Jim turns his at­ten­tion to his old HW55, and its con­ver­sion from a leather to syn­thetic pis­ton seal

Airgun World - - Technical Airgun -

Be­fore the in­tro­duc­tion of the syn­thetic pis­ton seal, the first of which ap­peared in the late 1970s Fein­werk­bau Sport, leather was the ma­te­rial of choice, and had been for al­most a cen­tury. In many ways, leather is an ex­cel­lent ma­te­rial for pis­ton seals, not least be­cause one can be fash­ioned by the DIY in­clined from re­cy­cled leather from shoes, belts or fur­ni­ture, but leather has a draw­back in­so­far as it has to be ‘wet’ lu­bri­cated.

For many years, leather seals were lu­bri­cated us­ing neats­foot oil, an an­i­mal fat ex­tracted from the shin and foot bones of cat­tle, and which is un­usual for an an­i­mal fat in re­main­ing liq­uid at low tem­per­a­tures. In some re­spects, neats­foot oil is ex­cel­lent for seal lu­bri­ca­tion, be­cause it soft­ens the leather to al­low it to con­form to the cylin­der, and con­di­tions it, al­though on the flip side, it ox­i­dises in time so the leather hard­ens, and it auto-ig­nites at 442C. As we know all too well, au­toigni­tion is as­so­ci­ated with diesel­ing.

Neats­foot oil slowly gave way to var­i­ous light min­eral oils, which also auto-ig­nited, some at lower tem­per­a­tures than neats­foot oil. As the late Mike Wright es­tab­lished, auto-ig­ni­tion cre­ates a pres­sure spike in the com­pres­sion stroke that can get the pel­let mov­ing ear­lier, in­creas­ing the time that the pis­ton and pel­let are trav­el­ling in the same di­rec­tion, in­creas­ing muz­zle en­ergy, and many spring airguns with short pis­ton strokes, in­clud­ing the HW55, could only pro­duce rea­son­able muz­zle en­ergy by har­ness­ing auto-ig­i­ni­tion. New airguns were de­liv­ered with a plen­ti­ful sup­ply of oil to en­sure pro­longed diesel­ing, and ad­vised that a cou­ple of drops of oil be ap­plied through the trans­fer port at reg­u­lar in­ter­vals.


Through decades of trial and er­ror, air­gun man­u­fac­tur­ers had es­tab­lished that their diesel-de­pen­dent guns pro­duced max­i­mum muz­zle en­ergy with wide di­am­e­ter (typ­i­cally 4mm) trans­fer ports, al­though for a va­ri­ety of rea­sons, when they started to sup­ply guns with syn­thetic pis­ton seals, they dis­cov­ered that the guns gave their best with far nar­rower trans­fer ports, typ­i­cally from 2.9mm to 3.2mm. Again from Mike Wright, we know that the larger di­am­e­ter trans­fer ports of diesel­ing airguns de­lay pis­ton bounce, max­imis­ing the ‘cylin­der pulse’ – the pis­ton travel be­tween pel­let start

and pis­ton bounce – and the en­ergy given to the pel­let.

More re­cent airguns with syn­thetic pis­ton seals tend to have longer pis­ton strokes, so don’t need to diesel, nor do they need to de­lay pis­ton bounce by hav­ing a large di­am­e­ter trans­fer port. In fact, if any­thing, they need nar­rower trans­fer ports to ad­vance the on­set of trans­fer port, mass air flow chok­ing, be­cause that’s when the air’s in­ter­nal ki­netic en­ergy is con­verted to ‘flow’ en­ergy. So, to gain the most from con­vert­ing a leather sealed air­gun to syn­thetic, the 4mm di­am­e­ter trans­fer port usu­ally needs to be sleeved down to nearer 3mm.

The HW55’s con­tem­po­raries and sta­ble­mates the HW50 and es­pe­cially the HW35, which sold in huge num­bers, are can­di­dates for trans­fer port sleev­ing, as well as the An­schutz 335 and others, if switch­ing from a leather pis­ton seal.


Most leather pis­ton seals are at­tached by a cen­tral fi­bre washer, it­self held to the pis­ton face by a ma­chine screw run­ning into a threaded hole in the pis­ton. The con­ver­sion en­tails sim­ply re­mov­ing the leather seal, and fit­ting an adap­tor us­ing the orig­i­nal ma­chine screw, then the new seal.

The most pop­u­lar adap­tor is the one that al­lows Weihrauch seals to be used, which also gives ac­cess to var­i­ous af­ter­mar­ket seals de­signed to fit Weihrauch airguns. If you have ac­cess to a lathe and an of­f­cut of Ac­etal or sim­i­lar en­gi­neer­ing plas­tic rod, an adap­tor can be turned in a few min­utes at a cost of pen­nies; if not, ex­pect to pay up­wards of £10 for a com­mer­cially made one.

Many peo­ple who have con­verted old spring airguns from leather to syn­thetic seals have re­ported a fall in muz­zle en­ergy as a re­sult. To a small ex­tent, this is due to the al­ready short stroke be­ing made a mil­lime­tre or two shorter by the seal and adap­tor, but the main rea­son for the fall in muz­zle en­ergy is the re­duc­tion in the cylin­der pulse due to the lack of diesel­ing.

