Beeman R8: Part 6
by Tom Gaylord
Writing as B.B. Pelletier
The Beeman R8 looks like a baby R1.
This report covers:
- Don’t over-lubricate
- Rail Lock mainspring compressor
- Lube the piston seal
- Clean the mainspring
- Finish the assembly
- What is it like now?
- Velocity test
Last Friday’s report set us up for today. I was discussing “tuning” airguns before knowing how they performed. I didn’t do that with this rifle, but that discussion loosened up a lot of minds, and I got many suggestions of what to do with the R8. I wanted to disassemble it and remove most of the “special” grease I had applied, and then Gene Salvino of the Pyramyd Air tech department and I had a long conversation about what was happening with that rifle.
Gene said it is very possible to put too much of that grease into a lower-powered spring gun. He said if you do that you’ll get exactly the result I got with the R8 — a reduction of several hundred feet per second in the velocity.
He didn’t say this, but here is what I am guessing happens with this grease. At room temperature and when it is just sitting out loose it is tacky, but no more so than many other greases. But put it under the pressure of an airgun piston moving fast and the grease stiffens up. Other tacky greases don’t change under pressure, but this grease is a special blend and that is why it is so good at what it does — which is reduce vibration in a spring gun. That’s just my guess — not a known fact. But it explains why the gun behaved like it did when I used too much.
I won’t document the teardown a second time, because it’s there in Part 5. But I will note some things that were different this time.
Rail Lock mainspring compressor
I like the Air Venturi Rail Lock Mainspring Compressor a lot! It is so small and allows so much access to the spring tube of the gun where work needs to be done. This time I installed it and had the rifle apart in minutes. Let me just tell you about the threads on the end cap to illustrate how easy it is to use. I expected to be able to feel when the spring tube threads released the end cap, because it was still under a lot of tension from the mainspring, but I could not feel the release. That tells me the compressor is under complete control at all times.
This time I tightened the screws in the rear of the compressor and noted that the white Nylon tip did not walk around the end cap as it came out from the gun. There was some walking when the end cap went back in, until the threads were engaged, but it was simple and easy to control. At no time did I lose control of what was happening.
Thus far I have only tested the Rail Lock compressor on a Weihrauch rifle that has a threaded end cap. There are many more types of spring guns to disassemble, so the compressor report is just getting started.
Once the rifle was apart I began degreasing everything. I cleaned the inside of the spring tube with a long dowel that had some paper towel on the end, held by a rubber band. This towel was saturated with isopropyl alcohol. There wasn’t much grease inside the tube, but it took a LOT of scrubbing to remove! This grease is sticky!
After cleaning the inside of the tube I dried it with another bit of paper towel. This job took the longest of all, because that grease was holding onto the honing scratches inside the tube.
Following that I cleaned the piston — inside and out. That was a simple wiping job. I also want to show you the piston seal, because several of you asked about it last time.
According to Gene Salvino, this much of their special grease on the piston and mainspring will really slow down the action of a weaker spring gun if the piston seal is too tight. If it’s not too tight a little of their grease works well, but I didn’t know whether it was too tight or not, so I assumed it was. I removed all the grease from the piston, inside and out, and about half from the spring.
This is a parachute piston seal. When the seal goes forward and compresses air, air gets in the groove and presses the sides of the seal against the compression tube wall.
Please don’t examine the picture of the piston seal above and think you see imperfections. I examined it thoroughly, both last time and this time, and it is in perfect condition. I’ve seen enough bad piston seals to spot one.
Oh, I did clean out the groove in the seal after that picture was taken. It took a stout paper clip and lots of paper towel.
Lube the piston seal
Okay, Gene’s next tip for me was to lightly lubricate the piston seal with 85W-140 gear oil. Don’t use anything else! The synthetic gear oil is too runny for this job. It doesn’t take much! I used just 2 drops. This is a special treatment that’s only for those piston seals that are too tight.
Use two drops of this gear oil on the clean piston seal.
It took 20 seconds for one drop of this gear oil to run like this. It is viscous!
I spread the 2 drops evenly around the piston seal that is larger than the piston body behind it. The seal touches the wall of the compression chamber; the piston body never does.
I also used a second 2 drops of gear oil around the enlarged rear skirt of the piston. The spring guide is supposed to keep the piston body from ever touching the inner walls of the spring tube, but a little oil back there doesn’t hurt.
Clean the mainspring
The mainspring was already coated with the special grease. All I did was remove about half of it, or maybe a little more.
I wiped about half the grease off the mainspring.
This is what the cleaner spring looked like when it was back inside the gun.
Finish the assembly
The remainder of the assembly went well, except for one thing. As I was grabbing the end cap to turn it in the threads I rubbed my index finger against the side of the trigger slot and sliced it open. Oh, well. They say the job’s not done until you leave some DNA inside!
What is it like now?
With a lot (3/4?) of the grease gone from the rifle I wondered what it would shoot like. Would there be some spring buzz? Nope! It is just as smooth and quiet as before. So Gene got that part right, at least.
Now let’s see where the velocity is.
I didn’t finish the test. Obviously the rifle did not change significantly. Man, was I disappointed! I felt certain I’d have dramatic results to show you today. I even replaced the breech seal, though in truth the seal that was in the rifle looked fine.
The breech seal that was in the rifle is standing proud of the breech. It appears to be good.
The seal that was in the rifle is on the left. You can see a little flaring at the top, but it looks pretty good next to the new seal on the right.
This is the groove the breech seal sits in. As you can see, the depth is not suitable for a o-ring.
After the “test” I called Gene Salvino and had a long talk. We came to the conclusion that either the piston seal that I think is okay really isn’t or the mainspring is tired, or both. I have tuned a couple hundred spring guns in my life. Gene does that many every year. He is much better acquainted with the details of the work I am doing.
He is sending me a new seal and spring. I will install the new seal first, followed by the new spring, if necessary. That way we will know which one was the culprit.
I can think of one other possibility. The honing I see in the compression chamber does not seem to be done correctly. The scratches run perpendicular to the movement of the piston, where they should be a series of crosshatch scratches that run on an angle approximately 45 degrees to the piston’s axis. It is possible that the bore of the compression chamber was enlarged by aggressive honing and doesn’t allow the piston to compress all the air that’s inside. I hope that’s not the case.
This series started as a look at an historic airgun, but has turned into an advanced tutorial on tuning a spring-piston air rifle. I don’t enjoy reporting negative results, but things like this do happen and you need to know that if you ever plan to tune airguns.