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

Autumn 2013, Little update what I have been upto

So…..the lathe blew up, typical Chinese lathe so my mates tell me, DC motors and carbon brushes that over heat…etc etc, so its all stripped out and will have a new 1HP 3ph motor installed with Inverter/remote start stop/speed control.

This means not much springer tinkering while i saved for the new motor etc…BUT that does not mean no tinkering ;)

During the down time I rebuilt an old Mk1 Theoben Rapid ( yes a PCP) and it rekindled my love of the dark side…and i must admit it opened my eyes to just how good the old Mk1 Rapid is. Now I own 2 Rapids, this old Mk1 and a much newer Mk2 based MFR, the Mk1 is now shooting superbly however the regulator in the MFR is somewhat ill and has been sent to KevG here in the UK for s sprinkling of his pixie dust, I could have tackled the reg rebuild myself BUT Kev is a dab hand at Theoben regs, he knows exactly how they tick and what to do to them to improve them…he has forgotten more than I will ever know so why stress when he does such good work ;)…so back to the Mk1.

Early Mk1 Rapids are not regulated, this is ok but I wanted mine regulated, for a while I ran it with a paintball regulator and tank, this worked well but i always had a small leak that was very slow but very annoying…so I hatched a plan. KevG offers brand new built regs for the Rapids which he calls Micro core, what this basically means is he redesigned the regulator with a much smaller Belleville washer stack..and it just so happens its now small enough to fit in the bottom chamber on the old Mk1 Rapids with no modification to the chamber. Now, Kev is now offering a bespoke reg for the early Mk1′s, I opted to re drill the porting in the block of the Rapid to suit the regulator installation, I did this as I wanted the reg to cycle well on each shot. So new regulator to hand, a fresh ported Rapid valve to 6mm, a new .177 barrel with fresh porting set to 6mm taper to 3.9mm (0.177) and a rebuild was in order. I am not going to go into a ton of details on the build as many have rebuilt rapids and they are very easy to work on….however I must say im more than impressed with the efficiency of the gun, with a 235bar fill 400cc tank and 16inch barrel the gun will produce 370 shots before the pressure drops and it goes off regulator, a true fill it once a year gun if there was ever one created :)

However….rebuilding the mk1 had me looking at the MFR (where i found the regulator was occasionally sticking) and….it had me looking if I could improve the efficiency of the Rapid action any.  The mk1 has 400CC, this will soon drop to 200CC as I plan on a stock change, the MFR is already set with a 200CC tank and again is a .177.

Two of the forums I frequent have some pleasantly technical  posters, the BSAOG forum has a lad called rkr and Airgunforum.co.uk has a chap called Cloverleaf, now both of these guys have used a method of recording a shot cycle to either show hammer bounce, lock time, or both, I have no clue if one of these guys was the first to do so online but I will thank them for posting their results as it has shown recording the muzzle blast from a PCP is a great way of learning how efficient or inefficient the rifle is…so thanks lads ;)The program of choice is Audacity, its a free download, and would you believe you use buddy headphones but plugged into the mic socket on your computer to do the recording. Now I have used audio editors for a good while now so it was reasonably easy for me to navigate round and start recording and zooming in on the waveforms the sound recording made. It also makes it easy to see blast duration, if there is more than one POP (so hammer bounce).

So…lets see what a recording from the MFR looks like:

rapid

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I used both mics on the muzzle as I was just interested in air wastage thru double or triple tapping of the valve during a shot cycle, as you can see there are actually around 5 taps of the hammer with 3 releasing lots of air, which is not what is best for shot count, quietness and efficiency shot to shot. You can see the muzzle blast duration is just a nats over 0.1secons long…keep this in mid as we move onto something I developed over the summer.

During the summer months I decided to rebuild 3 of my BSA R10′s, they suffer from worse hammer bounce than the Rapid you see here, in fact much worse, unfortunately I do not have a before modification waveform to show you but i do have some after modification ones and they are quite something.

