Give Us a Tune
Jim applies a basic, low- cost tune to his three-year-old Walther LGU under-lever
Jim Tyler tunes his faithful Walther LGU and demonstrates just how easy it is to do
Like all full-sized European spring airguns, the LGU was designed with one eye on the large North American market, where there is no limit on muzzle energy, and where high muzzle energy has long been a major selling point. At the same time, the company will have been under no illusions regarding the importance of its own Eurozone markets, the countries of which have a range of muzzle energies from 6 ft.lb. (7.5J) to 16 ft.lb (20J) and more, because Eurozone LGU prices, unlike prices in the USA and UK, are not affected by currency fluctuations. The company had to try to design a spring airgun that would be capable of high muzzle energy, but shoot well down to 6 ft.lb., and part of its solution was to make the rifle with two different piston-stroke lengths.
The LGU is supplied with a piston rod that gives a 70mm piston stroke for 6 ft.lb. markets, and one that gives 88mm for the rest of the world, so LGUs sold in the UK have the same 88mm of available stroke used in the USA, and other countries where the rifle achieves considerably more than 12 ft.lb. I have seen a figure of 23J (circa 17 ft.lb.) quoted for these export markets, and the differences between them and our 12 ft.lb. rifles are interesting.
Obviously, the LGU as sold in the UK is fitted with a spring that makes less energy available to the piston than rifles intended for unrestricted markets, no surprises there, but there is a surprise when we look at the transfer port.
The transfer port is very short at 5.5mm which, from my experiments into transfer port length, would suggest that it is very energy efficient but, in order to tame the muzzle energy, the port diameter has been set at a tiny 2.6mm, which negates the potential energy gain from having such a short transfer port.
It’s easy to visualise a narrow transfer port throttling air flow and reducing muzzle energy, but there’s more to it than that. From Mike Wright we learned that the diameter of the transfer port dictates the point in the shot cycle at which mass air flow chokes at Mach 1, and the 2.6mm port of the LGU chokes the air flow far too early in the shot cycle, reducing the mass air flow, as well as the contribution from the air’s ‘thermal’ energy, to muzzle energy. Little wonder that most LGU tunes will start with opening up the transfer port.
“The primary (rearward) recoil seemed little if at all affected by the enlargement of the transfer port diameter”
Why such a narrow transfer port, though? My guess is that the company wished to fit the same mainspring they were already using in the break-barrel LGV, which is common practice in the trade, and it is popular because the greater the number of springs in an order, the lower the unit price. There is one huge difference between the LGV and LGU internals, though, and it is the transfer port which, on the LGV, is no less than 26.9mm long, which lowers energy efficiency, which is compensated for by the mainspring, which is fairly stiff. Put a 12 ft.lb. LGV mainspring in an LGU with a much more efficient transfer port length, and you’ll need to take steps to reduce the efficiency, such as drilling the port 2.6mm.
THE APPLIANCE OF SCIENCE
The question is, what diameter the transfer port should be, and most will approach the subject using the finest tradition of ‘suck it and see’ engineering, meaning open it up a bit and measure the muzzle energy, then repeat until the muzzle energy starts to drop, by which time they’ve obviously gone a tad too far. There has to be a more scientific way.
Applying Mike Wright’s three experimental formulae to suggest transfer port diameter yielded, as ever, three different results: 2.9mm, 3.3mm and 3.59mm, the last of which, based on trying to match the cylinder and pellet pressure pulses at the onset of choking, I discounted due to the very short length of the transfer port, and the fact that any port diameter greater than 60% of the length (3.3mm) would fail to capitalise properly on the air’s elevated internal energy (heat). The figure of 2.9mm is based on attempting to time the choking to coincide with the pellet’s attaining 80% of muzzle velocity, and the 3.3mm figure is based on estimations of the cylinder pulse and compression stroke: neither is infallible, but both give a starting point.
The science, then, was pointing at somewhere between 2.9mm to 3.3mm as the best diameter, and I decided first to try a compromise 3mm.
The energy efficiency when the mainspring was new, calculated as the percentage of energy made available to the piston during the compression stroke compared to that of the pellet at the muzzle, was 37.9%, which is a tad low for a rifle with such a very short transfer port. As time went by, the mainspring lost a few millimetres in length through creep, although it wasn’t weakened in any way, and was perfectly serviceable, so the preload reduced, and the efficiency fell to 34%, rising to 37% as I increased preload to compensate. With the 3mm transfer port, and with the preload washers removed to leave just 23mm of preload – when the spring was new it had been 28mm – efficiency jumped to just shy of 43%. That, as they say, is more like it, and the port will probably stay at 3mm, depending on the shot cycle.
The primary (rearward) recoil seemed little if at all affected by the enlargement of the transfer port diameter, though the compression stroke and hence recoil will definitely be a fraction longer, but the following forward surge, during which, the pellet exits the muzzle, was reduced to just over 60% of that with the 2.6mm port, which tells me that more of the energy from the compressed air is driving the pellet, rather than driving the piston bounce.
