics as the 55 but for it running a synthetic seal as standard.
    Iโ€™ve been advised to open the long transfer port to 3.175 (currently 2.8) and am skeptical about this.
    As for not being able to get enough spring in your 55 have you considered having 6-8mm off the spring guide seat of the rear screw in block, this would allow for a further 2 coils when compressed.

  • never thought of machining the block back, I will look at it now though …thank you ๐Ÿ˜‰
    TP dia…I would increase the stroke first, this will lower the static compression anyway. I would say 2.8 will be fine, infact a reduction may have a nicer shot cycle feel.

    Long stroke it a little first,I would move to an O ring piston cap and then look how far the slot could be pushed back on the piston…I hit 75mm stroke or there abouts.

    I will post a pic of a long stroked 55 piston I have as a spare for you to the blog.

  • I think we are in conversation over on airgunforum lol

  • How much would you want for a converted piston?

  • Hi Tony, given the length of the transfer port in the HW55/50 is it viable to counterbore it from the rear to increase volume and leave a 10mm deep port at standard diameter at the front?

  • Paul…you would need to remove the breech from the main tube, machine back the breech as much as possible on a lathe, then re braze the beech back in.

    Ton of work man.

    If we could gain 2mm however it would be worth it, long stroking the piston and adding 2mm at the breech end would give enough for 11fpe easy.

  • Point taken, I’d thought about using the dremel on the flexi cable mounted in a shaft to open up the rear of the port.
    Is there an other pistons that I could look at modding save the risk of ruining the one thats in now?

  • no…all modern HW’s are 26mm i think and the cocking rods change in length.

    a custom drop in piston would be the ideal alternative if you want to save the piston you have right now.

    I don’t do them however

  • One last school of thought, cut the latch rod and reduce by 6mm, reduce the piston skirt by the same 6mm, drill and tap the latch rod m5 to reconnect.

  • the issue is not the latch rod Paul, its the cocking slot. You have to machine the nose of the piston back to increase the stroke, I did this on my 55 to place the o ring 1mm into the cylinder forward of the cocking slot…then you push the slot backwards in the piston the same amount so that the cocking lever clears it when the rifle is uncocked…if you do not do this obviously the piston smashes into the cocking arm.

    The rod can stay as is.

    Its easier this way being honest.

    Here is my spare HW55 piston, I actually filled the slot back further than needed then increase the size of the bronze head to add weight to the piston..again it seals 1mm into the comp tube forward of the cocking slot.

  • I’ve taken the vernier to my 50 today and I have less stock stroke that you do! 66mm is all I have.
    I’ve decided that I can fabricate a custom.
    off the shelf components like 24mm seamless tube for the body, 10mm silver steel for the latch rod.
    The only thing I would need is a steel head turned, I intend to use steel so I can tig weld the head to the body.
    I’m thinking along the lines of my meteor but one difference between the meteor and your custom in the picture is the lack of any cushioning like in the meteor.

  • I can gain around 8mm stroke i bet on that piston, so get you up around 74mm.

  • If I could afford it the gun would be in your workshop right now.
    I think I can get around 36cc out of it, hopefully I can run a much shorter titan spring and get a smooth shooter of around 9-10ft/lb, that would be ideal for HFT I think

  • Given the near 37cc of your long stroked 55, would having that back block machined down to allow another 2 coils mean you would theoretically have the volume + spring power to achieve nearer the 12ft/lb?

  • maybe, piston weight is an issue though, the 55’s piston is seriously light weight

  • does the phosphor bronze head also reduce the amount of available spring space too?

  • no..the alloy part of the 55’s piston does however. Ultimately the 55’s piston could be redesigned and I may do this when I have the mill im saving for.

  • I’ve sent you my email addy via pm on airgunform mate.
    If you are going to rework my piston then winter is the time to do it

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