R10_22_newhammer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Again for this waveform both mikes were at the muzzle, what you need to focus on here is the blast duration, 0.0160secs, the MFR (although .177) has a blast duration of around 1/10th sec, this R10 has just a nats over 1/100th of a sec. I have a feeling some of you will question/may say I just chopped the waveforms…trust me I just edited them to the first silence, what you see here is the total sound both rifles make and its duration in full. Also note on the R10 is just 1 pop, not 5 pops, this means the R10 is seriously more air efficient now, even more than the Rapid MFR.

Next up an older test on a custom BSA Scorpion SE /R10 hybrid in .25calibre i built for pest (rat) control, like the .22 R10 this one has a custom hammer but the action is unregulated.

25cal_hammer_test1

 

 

 

 

 

 

 

 

 

 

 

 

For this recording I set Audacity to record at the breech and the muzzle, the top waveform is the breech, the bottom waveform is the muzzle, the gain was also higher so I made some notes on the capture to try and explain what I feel is happening on the waveform, the action muzzle blast lasts for 0.015 seconds, the bottom waveform as what we shall call a pre peak which is the hammer ping as its striking the valve, then the muzzle blast hits the mics and lasts for just over 1/100th of a second…just like the .22 R10 above. Now the top waveform is a little more complicated, the first peak i feel is trigger sear noise as i release the hammer, the green area is hammer to valve strike, then i get the black area which I can only put down to the hammer being dragged away from the valve..it clearly does not open the valve again IMO although if it does its for a seriously short duration.

The .22 R10 now runs without a regulator BUT with the anti hammer bounce hammer system in place, I just replaced the 200CC tank with a 280CC one and gained my lost shot count back easily, in fact the rifle produces a higher shot count unregulated than the rifle achieved with its problematic regulator it had originally using the 200CC tank. The 0.25 Scorpion SE/R10 hybrid is basically a buddy bottled .25SE in an R10 stock, again it features a 280CC tank and from this one I have seen over 300 shots, in fact I gave up counting ;)

If you are into PCP tuning, consider giving Audacity a try, use some cheap ear bud style headphones as mics and play around with mic gain etc so you can record a shot and stretch the waveforms out and look for hammer bounce and multi valve opening…you don’t now need a high speed camera to watch the valve you can listen to it and see the waveforms it creates.

To finish, the plan is to recreate a similar set up with the hammer in the MFR and the Mk1 Rapid as I have done with the R10′s, I will then report back and try to show a before and after comparison, im hoping I drop down from a pop lasting 1/10s to one lasting 1/100s like the R10′s are doing. Also…people always ask which silencer is best on a PCP, or what mods make them better, consider if the rifle has multiple hammer/valve strikes the silencer is working really very hard to mute the long duration sound..often the shot will sound like a short fart rather than a single pop. If you instead work on reducing the hammer strikes the silencer you use on the rifle will suddenly become way more efficient, the .25 here is now so quiet with a standard HW silencer its like i doubled the length of that silencer…

Now I just need to get the lathe rebuilt ;)

 

Happy tinkering :)

 

 

 

 

 

 

 

 

 

 

 

 

HW77, the parts, tuning and upgrading, even on a brand new gun.

Edit, seem this was not loctite or glue but excessive bluing salts hardening on the threads between the trigger block and the action. I have been advised a good soak in dewattering oil/release agent should have helped. Regards how long to soak…i was told at least 1 full day.

After waiting months for a new 77k SE laminate to arrive I took delivery and started testing. I pushed around 30 pellets thru it before chrono tests started and was a little shocked. This rifle is a Hull Cartridge supplied rifle so is full UK spec, the issue is it was UK spec and some.

Initial tests were with the new Brocock pellets supplied by Crosman, they are infact just Crosman premiers with a stated head size of 4.52. I had pushed these thru the gun to settle it somewhat, there was no dieseling at all, i had guessed the piston was shod with one of the new type seals from HW which are usually pretty efficient. Now, I shot an additional 30 pellets after the settle is batch over the SKAN, I had not weighed the pellets and i was only after a rough idea what the gun was delivering. This is where it gets a little serious, it was pushing 12.8 to 13.1fpe with the Premiers, so I swapped to 4.52 Exact RS 7.3gr, now we had 13.4…so i dug out the last of my Hobby and hit 13.8fpe out the limited number I had. Now remember this is a brand new UK spec gun.

