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

Revisiting the Diana 52 Carbine

Event though I had the D52 shooting quite nice, i knew it could be better…with the Walther LGV piston being such a universal base for conversions I decided to see if it could get it to fit in the D52 and by making up a new longer piston rod reduce the stroke…and of course sleeve the compression tube down to 25mm πŸ˜‰

First job was the sleeve the compression tube, did not get any pics of this as we all know whatΒ  a piece of steel tube looks like machined square on both ends and cleaned/polish. The tube used is 28mm OD 1.5mm wall high precision hydraulic steel tube, machined square and true both ends, the comp tube open end is relieved a little so the piston is easy to insert, the end that sits on the breech is as flat and true as I could get it…the tube is pressed home and secured with loctite thread lock. Cut this sleeve +2mm length, measure the internal length of the compression tube and just add 2mm, it will clear the skirt on the piston by around 4mm πŸ˜‰

Now..the D52 has a central transfer port, this means the Walther Piston seal can NOT be used for this conversion, I opted for the Vortek Vac25 seal (although a second hand one showing some signs of wear..will get changed when the next order comes in). If you are considering this conversion then you really do need to consider the added cost of a replacement seal as well as buying the LGV piston.

Next job is removing the rear bearing on the piston and machining a new one. The ID is the diameter of the groove cut into the piston, for me this was 27.20mm, the OD is 31mm, the width is around 4mm but measure the groove as you machine a new bearing to suit. So you are replacing a 30mm OD bearing with a 31mm OD bearing πŸ˜‰

Once done you have to remove the OEM rod from the piston, I held the rod in the lathe and heated the nose of the piston till it started smoking then using some grips wind the piston off the rod..they are loctited on. The thread is M8x1mm so if you want to clean out the threads in the piston nose you are going to need an M8x1mm tap. Next reproduce what is on the end of the OEM piston rod onto a nice fresh piece of 10mm OD silver steel bar that is precut to 210mm long for .22Β  (205mm for .177)


The rods total length is 210mm, the sear engagement point lands at 207mm for .22 and will be at 202mm for .177.


This is the piston nose end, top OEM bottom the one I was identical but i had to lengthen stroke by 3mm hence the thread on the bottom self machined piece is shorter.

As you see what you end up with is quite a bit longer, remember this rod here is for a 70mm stroke for a .22 D52 to UK spec.


Here is the trigger sear end, copy exactly what is on the Diana piston remember…NOT the walther piston ;).


Remember you have to harden this after polishing.

All built up with fresh loctite on the threads you should end up with something like this.


Now, the spring to use is the full length spring you get with the D52 Ultra K here in the UK which pretty much is the full FAC spring, I used the OEM rear guide but shortened it by 10mm (off its nose) so I could install a top hat. This top hat has 3mm step on the end and I added an additional 3mm of preload, total preload on the spring is approx 45mm. Diana deliver the guns dry internally, make sure to grease the spring up and the guides, the piston seal just needs a tiny smear of moly lube or SM50 with the same on the front nose bearing on the piston..on the rear bearing i used high moly content grease.


So thats about it, rebuilt the rifle and just started tests, its touching 11fpe with 14.5gr RWS Superdomes. I expect it will gain 0.1 to 0.3fpe as it shoots in. The shot cycle is nothing short of amazing, 70mm stroke on this 258gramm piston in such a large gun means it bearly recoils…silky smooth to shoot.

I will update once I shoot the rifle at the local club over the coming week. I have to say it feels NOTHING like a stock Diana 52 to shoot, it feels much like a heavily tune HW77 mk1, for me for UK use this has to be the ultimate tune for the D52.






Final update for the .22 AGT80

Finally settled the .22 down, here is how the rifle ended up.

Refitted 16inch HW90 barrel (same as 16inch HW80 barrel although the bare 90 barrel is slightly lighter weight)

Added 280Gram barrel shroud (finishes flush with the end of the barrel) its just to add weight

Originally the short stroke using the LGV piston seal was 11mm, I have now swapped the LGV seal out for a 25mm Vortek seal which added just over 2mm rounding down I have increased stroke by 2mmΒ  so now only -9mm overall.

Changed the spring for new unit with 3.25mm wire, 20.5mm OD, approx 240mm free length (I will count coils and update) This spring has fewer coils per inch so is stiffer, its also slightly longer than the spring I was using originally and needed no extra preload. Power remained the same with RWS Superdomes however power increased with AA fields by 0.2fpe.

End result is a slightly faster firing cycle, hold sensitivity has massively reduced although the .177 is almost not hold sensitive at all…the .22 is now MUCH easier to shoot. I will swap the straight setback trigger bade for an extra set back unit from Rowan Engineering which I feel will help a great deal also.

