Give Us a Tune

Jim ap­plies a ba­sic, low- cost tune to his three-year-old Walther LGU un­der-lever

Airgun World - - Contents -

Jim Tyler tunes his faith­ful Walther LGU and demon­strates just how easy it is to do

Like all full-sized Euro­pean spring air­guns, the LGU was de­signed with one eye on the large North American mar­ket, where there is no limit on muz­zle en­ergy, and where high muz­zle en­ergy has long been a ma­jor sell­ing point. At the same time, the com­pany will have been un­der no il­lu­sions re­gard­ing the im­por­tance of its own Eu­ro­zone mar­kets, the coun­tries of which have a range of muz­zle en­er­gies from 6 ft.lb. (7.5J) to 16 ft.lb (20J) and more, be­cause Eu­ro­zone LGU prices, un­like prices in the USA and UK, are not af­fected by cur­rency fluc­tu­a­tions. The com­pany had to try to de­sign a spring air­gun that would be ca­pa­ble of high muz­zle en­ergy, but shoot well down to 6 ft.lb., and part of its so­lu­tion was to make the ri­fle with two dif­fer­ent piston-stroke lengths.

The LGU is sup­plied with a piston rod that gives a 70mm piston stroke for 6 ft.lb. mar­kets, and one that gives 88mm for the rest of the world, so LGUs sold in the UK have the same 88mm of avail­able stroke used in the USA, and other coun­tries where the ri­fle achieves con­sid­er­ably more than 12 ft.lb. I have seen a fig­ure of 23J (circa 17 ft.lb.) quoted for these ex­port mar­kets, and the dif­fer­ences be­tween them and our 12 ft.lb. ri­fles are in­ter­est­ing.

PORT

Ob­vi­ously, the LGU as sold in the UK is fit­ted with a spring that makes less en­ergy avail­able to the piston than ri­fles in­tended for un­re­stricted mar­kets, no surprises there, but there is a sur­prise when we look at the trans­fer port.

The trans­fer port is very short at 5.5mm which, from my ex­per­i­ments into trans­fer port length, would sug­gest that it is very en­ergy ef­fi­cient but, in order to tame the muz­zle en­ergy, the port di­am­e­ter has been set at a tiny 2.6mm, which negates the po­ten­tial en­ergy gain from hav­ing such a short trans­fer port.

It’s easy to vi­su­alise a nar­row trans­fer port throt­tling air flow and re­duc­ing muz­zle en­ergy, but there’s more to it than that. From Mike Wright we learned that the di­am­e­ter of the trans­fer port dic­tates the point in the shot cy­cle at which mass air flow chokes at Mach 1, and the 2.6mm port of the LGU chokes the air flow far too early in the shot cy­cle, re­duc­ing the mass air flow, as well as the con­tri­bu­tion from the air’s ‘ther­mal’ en­ergy, to muz­zle en­ergy. Lit­tle won­der that most LGU tunes will start with open­ing up the trans­fer port.

“The pri­mary (rear­ward) re­coil seemed lit­tle if at all af­fected by the en­large­ment of the trans­fer port di­am­e­ter”

MAIN­SPRING

Why such a nar­row trans­fer port, though? My guess is that the com­pany wished to fit the same main­spring they were al­ready us­ing in the break-bar­rel LGV, which is com­mon prac­tice in the trade, and it is pop­u­lar be­cause the greater the num­ber of springs in an order, the lower the unit price. There is one huge dif­fer­ence be­tween the LGV and LGU in­ter­nals, though, and it is the trans­fer port which, on the LGV, is no less than 26.9mm long, which low­ers en­ergy ef­fi­ciency, which is com­pen­sated for by the main­spring, which is fairly stiff. Put a 12 ft.lb. LGV main­spring in an LGU with a much more ef­fi­cient trans­fer port length, and you’ll need to take steps to re­duce the ef­fi­ciency, such as drilling the port 2.6mm.

THE AP­PLI­ANCE OF SCIENCE

The ques­tion is, what di­am­e­ter the trans­fer port should be, and most will ap­proach the sub­ject us­ing the finest tra­di­tion of ‘suck it and see’ en­gi­neer­ing, mean­ing open it up a bit and mea­sure the muz­zle en­ergy, then re­peat un­til the muz­zle en­ergy starts to drop, by which time they’ve ob­vi­ously gone a tad too far. There has to be a more sci­en­tific way.