Normally I would just take it back, the fact I had waited 4 months for the rifle and the fact I tune rifles as a hobby I decided it would just need a coil or 2 off the spring and all wood be good…how WRONG I would be.

To start HW are now loctiting the trigger blocks on the rifles with what amounts to concrete, I had to apply so much heat to the trigger block it destroyed the spring guide…and i was worried that I would damage the bluing. Once the trigger block was finally off and the melted spring guide was removed from the spring I proceeded to continue the strip of the rifle. Loosening both the side and top crew off the rear open sight I was able to slide the sight off the dovetail rail, only to find the action scratched underneath from where the sight had been installed…now I was really not happy. Anyway, out comes the piston and comp tube so I can check lube etc.

The spring is 255mm long, the transfer port is around 3 to 3.1mm in dia, the spring guide is melted at one end so now scrap. I check inside the piston and additional preload has been applied in the form of a steel washer 5mm thick which also added to the weight of the piston. So…I decide I will fit one of my own custom springs and rebuild the guides, adding a tophat and slip washer.

The rear guide I installed is a little different to the stock guides, I like to machine the piston rod hole in the trigger block to 14mm dia and 12mm deep, I then machine a guide with a 21mm flange face for the spring but a rear section 14mm dia and 12mm deep that pushes into the trigger block with a good interference fit. The guide length is 20mm short of touching the bottom of the piston so I can install a long tophat and play with preload by machining washers that slide down the top hat and not sit beneath it, this saves on weight…all preload washers are Acetal also to keep weight down.The piston was fitted with a new style seal so I left this alone, it was cleaned however and a test lube installed (will report on this later). Any traces of the molten rear oem guide were cleaned away, the new rear guide is glued into the back trigger block and then the spring is greased with Millers red rubber grease. I don’t run tight rear guides, I run guides that are a glide fit down the spring, the twang is removed with the grease ;) tight guides end up breaking…plus they sap power.

I just happen to have a spare 26mm comp tube, this is already modified with a 3.9mm transfer port, so this was installed. The spring is a custom spec 240mm long unit with 33 coils, these springs will be available to buy soon and I will have details for from where here on the blog. The rear guide applied 3mm preload off the face of the trigger block, the front tophat has a 2mm flange and a 1mm slip washer, and additional 2mm slip washer is installed on the rear guide only, so total additional preload from the guides setup is 8mm. Power is bang on 11.3fpe with 8.4gr Exacts, RS slightly lower and Premiers just under 11fpe. Hobby ran out but im thinking around 11.2 or so…the reason being I altered the peak pressure with the larger transfer port so they will not be as efficient.

With the gun now legal, although scarred, and the firing cycle much improved I noticed the underlever catch was ringing when operated and on the firing cycle, this is due to the spring on the ball catch vibrating in the tube. The cure is to drill out the dimple on the under lever (which is around 50mm from the end of the lever) with a 3mm drill bit, go in 3mm also. You can now grip the ball in a vice and drag the ball catch unit out the lever. I now disassemble the ball catch, shorten the spring by 1.5 coils, re round the ball using a lathe file and then polish before rebuilding the catch and packing the spring with good moly grease. next tap the hole in the under lever m4 and de burr the inside of the tube. Once the inner of the tube is clean slide in the ball catch, lining up the dimple you have drilled into the aluminium bush and lock in place with a short m4 grub screw and loctite…dead easy mod that does 2 things:

1 it stops the ringing noise

2 you weaken the spring a little so its easier to drop the lever off the catch and replace it to the catch..which also stops it wearing the catch as much ;)

The last job tackled was the trigger, I wanted to fit a set back blade, usually an easy job, however after removing the oem blade there was already a dimple worn into the bottom sear (the one the roll pins in the trigger blade act upon), all I can think is HW do not harden the bottom sear or if they do its not hard enough. Anyway, I decided a full trigger strip was in order so I can stone the sear to have a flat surface for the roll pins to rub against.