For now its sat back in an OEM stock, I find it easier to shoot in this stock over the CS800 it usually sits in, if this proves still the same the CS800 will be up for sale. If the rifle does stay in an OEM beech stock I will modify it somewhat like I have with the 0.177 in the mk1 stock…or i will look our for a mk1 HW80 stock and modify this.


Final thoughts on tuning the HW80, the 305mm Carbine barrel thats popular now is to short, I tried 2 on this .22 with one being fully shrouded, while good barrels they just did not help balance the rifle well and are not as efficient as a 16inch barrel. I could tell right away with the 16inch barrel reinstalled the gun had settled down considerably.

The .22 HW80 LGV piston conversion needs more work that the .177 for UK sub 12fpe use, for my USA readers or those with FAC you could leave the stroke at 79mm for the .22 but run the rifle up around 16 to 18FPE (It will do this power ;)) I would imagine the rifle would not feel any different to the firing cycle i have with the .177…seriously smooth to shoot. For UK use the .22 looks to need 70 to 71mm stroke (approx 34.5CC of air)

The single thing I may try is converting the piston to an O ring seal, this will quicken the shot cycle even more and may dial out what hold sensitivity there still is…its slight though so may not be worth the effort….its under debate πŸ˜‰









HW80 LGV Piston Hybrid, news for .22 conversions (AGT80)

When I converted the .177 80 to LGV piston I also completed a .22 conversion at the same time, till now its been somewhat hold sensitive, to the point I thought the barrel was faulty…tried 3 barrels and all had the same issue…so looked deeper.

The .177 conversion is much quicker than the .22, if you keep the 79mm stoke the conversion gives with the .22 conversion you may have to lose 2 full coils off the LGV spring, its nice to shoot but you may find like I did the hold you are used to is just not good for accuracy. A little frustrated by the gun I decided to shorten the stroke in the hope that less air more spring would get the gun to behave a little better…so I decided to just jump right in and go down 10mm stroke, which is a lot!

I in fact went down 11mm to 68mm stroke, in the thinking I could shorten the rod 1mm at a time if more stroke was needed…as it stands there is no need, the AGT80 conversion is so efficient it will make 11.5FPE in .22 from 33CC…I kid you not πŸ˜‰ Best pellets for power look to be 14.5gr Superdomes, Hobby lands at 11 to 11.2, AA fields 10.8, 19.2gr Baracuda Hunter Extremes give 11.3 ;), I will be trying more pellets over the week. Not shot Superdomes from this barrel before so praying they are accurate.


With this 11mm drop in stroke you still need to lose 1 full coil off the LGV spring. I went 2 full coils and added 8mm of preload so going 1 full coil off should be just about right. You could always go 1 coil off and 1 crushed coil and tune up with preload washers, however remember the piston is already heavy, use synthetic washers to pack up preload and consider adding them on to the rear guide not in the piston BUT remember they have to have an OD that will fit into the piston πŸ˜‰

The drop in stroke is not needed for the .177 conversion, although those wanting to quicken up the conversion even more way consider losing 3mm to 5mm off the stroke on the .177 conversion.

Will update once the gun has settled down.







Rarely show a finished gun on here…here is my project 80/LGV hybrid.

Just 2 quick pics to show how she looks now, bluing by Collin Malloy RFD here in the UK and because I like the Mk1 80 stocks I decided to just rework the woodwork she was in and have something a little different.

The dark wood I inlet into the stock is India Rosewood, I had it to hand and thought the contrast would be nice..its different although I know some will not like it.

So, final specs for the gun.

1980 HW80, started life as a .22, action was painted, stock was dinged and scratched, cost me Β£120

New 410mm Long .177 barrel, dovetails filed off

Self made shroud, not showing the internals although they are not special, extends the barrel 50mm…very quiet

Rowan Engineering fully setback trigger blade.

Walther LGV (the new one) piston, reworked main sear on trigger, 25mm sleeve, 3.6mm transfer port, reworked LGV mainspring, power set to 11.2fpe for UK use.








Just put her back together after getting her back from Collin, have to say I really like this one.


Total outlay for the project.

Gun 120

Barrel 90

Trigger 36

Bluing 60 (no barrel was blued hence cheaper)

Rosewood 10

Oil 50p

Steel for shroud 4

cocking shoe 8

PistonΒ  and spring 45

Guides material 4

total Β£369.50

Normally i would not have to buy a barrel so normally this type of project would have cost Β£250 to Β£280 all in to better than new condition.

Obviously my time machining was free πŸ˜‰



back to the 80/LGV Hybrid..the cocking shoe.

Sorry guys, got side tracked and forgot to update the build with how you modify the new style cocking shoe from HW to fit with the LGV piston etc…well here we go.

