Hw 55 Tyrolean - Part 5
How to measure a spring
by B.B. Pelletier
Annoucement: There's a new article on Pyramyd Air's website. "How to mount a scope" includes a short intro and avery comprehensive video guide for mounting scopes on just about any airgun.
On to today's blog.
With all the interest you guys have in tuning springers, I thought this would help. BG_Farmer has expressed curiosity about the spring selection process. Actually, there are aftermarket springs that are much higher quality than factory springs, and I would use good ones whenever I could, but you have to know if they fit.
So, although I'm writing this about the HW 55T, it applies to any mainspring in any gun.
Step 1. The spring guide
The mainspring must fit the spring guide as close as possible to reduce vibration and to lengthen spring life. When the gun's cocked, the coils compress around the guide, which prevents them from kinking. The piston rod fits inside the guide and passes through (usually) to contact the sear.
If you have your new spring on hand, you can just try the fit; if not, you must measure the inside of the old spring with a caliper. It takes practice, but many calipers have the ability to measure both outside and inside diameters. Measure the spring at both ends, because the dimensions will seldom be the same. If the factory mainspring fits the guide loosely, as many do, you'll have to make an educated guess how much smaller you can go. Remember, as a coiled spring is compressed, both the inside and the outside diameters increase a little.
Step 2. Inside the piston
As the piston compresses the spring, it begins to fill the inside of the piston. The spring guide is also entering the bottom of the piston and traveling toward the top as the piston goes to the rear. So the mainspring ends up sandwiched between the spring guide and the inner wall of the piston. And it expands slightly as it compresses. Do not fit the spring to the piston tightly while it is not compressed or it will not fit well when the gun is cocked.
Step 3. Compressed spring length
Determining the length of the fully compressed mainspring takes only simple arithmetic. First, count the number of coils in the spring. Start at the top where the spring begins and go straight down the side to the bottom. Don't be surprised if there's a fraction of a coil remaining after all complete coils have been counted (include that fraction in your count).
Next, with your calipers, measure the wire diameter of the mainspring. Again, this takes a little practice, so do it several times until you get the smallest diameter.
Now, multiply the number of coils by the wire diameter to get the length of a fully compressed spring.
Wayne's HW 55T mainspring has 31.75 coils with a wire diameter of 0.109". Multiply those numbers and you get a compressed length of 3.46". The spring I hope to use for the tune has just less than 32 coils of wire with 0.118" diameter. That works out to a compressed length of 3.776". The new spring is 0.316" longer than the spring that was in the gun when it's fully compressed. Will there be enough room for it? Perhaps not, but I can always cut off a coil to make it fit. I would have to flatten the end of the spring at the cut, but that would reduce the length even more.
Now, both ends of the spring are ground flat, and my methods didn't take that into account, so both springs are a little shorter than calculated. That may be handy to know in just a moment.
Step 4. How long can the spring be?
Determining the total possible length that's available for the mainspring is easy with a Weihrauch rifle. Install the trigger in the end cap and slide the piston through the end cap until it cocks the trigger. The maximum spring length is the distance from the face of the end cap to the inside top of the piston, minus the thickness of the spring guide base and the base of any top hat guide, if there is one. In our case, there's a top hat, so its base thickness is also subtracted from the total.
I measured the total available length at 3.947". The spring guide rim is 0.10" thick and the rim of the top hat is 0.120" thick so the total space available is 3.947" minus 0.220", which leaves 3.727" The new spring is probably too long to allow the gun to cock. I'll have to cut off one coil for the fit.
Is this method exact? No. It can be off several thousandths for each measurement. Because the measurements are so close and also because I said the springs are really a trifle shorter than I calculated, I will try the spring as it is. I don't think it will allow the gun to cock, but testing will tell for sure. If I have to, I'll remove one coil. I'll let you know.