Ap­ply­ing Mike Wright’s three ex­per­i­men­tal for­mu­lae to sug­gest trans­fer port di­am­e­ter yielded, as ever, three dif­fer­ent re­sults: 2.9mm, 3.3mm and 3.59mm, the last of which, based on try­ing to match the cylin­der and pellet pres­sure pulses at the on­set of chok­ing, I dis­counted due to the very short length of the trans­fer port, and the fact that any port di­am­e­ter greater than 60% of the length (3.3mm) would fail to cap­i­talise prop­erly on the air’s el­e­vated in­ter­nal en­ergy (heat). The fig­ure of 2.9mm is based on at­tempt­ing to time the chok­ing to co­in­cide with the pellet’s at­tain­ing 80% of muz­zle ve­loc­ity, and the 3.3mm fig­ure is based on es­ti­ma­tions of the cylin­der pulse and com­pres­sion stroke: nei­ther is in­fal­li­ble, but both give a start­ing point.

The science, then, was point­ing at some­where be­tween 2.9mm to 3.3mm as the best di­am­e­ter, and I de­cided first to try a com­pro­mise 3mm.

EF­FI­CIENCY

The en­ergy ef­fi­ciency when the main­spring was new, cal­cu­lated as the per­cent­age of en­ergy made avail­able to the piston dur­ing the com­pres­sion stroke com­pared to that of the pellet at the muz­zle, was 37.9%, which is a tad low for a ri­fle with such a very short trans­fer port. As time went by, the main­spring lost a few mil­lime­tres in length through creep, al­though it wasn’t weak­ened in any way, and was per­fectly ser­vice­able, so the preload re­duced, and the ef­fi­ciency fell to 34%, ris­ing to 37% as I in­creased preload to com­pen­sate. With the 3mm trans­fer port, and with the preload wash­ers re­moved to leave just 23mm of preload – when the spring was new it had been 28mm – ef­fi­ciency jumped to just shy of 43%. That, as they say, is more like it, and the port will prob­a­bly stay at 3mm, de­pend­ing on the shot cy­cle.

SHOT CY­CLE

The pri­mary (rear­ward) re­coil seemed lit­tle if at all af­fected by the en­large­ment of the trans­fer port di­am­e­ter, though the com­pres­sion stroke and hence re­coil will def­i­nitely be a frac­tion longer, but the fol­low­ing for­ward surge, dur­ing which, the pellet ex­its the muz­zle, was re­duced to just over 60% of that with the 2.6mm port, which tells me that more of the en­ergy from the com­pressed air is driv­ing the pellet, rather than driv­ing the piston bounce.

What was of es­pe­cial in­ter­est was the piston land­ing ve­loc­ity at the end of the sec­ond for­ward stroke af­ter piston bounce, which was just 1.5 M/s. In pre­vi­ous tests with the orig­i­nal 2.6mm trans­fer port, and spring preload of 33mm – which had given the same muz­zle en­ergy – the piston land­ing had been 3.6 M/s, which was due to the spring stor­ing more en­ergy dur­ing piston bounce, cour­tesy of the greater preload, and to the con­sid­er­able gain in en­ergy ef­fi­ciency.

To be hon­est, the piston’s sec­ond (fi­nal) land­ing is not some­thing I have ever been aware of feel­ing when shoot­ing a ri­fle, other than ex­cep­tion­ally harsh land­ings above 6 M/s when, af­ter a few shots, I can feel a tin­gling sen­sa­tion in the pad of my trig­ger fin­ger and, be­cause the pellet is long gone by the time of the piston land­ing, it can­not in it­self di­rectly af­fect ac­cu­racy. How­ever, a harsh land­ing is one of the fac­tors that can trig­ger a pe­riod of fairly fren­zied main­spring ac­tiv­ity, which does the sight pic­ture no favours, pos­si­bly ham­per­ing fol­low-through, lead­ing to longer term less­ened ac­cu­racy if the shooter suc­cumbs to the temp­ta­tion to pull the eye from the scope to look di­rectly at the tar­get in the hope of see­ing the pellet hit.

BAD VI­BRA­TIONS

As the spring neared its set­tled length of 233mm, spring vi­bra­tion had started to ac­com­pany ev­ery shot, and en­larg­ing the

trans­fer port in­creased the noise vol­ume, pre­sum­ably as the spring’s nat­u­ral res­o­nance reached some crit­i­cal point. As spring twang goes, this was not par­tic­u­larly loud, and au­di­ble only to the per­son shoot­ing the ri­fle, but I find it very dis­tract­ing, so it had to go. Al­ter­ing spring preload can stop the twang, but in­creas­ing it would have taken the muz­zle en­ergy in the wrong di­rec­tion – it was al­ready 11.6 ft.lb. – so I re­duced it by 1mm to 22mm, and the twang in­creased in vol­ume. The muz­zle en­ergy dropped to 11.4 ft.lb., which was where I wanted it, so I de­cided to leave the preload at 22mm and dampen the vi­bra­tion with grease.