The new style cocking shoe does not ride down the slot in the piston any more, it slides down the slot in the action..and yes it scores the tube and yes it removes the bluing but it can still be silky smooth and for us with this modification its ideal.

lets have a look at the shoe.
























As you see its a lot different to the old style cocking shoe πŸ˜‰ BUT we have to modify this shoe to suit the LGV piston and the stroke of the piston which is related to the length of the cocking lever which acts upon it.

The following pics show you what to remove, i coloured the area to be removed in black, i do this with a dremel and cut off wheel, followed by a small sanding pad and a sharp fine file to round off the corners…then polish it smooth. You are looking to remove around 6 to 6.5mm of material, it does not need to be rounded any more but does need to have the sharp corners rounded off, it now pushed back the shirt of the piston which is acting square against it. if you do not remove this material the piston will slam into the cocking shoe and cocking arm on the firing this is most important.

























This should give you an idea how much material has to come off, take your time, make sure to deburr and smooth everything off.

You will also need to polish the sides of the cocking slot in the action, around 3mm either side of the slot will get marked up by the shoe so polish this back by taping 3mm to the side of the slot and polish the metal work from the tape to the slot… for the full length of the slot.

I have found a wonder lube for the cocking shoe, its also nothing short of incredible for trigger lubing and would you guess fishing rod reels which is what it was designed for πŸ˜‰

Some of the airgunners in the USA are already using this for triggers, its available in the UK also…you need a few drops on to clean metal, spread it out..leave it to sit a while as it bonds to the steel, then feel the smoothness, its incredible πŸ˜‰

here are some pics of the shoe in place with the cocking arm:











As you can see I have the action taped off and polished either side of the slot..its sat with another application of Hot Sauce (its red if you did not guess ). You can see which way the shoe is orientated. Things to watch out for are the cocking arms may need some relieving so they will slot into the new shoe, and if you fail to remove enough material off the shoe the piston skirt will slam into it in the firing cycle..we are adapting components here so look out for this as you build..measure twice and cut once πŸ˜‰

You may want to polish the sides of the cocking shoe and the sides of the cocking slot in the action, all will do no harm and aid the smoothness, the 2 I have done here now are ridiculously smooth to cock and returning the barrel to the latched position is resistance free…really very very slick.

On the new 80’s they have this shoe already and they have a spring that acts upon the cocking lever, not sure what this spring does so if you have an idea or know what it does leave a comment so I may add it to this post..the more info we get the better πŸ˜‰

With the shoe modified and installed, the compression tube installed and fixed in place, the trigger sear modified and polished you should now have nothing more to do other than set the power. I have found the LGV spring does need shortening a little (2 coils for .22 and 1 coil off for .177), make sure you set the gun up over a chronograph and be prepared to get spring tweaking…it took me 20mins to get both guns right where they are awesome so its not a huge task πŸ™‚

If there is anything else you want to know about the conversion, add a comment and i will do my best to explain/add info.








HW95, Tuning for dedicated UK spec.

Lucky for me there are a few guys willing to send in their gun so I can tinker a little…obviously I have to return them shooting better than they did when they arrive ;). This time round (in between creating another HW80/LGV Hybrid) I’m going to take a look at the HW95.

The example sent to me is a slightly older one, not the newer spec with new style cocking shoe etc, the gun is like brand new however with very little wear signs…so no repairs to do πŸ˜‰

Measurements taken put the piston stroke at 85mm, 26mm piston, 45.1471428CC swept volume…

The transfer port is 3mm diameter and 15mm long (to long really HW…stick to 10mm or less if you can ;)) 0.106071CC

Out the box we have a Static compression of 425:1. NOT counting the lost volume in the parachute grooves on the piston seal.

The plan is to bring this HW95 to pure UK spec, max efficiency at around 11.3FPE, so my plan is to short stroke the piston down to 74mm, I will do this with an Alu piston cap and O ring seal to minimise lost volume, adding bearings to this cap (should have enough room) or may even go with a full circumference bearing. The piston will be sleeved and a rear bearing added so ensuring zero metal to metal contact. The transfer port will be opened to 3.3mm and polished, couple this to a reduction is swept volume to around 39CC we should see 306:1 static compression ratio. The flow through the transfer port will be enhanced by the larger diameter so I have no fear the action will still make good UK spec power levels…the plan is for it to land pretty much bang on 11FPE or so with a 230mm spring 3mm wire 31 active coils (33 counting the flattened ends) with little to no added preload over the stock amount…we shall see πŸ˜‰

This may be a slow project as a few other jobs have to be finished up to free up time…Once the time is available you should see a full update.






TX200 short stroke with Walther LGV piston

TX200 Mk3 with Walther LGV piston…Bring on the short stroke drop in.

We had a long thread about short stroking the TX200 Mk3, the -14mm of the mk1 piston makes the TX quite nice but it requires a new rod or piston cap to reduce the stroke on a mk3 piston as mk1 pistons are no longer available.