What was of especial interest was the piston landing velocity at the end of the second forward stroke after piston bounce, which was just 1.5 M/s. In previous tests with the original 2.6mm transfer port, and spring preload of 33mm – which had given the same muzzle energy – the piston landing had been 3.6 M/s, which was due to the spring storing more energy during piston bounce, courtesy of the greater preload, and to the considerable gain in energy efficiency.
To be honest, the piston’s second (final) landing is not something I have ever been aware of feeling when shooting a rifle, other than exceptionally harsh landings above 6 M/s when, after a few shots, I can feel a tingling sensation in the pad of my trigger finger and, because the pellet is long gone by the time of the piston landing, it cannot in itself directly affect accuracy. However, a harsh landing is one of the factors that can trigger a period of fairly frenzied mainspring activity, which does the sight picture no favours, possibly hampering follow-through, leading to longer term lessened accuracy if the shooter succumbs to the temptation to pull the eye from the scope to look directly at the target in the hope of seeing the pellet hit.
As the spring neared its settled length of 233mm, spring vibration had started to accompany every shot, and enlarging the
transfer port increased the noise volume, presumably as the spring’s natural resonance reached some critical point. As spring twang goes, this was not particularly loud, and audible only to the person shooting the rifle, but I find it very distracting, so it had to go. Altering spring preload can stop the twang, but increasing it would have taken the muzzle energy in the wrong direction – it was already 11.6 ft.lb. – so I reduced it by 1mm to 22mm, and the twang increased in volume. The muzzle energy dropped to 11.4 ft.lb., which was where I wanted it, so I decided to leave the preload at 22mm and dampen the vibration with grease.
The problem with using grease on the mainspring is keeping it on the mainspring rather than it being flung off, so I normally run springs with minimal, if any, grease, but this case needed a heavier application, so I used a common or garden moly CV grease which I thickened using fumed silica.
Fumed silica looks like a fine white powder, but the tiny ‘granules’ are like bits of 3-D Velcro, with tiny hooks that latch on to their neighbours so that, when suspended in grease, they act like an internal mesh and prevent the grease from flowing, or being thrown off the spring.
Sounds good, but fumed silica must be treated with great caution, The particles weigh so little that even gentle air movements will get them airborne, and they can cause severe respiratory problems if breathed in. I only open the container indoors, with all doors and windows shut, with it inside a cardboard box, and I use a dust mask just to be sure. So that I don’t have to deal with the raw material too often, I mix up a very rich solution of fumed silica and CV grease, which renders the fumed silica safe, and dilute some of it with more grease when needed. Please note that fumed silica enriched grease is purely for damping mainspring vibrations, and not for lubrication.
The LGU piston seal is fairly hard and has higher friction than many, so some owners fit alternative seals, either the 25mm HW seal as used in the HW30, or an aftermarket alternative. I could have used an HW seal, or one of the many experimental seals I’ve made from polyurethane, but decided to retain the LGU seal, and reduce its friction.
Many people reduce tight piston seal friction by reducing the seal’s diameter, which works, but leaves the danger that, in very cold weather, the seal can shrink enough to lose contact with the cylinder wall, potentially causing all manner of problems and ruining accuracy. There is another way. I measured piston seal kinetic (sliding) friction in a series of experiments last year, and found that the presence of even the tiniest trace of ‘wet’ lubricant on the cylinder wall more than halved kinetic friction which, in the case of the LGU, can mean a ft.lb or more at the muzzle. Rather than apply lubricant (a molybdenum rich grease) directly to the piston seal or cylinder wall, I apply it to the front piston bearing area, so that a trace is left on the cylinder wall every time the rifle is cocked, and that is all that’s needed.
Sceptics may think that the higher muzzle energy with wet cylinder lubricant points to dieseling, but from previous research I know that if dieseling contributes to muzzle energy, it benefits only higher start pressure pellets, and not the very low start pressure Air Arms Diabolo Field and Express pellets I used with the LGU. The litmus test is to measure the muzzle energy with high and low start pressure pellets of similar weight; RWS Hobby and Falcon Accuracy Plus, or RWS Superdome and Air Arms Diabolo Field. If the high start pressure pellets have the higher muzzle energy, they are diesel-assisted. In the LGU, they do not.
In summary, opening the transfer port to 3mm, coupled with refreshing cylinder lubricant, which might need refreshing very occasinally, has restored both my LGU’s performance, and improved its manners.
The LGU is ridiculously easy to strip. First, remove the four stock bolts.
I use moly grease around the front piston bearing.
The cocking link simply lifts out of the cylinder.
You can then pull out the piston and cylinder complete.
There is very little pre-load, so no sash cramp needed.
Take care not to damage the piston seal as you ease it past the cocking lever hole in the cylinder.
Tighten the rear stock bolts before offering up the front stock screws to ensure that the holes are aligned.
I was looking for 11.4 ft.lb., and that’s what I got.
This is the calculated recoil cycle of the rifle fitted with a 0.5kg scope.