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.
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.
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.