The 2mm dia pin that holds the blade and it rocks on was also bent, 100 or so shots should not do this, however I now use hardened ground pins 15mm long and grip washers on either side of the trigger cassette so the pin was discarded (this does mean I alter the inletting on the stock) The sear was stoned and polished, the grease removed and fresh lube applied and the trigger rebuilt, first stage travel set to 2mm and the second stage let off set as fine as is safe, it breaks like glass now ;) Then the stock inletting is altered to take the wider blade pin and grip washers and the action dropped back into the stock…this is when i noticed the inletting for the breech area was incorrect and was rubbing on the breech so much is was splaying the front of the stock wider…putting strain on the for end of the stock.

So, out comes the dremel again, the inletting was again altered so the action just slipped in with no resistance from the stock…this is where i noticed another fault and something you need to look for on all mk3 77 and 97 rifles. What you need to check is the stock lug (Part# 1332) finishes flush with the underside of the inletting for the trigger guard, on my gun here it was 2.5mm below being flush. The fix is to add a washer or washers to pack up the stock lug till its flush.

Now, you may be asking why is this important? The answer is the 2 screws that hold the trigger guard can now pull the trigger cassette out of true and induce wear and irregular performance from the trigger. Once you get that stock lug true with the inletting for the trigger guard you can actually do away with the small m4 screw at the rear and just use the larger locating screw at the front (Its what I do here) and the trigger cassette is allowed to sit true and deliver the performance you expect from it.

I will detail these modifications with pictures so you can see exactly what is needed, the 2 alterations to the trigger (the longer ground 2mm hardened pin with grip washers and the stock lug) really do make a long term difference to the feel of the trigger. This build has stock 81mm stroke and no bearings to the piston, I want to test some special lubricants to see how they effect a stock set up, the rear guide will be helping the piston fly truer anyway and is reflected in the felt shot cycle.

Last moan about the gun is the stock has sustained some damage to the left side during manufacture, this was then lacquered over, while it does not effect how the gun shoots it does mean the stock is not 100% on the outside, which upsets me. I waited a long time for this rifle and really expected better from HW, usually I rebuild old rifles that have seen better days, this was my first new 77k out of a previous 10 that I have owned and im being honest I wish I had bought second hand and just tried to pick up this new laminated stock separately.

To finish, any RFD’s or tuners reading this, you are going to have serious issues servicing HW rifles with screw in trigger blocks (new ones from around 6 months ago) going forward. You will need to heat up the trigger blocks to break the loctite and in doing so you will destroy the spring guide and potentially risk damaging the bluing on the rifle.  This obviously applies to the following rifles:

HW77

HW97

HW80

HW35

 

 

 

 

Diana 280 piston, where to correct if nose is out of round.

Decided to pick up another D280, this one is .177 and full rifle, the extra barrel length really helps the gun IMO….if you want the K version get a .22. So….again i checked the piston and again it was rubbing on the compression cylinder just behind the piston nose…so I figured I would show some pics of where to file a little off the piston if you do not have access to a lathe. Of course I machined my piston here on the lathe but this could be done by hand.

So the pics…:

 

IMG_4673 IMG_4675 IMG_4674

 

 

 

 

 

 

 

 

As you see its between the crimps, this is where the steel was bulging a little and could possibly rub on the compression cylinder. Not all pistons are bad, my first .177 was way worse than this one, so its worth checking while you are tuning the gun.

 

Something I dreamt up….recoil and surge dampener.

This would lose some spring room but in theory would counter piston bounce.

O ring placement would need to be experimented with to ensure enough compression happens, this is just a rough idea to get some creative juices flowing.

