Tech­ni­cal Air­gun

Ba­sic Tech

Airgun World - - Contents -

Jim Tyler con­tin­ues his new se­ries on ba­sics. This month, it’s spring air­gun tun­ing; all about main­springs and how to fit a new one -

The next step up from the ba­sic kit de­scribed last month is a kit com­pris­ing a pair of spring guides and some preload wash­ers, plus a main­spring, which can be either a new fac­tory stan­dard spring, or an af­ter­mar­ket one, and some go fur­ther and in­clude a new pis­ton seal.

Fac­tory main­springs are rarely matched to the in­di­vid­ual ri­fle be­cause, to keep costs down via econ­omy of scale, only one spring spec­i­fi­ca­tion is nor­mally bought in by man­u­fac­tur­ers for all cal­i­bres in which any model is of­fered, or some­times even more than one model of air­gun (usu­ally the spring is used in con­junc­tion with dif­fer­ent pis­ton weights and/or preload wash­ers to achieve the de­sired muz­zle en­ergy ac­cord­ing to cal­i­bre, bar­rel length etc.), so fac­tory springs are in most cases a com­pro­mise. In favour of fac­tory springs is that they are usu­ally of good qual­ity, and spec­i­fied to de­liver the tar­get muz­zle en­ergy over a long life.

The non-stan­dard springs sup­plied with other tun­ing kits are in some cases made to or­der for the spe­cific model and per­haps cal­i­bre of air­gun for which the kit is sold, but more com­monly they are af­ter­mar­ket springs, not de­signed for any spe­cific air­gun so much as to phys­i­cally fit in­side the widest range of airguns. Some of the kit sup­pli­ers who use th­ese off the shelf af­ter­mar­ket springs have to shorten them to bet­ter suit the in­di­vid­ual air­gun.

OPIN­IONS DIF­FER

Re­gard­less of whether a kit comes with a fac­tory or af­ter­mar­ket spring, it should of­fer the ad­van­tage over kits that don’t in­clude a spring of the guides be­ing guar­an­teed cor­rectly sized

“If you find that you can­not fit a spring guide short of ham­mer­ing it in, I’d sug­gest not fit­ting it…”

for the in­di­vid­ual spring pro­vided. Opin­ions dif­fer on what ‘cor­rectly sized’ means, though; some tuners think that the spring should grip the guide very tightly, which qui­etens the shot cy­cle at the ex­pense of slowing it as the spring fights fric­tion against the main guide; oth­ers pre­fer a slacker fit, which al­lows the spring to move freely. My own feel­ing is that the guide should be a slid­ing fit in the spring, which damp­ens spring vi­bra­tion while not rob­bing the pis­ton of en­ergy.

The in­ter­nal di­am­e­ters of the two ends of a spring usu­ally dif­fer, and if the main rear guide is too slack or too tight in one end, it’s worth try­ing it in the other. If you find that you can­not fit a spring guide short of ham­mer­ing it in, I’d sug­gest not fit­ting it, be­cause so tight a fit could cause the guide to break. Most kit sup­pli­ers will hap­pily swap com­po­nents that don’t fit, so speak to them.

So which of the kits that in­clude main­springs is best? It is the more ex­pen­sive op­tion, but a be­spoke spring will have been de­signed by the tuner to work with a par­tic­u­lar air­gun and cal­i­bre in the way he wanted it to, and that is usu­ally a bet­ter op­tion (pro­vided the tuner’s pre­ferred shot cy­cle is the same as yours), than an off the shelf spring cut to length, or the man­u­fac­turer’s stan­dard spring. The prob­lem is that there is no such thing as an ‘ideal’ spring for any air­gun

STIFF­NESS AND PRELOAD

In or­der to achieve a tar­get muz­zle en­ergy, a spring must either com­press the air to a very high pres­sure for a very short space of time, for which it needs to be stiff, or it can al­ter­na­tively com­press the air a lit­tle less, but main­tain near peak pres­sure for a longer pe­riod, for which it can be less stiff, but needs

more preload to de­lay and slow pis­ton bounce.

The shot cy­cles with shorter and stiffer, or longer and softer springs are rather dif­fer­ent, as I’ll ex­plain shortly, and this pro­vides air­gun tuners, and the mak­ers of tun­ing kits, with an op­tion re­gard­ing the feel of the shot cy­cle, though those who sup­ply tun­ing kits usu­ally opt for a stiffer spring with less preload, not for the shot cy­cle feel, but for a very good rea­son.

With some airguns, com­press­ing the spring while fit­ting the trig­ger or back block of­fers po­ten­tial for things to go wrong. In the case of airguns with screw in trig­ger blocks, the per­son fit­ting the kit has to si­mul­ta­ne­ously com­press the spring while turn­ing the block to en­gage the threads, and the spring or guide can slip, so there’s po­ten­tial for an ac­ci­dent. The sell­ers of tun­ing kits can have no knowl­edge of the ex­pe­ri­ence and com­pe­tence of their cus­tomers, so they tend to favour stiff springs with lit­tle in the way of preload, min­imis­ing the chances of ac­ci­dents.

SHOT CY­CLE

The shorter, stiffer spring favoured by those who sell kits that in­clude springs give fast pis­ton ac­cel­er­a­tion, and briefer (by in the or­der of a mil­lisec­ond) pis­ton stroke times, ap­par­ent to most peo­ple and usu­ally de­scribed as ‘feel­ing fast’, more like a gas ram than a spring ri­fle. Many seem to favour this shot cy­cle but, this be­ing the spring air­gun, there is a small price to pay, and it is that the ri­fle surges fur­ther, and for longer.

Longer, softer springs that achieve muz­zle

If you’re sure of what spring spec­i­fi­ca­tion you want, you can have a small batch made, as Mike Wright and I did for our .177 HW95s.

If you have ac­cess to a lathe, it’s easy to make your own spring guides and top hats.

Kits of­ten have stiff springs, char­ac­terised by thick wire.

Be­fore fit­ting a kit, check that the spring guide is par­al­lel, and re­ject it if not.

Al­ways mea­sure a spring’s free length be­fore fit­ting it, so you can check for creep or fa­tigue in fu­ture.

Hav­ing ma­chined this TX200 pis­ton and made an adap­tor for closed face seals, I made an­other adap­tor and went back to the stan­dard seal.

Most kits in­clude so-called ‘power wash­ers’. Best start with none fit­ted, test and add preload if nec­es­sary.

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