The spring-piston powerplant
Today, I’m going to do a fundamental report that I promised some time back. I’ll show you how a spring-piston powerplant works. I’ll also show you how to measure mainsprings so you can find replacements when yours wears out. We had a question yesterday morning from a blog reader named Peter:
Hi Tom! Been trying to locate a replacement spring online for about the last six months. What I have is a pre-war Diana model 45 underlever. I have read on different blogs that springs from other guns can be used; but when contacting these spring suppliers, they say that if it’s not in their online catolog it’s not available. At any rate, there must be someone, somewhere that would be willing to sell a spring that would work in this gun. What baffles me most is there must be hundreds of other people who have located springs for their old airguns, but in six months of searching have had no luck in locating their source. Tom, I need your help before I go completely bazonkoos!!!
Well there’s not much chance of a pre-war Diana airgun mainspring being available anymore, is there? World War II ended in 1945, which was 67 years ago, and the chances of finding a new old-stock mainspring are virtually impossible. However, all is not lost. There are plenty of mainsprings around today that can be made to work. All you have to do is measure the old spring, so you know what’s needed. I’ll come back to this, but first I want to discuss how the spring-piston powerplant works.
The spring-piston powerplant
There are a number of ways they can work, but the basics of the spring-piston powerplant is a spring pushing a piston that compresses air that then pushes the pellet. That’s it in a nutshell. There are no valves or moving parts, other than the piston and mainspring. The air is compressed ahead of the piston in a part of the spring tube called the compression chamber.
In the end of the compression chamber, there’s a hole called the air transfer port. It’s where the compressed air leaves the compression chamber and gets behind the pellet, because the breech is in line with the transfer port.
This side view of two air transfer ports shows how they work with the piston to channel high-pressure air behind the pellet. The port may be offset compared to where the breech is located in relation to the compression chamber.
This drawing shows how the pellet seals the end of the compression chamber, which is on the right in this view. The compressed air has nowhere to go, other than behind the pellet.
This Beeman R1 rifle has a special replaceable transfer port installed. That’s the reason the Allen screw is on top of the spring tube. But the transfer port here looks like most breakbarrel transfer ports.
When a spring gun is fired
Most spring guns, including those with gas springs, work the way I’m about to describe. When the gun is cocked, the mainspring is compressed by the piston, which is held in the rearward position by the sear. When the trigger releases the sear, the piston springs forward, compressing air as it goes. When the air is fully compressed it stops the piston before it hits the end of the compression chamber.
On some guns that aren’t properly designed or guns that have been tuned incorrectly, the piston actually slams into the end of the compression chamber because the air pressure isn’t high enough to stop it. This is why some pellets feel much harsher than others in a spring gun — because they either move too soon and are unbalancing the powerplant (allowing things to happen that shouldn’t) or they remain in place too long, allowing the piston to rebound off the high-pressure air in the compression chamber.
This is the question that started this report. A reader asked me to show what a spring-piston powerplant looks like. This is it, except for the outer spring tube.
These are the major parts of a spring-piston powerplant. They fit inside the spring tube and the mainspring fits inside the hollow piston. It also fits over the spring guide, which is shown in front of the trigger. Some guns also have a shorter spring guide at the front of the spring that fits inside the piston. Because of its shape, it’s called a top hat — shown above the end of the spring.
The piston is hollow, as you can see here. The mainspring fits between the central rod, which is used to engage the sear and the inside of the piston. The better the fit here the less the gun will buzz. A tin soda can could be cut to make a liner to fit inside the piston and take up any slack between the mainspring and the piston.
Here you see the piston in the cocked position. The trigger has caught the piston and will hold it in place until the trigger is pulled. The spring guide is not shown here so you can see this relationship, but it would be against the front of the trigger and extending into the hollow piston, inside the spring. The piston rod that you can see passes through the spring guide.
How to measure a mainspring
Pete asked where to get a mainspring for his pre-war air rifle. I told him he’ll have to get one that is close and make it work, Let’s take a look at what that means.
There are several measurements you need to know when measuring mainsprings. One is how fat the spring is on the outside, so you can determine if it will fit inside a piston. Be careful with this one, though, as springs will buckle when they’re compressed and become wider than they measure at rest. Always allow yourself some wiggle room until you get the hang of it.
The second measurement is the inside diameter of the spring. It has to fit over the outside of the spring guide and top hat, if there is one. You want the fit of the spring over the rear spring guide to be very tight or “nailed on” as Jim Maccari says. The spring will get a trifle wider when it’s compressed, so don’t worry if it’s very tight — as long as it fits over the guide.
Lastly, you need to know how long the mainspring will be when it’s fully compressed. That helps determine if you’ll be able to cock the gun or the spring will become coil-bound before the sear grabs the piston. Look at the picture of the piston in the cocked position above. The compressed mainspring has to fit in the space provided when the piston is in that position and the spring guide is in place.
This graphic tells you how to calculate the compressed length of a mainspring.
What if the spring is too long?
You can live with a spring that’s too long, as long as all the other measurements work. When the spring is too long, you cut off some coils to make it fit. I use a Dremel tool with a cutoff wheel for this, and I cut with the wheel at 90 degrees to the axis of the spring wire. Then, if I want a beveled end, I can grind the end of the spring on a bench grinder to get it flat again.
I don’t bother to collapse the last coils of the spring the way they come from the factory, but if I wanted to I would heat them red hot and whack them with a hammer until they bunch up. I would have to use heat-sink paste or a good wet towel above the spot where the spring was heated to prevent more of the spring wire from softening this way. Then I would quench the job in water when it was completed. But like I said, I don’t bother doing that and my springs seem to work fine.
Where to get mainsprings
Now that you know how to measure a mainspring, you probably want to know where to buy them. I know of three possible sources. First is Pyramyd Air, which sells many replacement springs for current and obsolete models. Some of the old Weihrauch, FWB and Diana mainsprings can be modified to fit many older airguns. The HW55 mainspring and the FWB 124 mainspring are both quite long and can be cut to fit many other guns.
The second source is John Groenewold, of JG Airguns. Contact him at https://www.jgairguns.biz. John buys vintage parts in the UK for many old classic airguns and often has parts for those oddball guns. He may even know if he has a spring that will work for you, even if it isn’t commonly listed anywhere.
Third and perhaps the best of all is Jim Maccari, also known as the Springman. Visit his website at https://www.airrifleheadquarters.com. Jim makes springs for many older airguns, plus he can sometimes find a spring that will serve for something other than what it was made for.