by B.B. Pelletier
Tune is slang for tuneup, and in airguns a tuneup can range from a quick lubrication all the way to a major overhaul of the powerplant and trigger. Everything in between these two extremes is also fair game. So, lesson one is that a tune can be anything that changes and hopefully improves the airgun’s performance.
I’m going to address a breakbarrel spring gun in today’s report. Other powerplants can also be tuned; but the steps are different, and the results will differ from what you get with a spring gun tune. Since the majority of airgun tunes are performed on springers, it’s appropriate to look at them first. And the breakbarrel is the No. 1 type of spring gun.
Victor asked what was meant by a tune, but I suspect that others would like to know what’s involved, as well, so today we’ll look at airgun tuning in all its complexity. Let’s begin with a brand-new spring gun and see why we would tune it and what might be done.
Smoothing the edges
Most new spring guns have sharp edges on all the mating powerplant parts. Sometimes, these edges interfere with the movement of the parts. These edges are worn down during a long break-in period, which is why a gun gains velocity as it wears in. But you can also remove these edges and burrs with small files, and that is one thing that a tuneup can do.
Key places to look are the cocking slot, the piston slot, the cocking linkage and, if there’s an interface between the linkage and the piston, that’s a prime place to look for burrs and sharp edges. The forward edge of the cocking slot is especially important, because it can slice a new piston seal when it’s installed…and that will ruin the seal. The end cap and sides of the trigger mechanism should also be checked.
The action forks that the pivot bolt passes through is another place to look for burrs and sharp edges, as well as the sides of the baseblock that the barrel is pressed into.
There are also burrs and sharp edges that don’t affect the operation of the powerplant. These do not go away with use and they can be left alone if you like. However, if you plan to take the powerplant apart in the future, these edges and burrs will be waiting to cut you.
Probably the most common thing done during a tune is lubrication. New guns can have either too much grease or not enough. And most of them have the wrong kind of grease. The factories use a general machine grease, but there are much better greases that can be used.
For metal-to-metal contact, nothing is better than grease that contains a high concentration of molybdenum disulfide. Moly isn’t a grease — it’s a solid particle that’s ground very fine and mixed with grease for application. When it comes in contact with metal under some pressure, the particles bond with the metal on the surface, forming a layer of extreme low friction. That layer is durable and allows other metal to slide across the surface it’s on.
We don’t appreciate how low-friction moly is, because the grease it’s in raises the coefficient of friction. But custom tuners are known to burnish certain parts of a gun — like the inside of the compression tube — with dry moly particles. This process takes a long time, as the moly doesn’t want to cooperate; but once it’s, done you have a surface with very low friction. Jim Maccari and I split a pound of moly powder, and my half was in several large bottles. It’s a lifetime supply for a full-time tuner!
Another place where moly powder comes into play is on the mating trigger sear surfaces. I’ll have more to say about this in a moment, but this is a custom tuner’s trick. The action fork and baseblock can also benefit from a burnish of moly.
I don’t burnish anymore, though. Moly grease, such as Air Venturi Moly Paste, will do the same thing over time as it gets worked into the action through the process of shooting.
But not every springer needs moly grease. The older guns with leather piston seals actually do better with a white lithium grease. The grease serves as fuel for the constant dieseling of all spring-piston guns, and leather seals burn more fuel than synthetic seals do. For this same reason, I lube the mainsprings of the lower-powered springers like a Diana 27 with the same white lithium grease.
Does it bother you that I said all spring piston guns diesel? Well, they do. Don’t confuse dieseling, which is normal and even good, with detonation — which is when you here a low bang. That’s too large an explosion for your gun, and you don’t want to do very much of it.
The barrel pivot and the forks through which it passes is another place to grease. The right grease (moly) applied here reduces the cocking effort by 10 pounds!
The mainspring is the other place that gets lubed, and often it’s to stop the vibration, though I’m going to tell you in a moment a better way to do. For this, people use black tar, or what Jim Maccari calls Velocity Tar. It’s just a very viscous grease with a high adhesion that feels tacky to the touch. Farmers and heavy equipment operators know it as open gear lubricant. Most of the different greases like this will slow your gun to some extent, but there are products like Velocity Tar which, if used sparingly, seem to not phase the velocity at all.