Pneumatic recoil and surge dampener

Busy with projects but will be bringing new posts and info soon

Most of the early projects at AGT have been with HW rifles, HW95′s and 97/77′s and HW80, of  late however I have entered into a love affair with Diana rifles, notably the M52/54 and the 440TH/430. I am sorry to say I won’t be testing or tuning the T05 or earlier triggers, I personally have no interest in them and update all my guns to the T06, its an excellent trigger and once set up correctly is the match of the Rekord from HW.

Recent rifles I have added to the collection are 2x 440TH”s which have been tuned in different ways, one has been sleeved to 25mm and fitted with a modified Walther LGV piston with 80mm stroke, the other has a Diana 52 piston (which shortens the stroke to 78mm) which is lightened with bearings added to 250g…and I have to say right now the 28mm D52 piston gun is the sweeter. I did try 68mm stroke on a 28mm bore but the cocking effort was was to high with the preload needed on the spring..the shot cycle however at 11.3fpe was stunning…nothing i have tuned here came close.

So, I compromised at 78mm stroke and it is still an exceptional rifle to shoot, extremely accurate. However I have lengthened the under lever by 75mm or so and reworked the barrel catch/muzzle break to give me a little more mechanical effort when cocking the rifle. I have decided to do the second 440 the same way to will detail the build here in case any of you guys wish to copy what I have done. Its a radical alteration to the 440 but is in my opinion needed as the cocking effort even on sub12fpe rifles in .177 is substantial….tennis elbow anyone?!!!

The other main Diana I have worked on is the M52 and the M54 Airking, I will detail the Airking tune after the 52 tune as they are very very similar (essentially the same gun). I managed to pick up a second M52 Ultra K with the intention of tuning it, selling it and using that cash to pick up an .177 M48 pro Blackline from Germany, this  as far as I am aware looks like it may have the ultra k 300mm barrel and is the same basic chassis as the 52 so i would just swap the stock for a custom one…however a mate gave me a .177 52 firebird as a “thank you” so I have reworked this instead. I will detail the build spec although I will be altering the gun inline with my testing, right now the gun has a 270g LGV piston with 69mm stroke, it makes power supremely easily BUT I have made the decision to push the stroke to 75mm, this will allow me to ease back on the preload a little on the spring…right now it just feels a little too fast. Also the old Firebird stock…I just can not get along with it, its too sporter for me so im looking to have a go at carving something similar to the M54 laminate stock with some alterations to make it suit me better.

One interesting thing I have found tuning Diana rifles for UK sub12 spec is the pistons I use, it does not seem to matter whether i stay 28mm bore or sleeve down to 25mm bore, the stroke looks to be the same with both (or close) as does optimal piston weight. With the 440TH under lever rifle the 28mm 52 piston gun is actually sweeter to shoot than the 25mm piston gun even though its power output is marginally higher. I did find the Diana OEM piston seal to be very good, however  its not as consistent as the Vortek red Diana seal which i found to be as consistent as an O ring…really quite stunning, being a large seal it gives the soft cushion however which many like. I may in the future explore a home brew 28mm piston for the bigger side lever 52, however right now its so efficient at making power the easy route is to renew the rod on the LGV piston and sleeve down to 25mm.

When I finish the second 440TH a decision will be made which I prefer and the other will be sold, it only makes sense to keep both while I experiment. Out the 2 52Ultra K .22′s I have again one will be sold once I finish fine tuning them…If Diana do produce a 52 Ultra k in .177 i may jump on one, the Firebird while nice has had a hard life and will never look like new.

 

So stay tuned, I think I will start with the Diana 52 revisit with .177 update over the next week or so.

 

 

 

 

 

 

 

 

 

Update on the HW95 tuning

Its been a while…remember this post?

http://www.airguntech.com/?p=465

Well Its been on the back burner for a while and I decided I have had the rifle long enough here and should be getting it finished. Specs for the build were exactly as I outlined in the post above except for the transfer port, this was opened to 3.6mm over 3.3mm.