So…I dropped a Walther LGV piston into a TX200 mk3 to experiment a little and report what I found.

Now the Walther piston seal is special, its designed to work well with offset transfer ports, you can NOT use this seal in the TX, way to much lost volume.

So..I swapped over to the Vortek piston seal, this had the added benefit of adding 2.5mm to the piston stroke.

So to start the specs of the Walther piston dropped in a TX200 with Vortek Vac25 piston seal.

First off I had Mattyboy measure up a Mk1 TX piston, front face of piston to sear engagement point on cocking rod.

= 165mm

next up I measured up a LGV piston.

LGV piston = 173.5mm

8.5mm difference in stroke

So the LGV piston in TX chassis would give 76.5mm stroke

which gives 37.56CC swept volume

Now my gun has a 4mm transfer TP at 4mmx9mm long 0.1131428CC

Static compression ratio 37.56/0.1131428 = 331:1

Now I have ONLY tested this with the LGV spring and it is to long, I will shortly have a TX200 Mk3 stock spring to test with so I will revisit this, keep in mind the LGV spring is to long so the gun was a little oversprung, the LGV spring did fit the tophat and rear guide well so those wishing to experiment could just shorten the LGV spring if they wish…

FTT 8.8gr (weighed) 10.8FPE
Daystate Li 8gr (weighed) 11.05FPE

I do not have any AA fields, they are efficient in this gun and I would expect them to land around 11.2FPE max , when I revisit this I will have AA fields/Exacts to hand.

Now, I did no reshaping to the sear engagement point on the rod of the piston, it is nearer to 90deg where the TX is slightly set back a little more, the rod could be reshaped and polished to match the TX rod as it is hardened all the way thru…me, i would not bother.

So to make it clear, the LGV piston is a drop in replacement for the TX200 piston to short stroke using the Vortek Vac25 piston seal to 76.5mm stroke

bear trap…right now mine here is disconnected, I doubt it would work correctly as I had issues with when i tested -18mm stroke and we are now at -22mm or so. When I revisit I will add it back and retest.

Those wishing to experiment, you could machine the piston head back a tad, machine up a bronze or alu piston head and sit a 21mm ID 2mm CS O ring up front and push the stroke to around 78mm, at 76.5mm it is making 11FPE, keep this in mind.

I measured up the bare TX200 mk3 piston at 227g and the LGV piston at 252g

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 ofΒ  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 =Β  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 πŸ˜‰

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 52Β  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 πŸ˜‰


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 πŸ˜‰












Trigger detail update for HW80/LGV

As promised some pics, this is right after 200 pellets put thru the gun and i have no wear marks on the sear or the piston.











Sorry about my grubby hands πŸ˜‰ this should give you an idea. Sorry no measurements as i did this by feel offering the piston to the trigger and seeing if it would engage..then removing it and grinding a tad more off etc. If pushed I will get things measured up.


It is more of a touchy feely job however, taking a little off at a time till it engages, contact is VERY good with a huge surface area of the sear holding the piston.


No idea why the second stage adjustment screw and sear look canted over, when back in the gun everything lines up true…will maybe have to investigate this a little further..the gun is really quite old (very early Mk1) 901457 manufactured 1980/ (early 81 possibility) so 32years old ish πŸ˜‰






3rd small update HW80-LGV

Promised some pics, here is what I ordered from Germany, its the 16J spec service kit for the Walther LGV, you get the piston, a spring, a rear spring guide and a washer and 2 tubs of grease for 44 euro + shipping…not to bad πŸ˜‰


A close up of the label.


Its pretty much the same as a TX200 piston, well made as I posted earlier. The grease is handy and the piston grease is actually quite nice with enough to do many guns.


Next up a quick pic showing some of the modifications needed to the main sear on the Rekord trigger. There is another modification needed but i do not have pics of this just now, they will be added on the next strip down.




Bottom sear is the stock untouched one, the top one shows some relief added, I also dished the sear so that it cupped the piston rod (the smaller diamater bit) which I will show on a later pic. I wanted to maximise surface contact as the sear is holding back a lot of force…the mods to the sear were minimal and im not to worried about it losing some of its strength.

Last pic is the rear skirt detail and piston rod…the rear bearing is sized for a 30mm compression tube so no machining is needed, drop it in and go πŸ˜‰



Next update i will show the addition sear machining and the alterations needed for the cocking shoe. I am considering a 75mm stroke version on a second HW80 with the same hardware, this will not need any modification to the cocking shoe, right now the gun has a standard piston with short stroke down to 60mm, the problem is this conversion is much much sweeter to shoot πŸ˜‰


More soon

Assign a menu in the Left Menu options.
Assign a menu in the Right Menu options.