Remove all the play
Okay, lubricating a gun to smooth the firing cycle is a redneck approach. Many people, including me, do it that way. But there’s a more elegant way if you’re willing to work. That way is to remove all the play in the various moving parts. The piston and mainspring are the primary parts involved.
The piston in a factory gun fits well inside the spring tube, but there’s a looseness to allow for manufacturing tolerances. The piston seal takes up a lot of the slack, but it’s located just at the front of the piston. The rear is free to move in all directions. While the space is small, this is where some of the vibration comes from.
To tighten the piston, it’s possible to put small bearings at the front and rear of the piston. These are usually small, round spots of synthetic material such as Teflon or nylon. Typically, three are placed at the front and three more at the rear. They are spaced evenly around the piston body, and the front ones are offset from those in the rear. If they fit the spring tube exactly, the piston rides on them, and then a moly coating really does its work.
The next critical fit is the mainspring, and here it’s sometimes possible to buy a spring that fits the spring guide in the rear and the piston rod in the front very tightly. Tuners call this close fit being “nailed on.” When you have a close fit like this, good moly lubrication is essential, or the close fit of steel on steel will cause galling, which is a form of burnishing that causes friction, vibration and excess heat.
If you can’t find a spring that fits this tight, you can always have a custom spring guide made that does fit the spring you have. Then, inside the piston, you can put a steel shim that fits between the mainspring and the inner walls of the piston. It’ll look shoddy; but once the powerplant is together, it’ll stay in place. And moly is essential here for the mainspring and the guide. This is called a “beer can” tune, because people often use cans to make the shim.
Another trick people use is to put shims behind the mainspring on the spring guide end. This puts the mainspring under more tension and gives more power. You have to make sure there’s enough room to cock the rifle when doing this, because it’s possible to shim the spring too much.
New airgunners assume that the stronger the mainspring, the more powerful the airgun. That isn’t always the case. Piston stroke has more to do with power than the spring rating. I always look for a weaker spring because I know it won’t subtract that much power from the gun. A coating of tar will do more to slow down a gun than a weak spring, as long as the spring fits well.
A final word on the mainspring is to notice that each end is usually a different size. Try to match the end with the spring guide or piston rod that fits best.
Piston seals used to be a real big reason for tuning a spring gun, because they wore out or melted from friction. Today’s seals are pretty well made, though there will always be some cheapies that come to market from time to time. The thing about the piston seal is to ensure that it fits the bore of the compression tube without adding too much additional friction. Some is unavoidable, but it’s easy to go overboard. The modern parachute piston seal that expands as it compresses air is very sophisticated, and shouldn’t be too difficult to size correctly. To reduce the diameter, put the seal on the piston and rotate the piston against sandpaper. Be careful to keep the sides of the seal parallel to the compression chamber bore while doing this. It usually only takes a minute or two for this job.
The trigger can be adjusted and lubricated during a tuneup. I lubricate with moly grease, because a trigger is not a part that works by friction. No matter how low you get the friction, the trigger should always be safe…but this is a place where home tuners often have problems. They either stone or file the mating sear surfaces and put a dangerous angle on them. Then, they lubricate them with moly. These are the triggers that slip when cocked.
People are also known to adjust a trigger to have too fine mating surfaces, and once more, they’ll slip when cocked. My advice is to lube first, then let the trigger work for several hundred shots before you adjust it. I would keep stones and files away from triggers unless you’re certain that you know what you’re doing.
This part is often overlooked and can sometimes give you a large boost in power. The breech seal doesn’t have to stand proud of the breech to work well. It all depends on how the gun is designed. But don’t overlook the possibility of improving performance by raising the breech seal a few hundredths of an inch.
I hope this report answers most of the questions you have regarding tuning an airgun. As I said at the start, a tune can be any of these things, or all of them. A professional tune is usually all, but you should discuss the specifics with your airgunsmith before letting him start the work.