by Tom Gaylord, a.k.a. B.B. Pelletier
This report was requested by blog reader Rob 8T2 and seconded by a number of others. I reported on the spring piston forward spring guide that’s known as a top hat, and he wondered if I was also going to talk about the AirForce top hat. Though they share the same name, the two items aren’t connected in any way. One is a spring guide, and the other is an adjustable valve stem for a pneumatic valve. The adjustable valve stem draws its name because, like the forward spring guide, it also looks like a gentleman’s top hat in profile.
The sliding breech cover has been pushed forward, cocking the striker and revealing the breech for loading. This is the original valve from a first-generation AirForce Talon without a power adjustment wheel.
To understand the AirForce top hat, we have to go back in time to before AirForce Airguns air rifles had power adjustment wheels on the left side. In January 2000, I reported on the new AirForce Talon, the first air rifle to be offered to the U.S. market by the new AirForce company. But I’d already owned the rifle I tested for two years. I bought it from the UK company, GunPower, in 1998, when it was configured differently and sold as their Stealth rifle. In late 1999, AirForce Airguns sent the parts to reconfigure my rifle to their new Talon specification, with an 18-inch .22-caliber barrel.
By that time, I already had many hours of testing and shooting on the rifle in its original Stealth configuration with its 12-inch barrel. It was during that time, by talking to AirForce owner and creator John McCaslin, that I learned how to adjust the top hat of my rifle to change the power.
You may find it amazing to learn that I could adjust that rifle to fire from 65 f.p.s. all the way up to 950 f.p.s. with 14.5-grain Eley Wasps. In truth, any velocity below about 400 f.p.s. was just a parlor trick. The rifle could do it, but all consistency was lost. From 400 to 950, though, it was fairly consistent.
How the top hat power adjustment worked
Power was adjusted by screwing in the top hat to shorten the valve stroke and the valve dwell time (the time the valve stayed open). The valve stroke became shorter because the wide flange at the base of the top hat contacted the top of the valve body and stopped moving. Then, the valve return spring started pushing the valve stem closed again, aided by the high-pressure air inside the reservoir.
Conversely, a longer valve stroke meant more dwell time and more air flowing out. At some point, however, the pellet left the 18-inch barrel, and the longer valve stroke stopped having any additional influence. Once the pellet’s out of the barrel, no amount of additional air can push it any faster.
To loosen the top hat for adjustment, unscrew a tiny 0.050″ Allen screw in the large knurled bottom flange of the hat, allowing it to turn on its threads. Once the desired clearance was reached, the small screw was tightened again. This screw caused problems because enthusiastic owners were over-tightening it, causing it to put dents in the hollow valve stem it contacted. In later years, AirForce started putting two screws in this flange to increase the locking pressure and hopefully reduce the damage to the valve stem.
The o-ring secret
Adjusting the top hat was a chore. One day, airgunsmith Tim McMurray told me about an easier, more convenient way. He said to slip a rubber o-ring around the top hat flange, so it rode in the space beneath the flange. It very effectively limited the amount of valve stem travel. Once I found out how good it was, I left it in place all the time. I wasn’t interested in sheer velocity. I wanted good accuracy at a reasonable level of power. Nothing has changed in 14 years, has it?
Talon SS puts an end to top hat adjustment
In November 2000, I wrote about the new Talon SS, which was the first AirForce rifle to have a power adjuster on the left side of the gun. My own SS was a pre-production prototype that didn’t have the power scale engraved on the side of the rifle; but after 14 years of continuous use, it’s still working fine and the air tank has never leaked.
The Talon was also updated with the power adjustment wheel at the same time. Now all AirForce sporting PCPs have power wheels and the top hats no longer need adjustment.
John thought that the power-adjustment mechanism would put an end to the fiddly top-hat adjustment, but it didn’t! By the time the power adjuster came on the market, there was a lot of interest in AirForce Airguns…and the internet was abuzz with homebrew ideas of how they should be set up and operated. People did use the new adjustment, but they also continued adjusting their top hats. Top hats continue to be adjusted and discussed right down to today!
The truth about the top hat
The truth is that the top hat is still a very influential part of the AirForce system. It does have tremendous impact on the rifle’s operation, though not always in the ways you read on the internet. Now that the rifles have the power adjustment wheel, the top hat has become more of a starting point or a setting that gives each rifle a potential range of power. The power adjustment wheel is what fine-tunes that range. The top hat is a set-and-forget kind of adjustment, only people are not leaving it alone.
Some valve stems are very thin, such as those found on the Hi-Flo valves. When the screws are over-tightened on these valves, they dimple all too easily. That’s one reason I advise owners not to adjust their top hats.
Top hats are set at the factory with feeler gauges. I’ve told several people I once discovered that an American quarter coin was exactly the right thickness to set the top hat for a Condor Hi-Flo valve. I’ve actually done that more than once. A difference of one or two thousandths of an inch from the factory spec usually isn’t critical. But when the difference grows larger than that, it does start to become critical. It depends on which rifle you’re talking about to determine if the difference is critical.
For the record, I left AirForce Airguns in 2005, and a lot of things have changed since then. I’m not qualified to give out factory specs on anything they make today. I don’t adjust my top hats at all. I use the power adjuster 100 percent of the time when I adjust, and most of the time I leave each gun set at the position that gives me the greatest accuracy.
You wanted to know about the AirForce top hat — there you go.