The 4mm di­am­e­ter trans­fer ports of many airguns with leather seals may de­lay pis­ton bounce slightly, ex­tend­ing the cylin­der pulse, but this alone is not enough to com­pen­sate for the loss of diesel­ing and con­se­quent early pel­let start. The loss of muz­zle en­ergy can be mit­i­gated to an ex­tent, though, by ad­vanc­ing the on­set of trans­fer port chok­ing through sleev­ing the trans­fer port.


Some years ago, I used my old HW77 and a range of trans­fer port in­serts to prove some­thing, prob­a­bly for the first time, that many had long sus­pected, which is that there is a gen­eral re­la­tion­ship be­tween trans­fer port length and en­ergy ef­fi­ciency, with shorter ports tend­ing to be more en­ergy ef­fi­cient. Air­gun

man­u­fac­tur­ers may have sus­pected the same, but pro­duced airguns with ports in ex­cess of an inch long re­gard­less, prob­a­bly due to po­ten­tial prob­lems seal­ing the cylin­der to the end plug with less plug length in­side the cylin­der, as well as con­cerns about the strength of the joint. The HW55 trans­fer port is not un­typ­i­cal at 25.9mm long.

Short­en­ing the HW55 trans­fer port is not prac­ti­cal be­cause it would en­tail re­mov­ing the end plug/breech block, ma­chin­ing and re­fit­ting it, and that would leave a short length of in­ter­nal thread on the cylin­der wall for the pis­ton seal to rub against. Other ri­fles may be dif­fer­ent, but with the HW55, I’m stuck with 25.9mm.

I had pre­vi­ously made and fit­ted an Ac­etal trans­fer port sleeve with a 3mm hole, but later re­moved it af­ter it crept for­ward a frac­tion into the breech, so de­cided to try a more sub­stan­tial al­ter­na­tive.

The sleeve needs to be 4mm ex­ter­nal di­am­e­ter with a length of 25.9mm and a cen­tral hole ide­ally of 2.9mm to 3mm. Un­less you have a model en­gi­neer’s lathe, then due to the po­ten­tial for flex caused by the length and di­am­e­ter of the in­sert, it would per­haps be bet­ter to start with a length of 4mm brass or per­haps mild steel rod, than to ma­chine a larger di­am­e­ter rod down.


My ma­chine is way too big and cum­ber­some for such pre­ci­sion work, al­though I’m sure a skilled ma­chin­ist could make it work, and a half re­mem­bered post on Air­gunBBS gave a cheap al­ter­na­tive that did not need a lathe at all, so Jon Budd, please take a bow. Jon’s so­lu­tion was to cut a length from an old 4mm di­am­e­ter ra­dio aerial, a pos­i­tively clas­sic ex­am­ple of ‘shed tuner’ in­ge­nu­ity!

As luck would have it, the aerial of my work­shop ra­dio had bro­ken off re­cently, so I re­trieved it from the waste bin, and found that one sec­tion was 4mm in di­am­e­ter. The in­ter­nal di­am­e­ter of 3.5mm was larger than ideal, but a step in the right di­rec­tion, so I cut off a 25.9mm length us­ing a fine cut­ting disc in a high speed drill, cleaned up the ends with a drill bit, and slid it into the HW55’s trans­fer port with a tiny dab of lock­ing so­lu­tion to hold on to it. Job done – cost £0.00.


My tar­get was the HW55’s de­signed muz­zle en­ergy of 7.5 joules (6ft.lb.), which it achieved eas­ily. I have pre­vi­ously achieved far higher en­ergy from the ri­fle, al­though at the cost of los­ing the sweet-shoot­ing char­ac­ter­is­tics of the orig­i­nal. When time per­mits, I’ll al­most cer­tainly ob­tain a length of 4mm brass rod and re­place the length of aerial. It and many clas­sics of the late 1950s to 1970s are more than worth the ef­fort.

The ideal port sleeve would be 4mm tube with a 3mm bore, but even sleev­ing to 3.5mm is a step in the right di­rec­tion.

Another al­ter­na­tive to a leather seal is to turn a syn­thetic cup seal like this, which is for an An­schutz 335.

Even af­ter molyb­de­num lu­bri­cants were widely adopted, lav­ish ap­pli­ca­tions to leather pis­ton seals en­cour­aged diesel­ing.

With the sur­plus grease re­moved, it’s plain that this seal has long passed its ‘sell by’ date!

With around 3mm of re­coil, the HW55 is a joy to shoot.

Tip: A cou­ple of blocks of Ac­etal in the cock­ing link pre­vent galling with the cylin­der.

A sharp twist bit cleaned up the cut edges per­fectly.

The sleeve ended up a frac­tion shy of 25.9mm, but that did not seem a prob­lem. I put a tiny dab of lock­ing so­lu­tion on the end of the sleeve be­fore push­ing it home.

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