Spring was to spec above, I had to make a fresh guide and tophat for it as its smaller ID than the OEM spring…the result with approx 50mm of preload is 11fpe..so bang on the money. Tested pellets werte Daystate Li’s (Crosman Premiers), Superdomes, Geco and Hobby and RWS Superfield. 10 pellets of each weas pushed thru the gun and then 10 shots over the chrono…10.8 to 11.1 with all of them, highest seen velcity was 11.3fpe and this dropped once the rifle settled with the pellet under test. Recoil is light, very little surge…its actually quite nice to shoot and I will range test it to see how it settles…right now im quite pleased how it came out and for me a blueprint tune is now set.

 

 

 

 

 

 

 

 

 

 

Colin Molloy contact details

Colin has a new number, please contact him only on this new number for now.

07565582549

He is still bluing but as far as I know is not rebuilding the rifles, so you send him a kit of parts and he blues those parts for you.

 

 

 

 

Could this be the ultimate piston seal?

Been looking for an alternative seal now for a while, something off the shelf, easy to find and that works as well as an O ring if possible with minimal lost volume. Scouring the Hydraulic seal suppliers I came across Kastas.

Click the pic for a link to the site, I highlighted the seal I will be testing next on 25mm pistons.

Kastas seal

 

The reason for this seal was its size, it is around the same size as an O ring but is made of polyurethane, the pressure groove is tiny so the lost volume should be minimal and the seal sits in an easy to machine flat bottomed groove. My plan is to test the seal with flat fronted pistons (bronze piston nose) running tight tolerances to minimise the lost volume even further.

Some addition pics of the seal.

IMG_4461

 

 

 

 

 

 

 

IMG_4464

 

 

 

 

 

 

IMG_4465

Will update with some results and installation pics as the seal, as you can see its pretty small.

 

Weihrauch HW35 LGV

Back in July2012 I had the idea of building a Weihrauch HW35 fitted with a Walther LGV rotary piston and that’s just how it stayed until Christmas — just an idea.

Then in December I was fed up one Sunday  and decided to de-braze a HW35 cylinder from its breech block in order to show how the cylinder was made and to highlight the grooves round the breech block which give rise to the porous cylinder problem on the 35.

01012006666

As can be seen the original Weihrauch breech  it pretty long with 20mm of the breech brazed inside the cylinder with a total transfer port length of 26mm. With the cylinder de-brazed and armed with a few LGV piston measurements from Tony I was able to work out if the conversion of the HW35 to rotary LGV piston was possible. Everything looked good to go so I ordered a 16joule Walther piston kit from Germany (via Tony) and a short length of 30mm OD X 25mm ID Hydraulic tube. Prior to machining the breech block plug down I was already aware that I was going to break through into the breech latch slot and the breech stop hole — I was quite happy that these problems could be overcome by welding.

So I machined the breech block down by about 20mm:-

25012006689

As can be seen I took the precaution of bolting a spacer in between the breech jaws to stop them distorting when machining or welding.

Once the holes in the rear of the block had been welded shut I machined the breech down to leave a 0.5mm ridge on which to locate the 25mm ID tube.

25012006694

I could then clamp the 25mm tube onto the breech block and weld it on and then machine down to end up with a breech block with a 6mm long transfer port attached to a 25mm ID X 30mm ID cylinder.

26012006698

Now before welding this back into the original HW35 cylinder I decided to machine the piston to suite its new home.

With the original LGV piston seal the piston was too long and would cause a large gap between the cocking lever end and the rim on the rear of the LGV piston. So I decided the simplest option was to machine the seal retaining hub off the front of the piston and machine up a new bronze O ring piston head.

29012006706

With the piston sorted I was then able to weld the new breech arrangement into the original HW35 cylinder.

I then machined the weld down and linished the joint.29012006709

So I ended up with a HW35 cylinder with a 25mm bore which opened out to 30mm at the cocking lever slot enabling an altered Walther LGV piston to be used

Machining the front of the breech block not only enabled the Walther piston to be used but it reduced the gun’s transfer port down to the length of a HW77 transfer port at 6mm.

 

 

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 ;)

First job was the sleeve the compression tube, did not get any pics of this as we all know what  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 ;)

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 ;)

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 .22  (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 machined..it 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.