Spring shortening....

Many of us, “Springer” airgun enthusiasts have at some time, needed to replace a main spring in our guns. Some would leave the job to a gunsmith, which is always the best plan if you don’t feel confident or competent to do the job yourself, but there is a certain feeling of deep gratification when YOU work on YOUR rifle, to set it apart from the others.

Fitting an aftermarket spring isn’t a difficult job for anyone with some general handyman skills, the right tools and a little knowledge about how the spring gun is put together.
The replacement spring will undoubtedly change the shooting characteristics of the gun, not least the muzzle energy. You will see the disclaimers on the spring packaging, saying that in fitting this spring the muzzle energy may exceed the 12ft.lb legal limit....!
So, if this happens with your new spring, you will need to shorten it.

Before we look at how to do that, perhaps we should try to understand a little more about the spring material and how it responds to the inevitable application of heat at the shortened end.

Spring Steels.

There are many different types of material that can be used in spring production. Each material has different compositions and properties. Steel spring material is typically 0.45 – 0.90% Carbon, with additions of Manganese, Silicon, Chromium and Vanadium. In general, the more the alloy content, the larger the diameter of wire or rod can be hardened through the entire section.
Because of the small diameter wires used in manufacturing airgun springs, there is no need to add cost by using highly alloyed steel. Typically, small diameter wire coil springs will be made from “Carbon” spring steel, or “Silicon-Manganese” spring steel. The composition of the steel wire (particularly the Carbon content) makes it able to respond to heat treatment. We can make it harder or softer, by a combination of controlled heating and cooling cycles.
For a spring steel to give us the elastic properties needed, it is necessary to first harden, then temper the steel.
Hardening involves heating the steel wire to between 870 – 930oC (cherry red/orange colour), then quenching in oil. The rapid cooling causes microstructural changes, resulting in a very hard and brittle material. In this condition, the spring steel is useless. To make the steel function as a spring, being able to compress without breaking, we will need to temper the steel, by re-heating within the range 425-500oC for a period of around twenty-thirty minutes. Effective tempering is both time and temperature dependent and “refines” the microstructure bringing about a reduction in the hardness and increase in the toughness, thus achieving that optimum combination we are seeking.
Airgun springs tend to be made from spring wire that is pre-hardened and tempered to optimum properties. Spring material needs to have the capacity to store energy by elastic deformation, so the metallurgical condition of the steel wire is of huge importance. Too hard and the spring will almost certainly result in brittle fracture; too soft and the spring coils will suffer “plastic” deformation when compressed, causing the coils to collapse.

Back to the job in hand...

You will have established from your chronograph, just how “Hot” your gun is with the new spring fitted. You will decide how many coils you will need to remove. This is somewhat down to trial and error as springs for different guns will vary.

When coils are removed to reduce the free length and consequently the “ESC” (Energy Storage Capacity) of the spring, the cut end will result in an “Open End.”(picture?)
This open end will need to be closed and then ground so that the end of the spring sits flat on the spring guide face, or at the bottom of the piston.(picture?) I have seen first-hand, how some very respected air gun manufacturers, or their UK agents, have reduced the length of what was an over powered spring and left an open end with just a small, flat “foot” ground onto the wire end! (picture)This is obviously not good practice and can result in scoring of the spring guide mating face, so a closed and ground end will be needed. To achieve this, the spring wire is heated then pressed firmly and squarely against a flat plate to collapse the end coil to make a closed end. Then the closed end can be carefully ground (I do mine on a vee block, dressing the closed end on the side of a grinding wheel) to make a flat face so that the spring can now stand vertical.
Many tuning guides will advocate polishing the closed ends of the spring to reduce end-load friction and consequently the torque generated when the spring is released; it’s probably marginal in improving the overall efficiency of the spring, but we are always seeking to reduce recoil on our spring rifles...aren’t we?

Sounds quite simple...?

At this point, I need to put my Metallurgist’s hat on again to warn against the apparent misguided information out there on how to do this properly..!!
You will remember that I earlier talked about hardening and tempering springs so that they are useful and not prone to brittle fracture.
When we heat the wire to collapse the open end, there is no need to heat more than a small area of one coil. Too often, the belief is that the whole of the last couple of coils will have to be bright red hot to collapse the end. Then, it is well known that springs are “hardened“ so, we dunk the end in old engine oil. The result...?, a very hard, very brittle end to your spring..!! It may look to have done the job, but we have not considered what metallurgical changes we are making to the steel. Didn’t I say that a hardened spring was...useless? Most times, you will see how useless when you use it in your rifle, only to find that the last two coils break off.
You know what was needed right? .......Tempering of course!
The problem is, to temper effectively the material will need to get up to 400oC minimum, for twenty minutes to remove the brittleness and render the hardened coils free from risk of brittle fracture.
Do we all have a furnace available to do this...sadly, not.
Another misinformed view is that having quench hardened the end of your spring, it is good practice to polish the end coil and warm it up with the blow torch until the polished steel turns blue. These temper colours depict the colour of the oxide film that forms at different temperatures. Blue temper represents a temperature between 290-310oC. Then a quick quench in oil or water and the steel is tempered. Sorry guys, it won’t be tempered anything like enough to prevent brittle fracture. Remember I said that the tempering process was both time and temperature dependent..? Well what I just described is neither hot enough nor for long enough to get the metallurgical changes we need.

There is an alternative solution....

I said previously, that there is no need to heat more than a small area of the wire to close the end of the coil. Maybe more surprisingly, you don’t need to heat to red hot either to soften the wire sufficiently to get the plastic deformation at the right point to close the end of the spring. The tensile strength of the wire is greatly reduced at temperatures well below red heat and more importantly, below the temperature that those microstructural changes take place that can give us that nasty.. “hard stuff!”

I have found most success with using the brazing attachment on my butane gas torch. The flame can be adjusted to a narrow, one inch length, with a clear blue area towards the tip of the flame. (photo?)
Play the blue (hottest) area of the flame on the first coil, at roughly 180-270o (half to three quarters of the coil) from the cut end of the wire.(photo?)
When the area gets to a dull red heat (650-700oC), you can quickly collapse the open end against a suitable flat surface. Let the end of the spring cool in air. This is a good time to go for a cup of tea.
If the coil doesn’t flatten completely, you can repeat the process. I’ve never had to repeat the process, but you won’t do any harm if you keep the hot area localised.
Finish by grinding the end flat, then use 240 grit, wet and dry on a flat sanding block to get a smoother finish. Those of us that seek perfection, will now polish the flat face with metal polish.....I don’t know how much benefit it brings, but it makes me feel good about my gun!

Jez Finnemore.