The prob­lem with us­ing grease on the main­spring is keep­ing it on the main­spring rather than it be­ing flung off, so I nor­mally run springs with min­i­mal, if any, grease, but this case needed a heav­ier ap­pli­ca­tion, so I used a com­mon or gar­den moly CV grease which I thick­ened us­ing fumed sil­ica.

Fumed sil­ica looks like a fine white pow­der, but the tiny ‘gran­ules’ are like bits of 3-D Vel­cro, with tiny hooks that latch on to their neigh­bours so that, when sus­pended in grease, they act like an in­ter­nal mesh and pre­vent the grease from flow­ing, or be­ing thrown off the spring.

Sounds good, but fumed sil­ica must be treated with great cau­tion, The par­ti­cles weigh so lit­tle that even gen­tle air move­ments will get them air­borne, and they can cause se­vere res­pi­ra­tory prob­lems if breathed in. I only open the con­tainer in­doors, with all doors and win­dows shut, with it inside a card­board box, and I use a dust mask just to be sure. So that I don’t have to deal with the raw ma­te­rial too of­ten, I mix up a very rich so­lu­tion of fumed sil­ica and CV grease, which ren­ders the fumed sil­ica safe, and di­lute some of it with more grease when needed. Please note that fumed sil­ica en­riched grease is purely for damp­ing main­spring vi­bra­tions, and not for lu­bri­ca­tion.

CYLIN­DER LU­BRI­CA­TION

The LGU piston seal is fairly hard and has higher fric­tion than many, so some own­ers fit al­ter­na­tive seals, ei­ther the 25mm HW seal as used in the HW30, or an af­ter­mar­ket al­ter­na­tive. I could have used an HW seal, or one of the many ex­per­i­men­tal seals I’ve made from polyurethane, but de­cided to re­tain the LGU seal, and re­duce its fric­tion.

Many peo­ple re­duce tight piston seal fric­tion by re­duc­ing the seal’s di­am­e­ter, which works, but leaves the dan­ger that, in very cold weather, the seal can shrink enough to lose con­tact with the cylin­der wall, po­ten­tially caus­ing all man­ner of prob­lems and ru­in­ing ac­cu­racy. There is an­other way. I mea­sured piston seal ki­netic (slid­ing) fric­tion in a se­ries of ex­per­i­ments last year, and found that the pres­ence of even the tini­est trace of ‘wet’ lubri­cant on the cylin­der wall more than halved ki­netic fric­tion which, in the case of the LGU, can mean a ft.lb or more at the muz­zle. Rather than ap­ply lubri­cant (a molyb­de­num rich grease) di­rectly to the piston seal or cylin­der wall, I ap­ply it to the front piston bear­ing area, so that a trace is left on the cylin­der wall ev­ery time the ri­fle is cocked, and that is all that’s needed.

Scep­tics may think that the higher muz­zle en­ergy with wet cylin­der lubri­cant points to diesel­ing, but from pre­vi­ous research I know that if diesel­ing con­trib­utes to muz­zle en­ergy, it ben­e­fits only higher start pres­sure pel­lets, and not the very low start pres­sure Air Arms Di­abolo Field and Ex­press pel­lets I used with the LGU. The lit­mus test is to mea­sure the muz­zle en­ergy with high and low start pres­sure pel­lets of sim­i­lar weight; RWS Hobby and Fal­con Ac­cu­racy Plus, or RWS Su­per­dome and Air Arms Di­abolo Field. If the high start pres­sure pel­lets have the higher muz­zle en­ergy, they are diesel-as­sisted. In the LGU, they do not.

In sum­mary, open­ing the trans­fer port to 3mm, cou­pled with re­fresh­ing cylin­der lubri­cant, which might need re­fresh­ing very oc­casi­nally, has re­stored both my LGU’s per­for­mance, and im­proved its man­ners.

The LGU is ridicu­lously easy to strip. First, re­move the four stock bolts.

I use moly grease around the front piston bear­ing.

The cock­ing link sim­ply lifts out of the cylin­der.

You can then pull out the piston and cylin­der com­plete.

There is very lit­tle pre-load, so no sash cramp needed.

Take care not to dam­age the piston seal as you ease it past the cock­ing lever hole in the cylin­der.

Tighten the rear stock bolts be­fore of­fer­ing up the front stock screws to en­sure that the holes are aligned.

I was look­ing for 11.4 ft.lb., and that’s what I got.

This is the cal­cu­lated re­coil cy­cle of the ri­fle fit­ted with a 0.5kg scope.

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