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DIY Tuning BB’s Diana 27: Part 3

Tuning BB’s Diana 27: Part 3

by Tom Gaylord
Writing as B.B. Pelletier

Part 1
Part 2

A history of airguns

This report covers:

  • Tuning time
  • Flat breech seal
  • New breech seal
  • Big lesson number one
  • Was there any change?
  • Oh, my!
  • Smooth
  • But wait…
  • Smoke tripped the chronograph start screen
  • Summary

What a story I have for you today! Talk about a trip to Serendib! I started writing before something incredible happened, so I will leave the start of the report as I originally wrote it and realign things when we get to the good part.

Tuning time

We learned in Part 2 that the Krytox grease that many people tout is no good for dampening vibration. After tuning Michael’s Diana 27 I wanted mine to be as smooth, so in Part two I cleaned out the 20+-year-old lithium grease from my rifle and applied Krytox, as several people have recommended over the years. But no dice. With Krytox my 27 buzzed again, though not as bad as a factory gun might. It also shot a little slower, though I think that might have been due to the breech seal and not the lube.

Diana 27 leather breech seal
The leather breech seal (arrow) is flat. It still works, but may not seal the breech as well as it should. Just to the left of the arrow’s point there is a small depression where some leather has fallen out.

Flat breech seal

I knew the breech seal was flat, but leather seals can be flat and still work. Mine, however, wasn’t just flat — it had a divot where some leather had popped out! Perhaps it wasn’t in tip-top shape any longer?

New breech seal

One way to tell if the old seal is bad is to install a new seal and test the gun. I learned from tuning Michael’s rifle that there is supposed to be a steel shim under the new synthetic breech seal, to raise it a bit higher. So, I ordered both parts for my rifle from T.W. Chambers & Co. and had them in less than two weeks.

Diana 27 breech seal shim
A new synthetic breech seal and steel breech seal shim were purchased from Chambers.

Big lesson number one

I usually remove breech seals with a dental pick. It works for both synthetic and leather seals alike. However, when the leather is dry-rotted, it doesn’t come out in one piece. It breaks up and comes out in many small pieces. That’s what happened to this seal. Even though it was lubricated the entire time it was in the gun, it somehow has dry-rotted and become useless. I had to scrape the pieces out with a pick and a small screwdriver blade.

When the breech seal channel was clean I placed the steel shim in first, followed by the new seal. The new seal sits proud of the breech, and I think it will seal better than the leather seal did.

Diana 27 breech with seal
With the shim the new seal sits a little proud of the breech face.

Was there any change?

I said I would check the velocity again after installing the new seal, but before I tore the gun apart and relubed it with Tune in a Tube. It’s always good to start from a known point. The last time I chronographed the rifle was after applying Krytox in Part 2. At that time RWS Superpoint pellets averaged 435 f.p.s. with a 51 f.p.s. spread. That velocity was down from the velocity I recorded when my lithium grease tune was on the gun. At that time it was 465 f.p.s. with the same pellet.

Oh, my!

When I tested it after installing the new breech seal the average velocity was 460 f.p.s. and the spread was 18 f.p.s. It went from 450 to 468 f.p.s. So, it’s back performing like it did before the Krytox was added. But that isn’t why I said, “Oh, my!”


The rifle is now smooth again! Apparently that dry-rotted leather piston seal was really letting the gun down and I never knew it. I suspected it was leaking because of the large velocity spread, but I had no idea that the Krytox tune would also smooth out when the breech was sealed better!

That is the first big lesson I learned today. I was all set to tear the rifle apart, clean it and lubricate it with TIAT, but now I don’t have to! Or at least I don’t want to. The Krytox tune is still not as smooth as Michael’s gun, but it’s back to being as smooth as it was with the lithium grease, and that makes me very happy. I owe those who recommended Krytox an apology for what I said in Part 2 and again at the beginning of this report. I still wouldn’t spend the money for Krytox, because it’s no smoother nor more powerful than a straight tune with white lithium grease that costs only a fraction as much — but it is a good grease.

But wait…

Yes, there is more. Something else happened today that you need to know about. I had oiled the Diana’s piston seal heavily for the Krytox test and it’s still pretty oily. And, when I chronographed today’s short test I was getting velocities like 183 f.p.s. and 201 f.p.s., interspersed with the 460s. I knew from seeing the pellet’s penetration in the duct seal trap that it wasn’t shooting that slow. So — what could it be? Well, it’s big lesson number two!

Smoke tripped the chronograph start screen

The rifle is dieseling, as all spring-piston guns must, but because there is a lot of oil on the piston seal, this one is smoking a lot. Apparently the smoke is escaping the muzzle ahead of the pellet and tripping the start screen. That starts the chronograph’s clock, so the time of flight appears to take longer. The solution is to back up the muzzle a foot from the start screen and everything works fine again. In those 12 inches the smoke dissipates and only the pellet is seen by the skyscreens.


I’m going to leave my Diana 27 tuned with Krytox for the time being. It’s back to being as smooth as I remember and I want to give the Krytox a chance to break in.

How about that? A return to normal velocity and the Krytox tune smoothed out — I got a twofer, just by adding a new breech seal! And, I don’t need to tear this rifle apart again. I guess that’s really a threefer!

What’s coming next is a look at the Diana 35. That was the air rifle that started this whole series of reports. There is a lot to tell you about the 35, but let’s let that wait until then.

27 thoughts on “Tuning BB’s Diana 27: Part 3”

  1. BB,
    I am a krytox convert. But I don’t think it was ever meant to dampen anything. Once you shim and get properly sized spring guides then it comes into it’s own.

    • Edw
      Did you shim up you spring anymore when you did the Krytox?

      My TR5 has a little thump when shooting now after shimming the spring. And no lube other than what came from the factory.

      I usually use lithium grease when I do springers. So I may do that next. I have been wanting to try Tune in a Tube also. So that may come also later on.

      And I was going to mention this the other day and forgot. I like to put a couple drops of silicone oil into the transfer port from time to time with my spring guns with synthetic seals.

      Have you thought about that with your TR5. And how did you do it. That is If you could.

      • I didn’t put anything around the spring, just .005 stainless discs on either end and the two brass colored washers. I didn’t have anything to sleeve it properly, but it all fits quite tightly as is and the spring can spin freely now.

        • Edw
          I didn’t put anything around the spring either. Just that 3/8″ spacer in front of the spring. And mine is actually pretty quiet shot cycle wise. Just makes a thump when it shoots. No vibration either. It’s actually quieter now with the spacer shim than it was without. So that’s why I haven’t been worried about lubing mine.

  2. B.B.

    What little I know is that Krytox does NOT mix well with Dinosaur lube. Lets see if the
    ‘real oily ” piston seal degrades it over time.
    How is the smoke getting past the pellet skirt? I could see the smoke delays the stop function of the chronograph. The device reads the trailing smoke as still part of the pellet. I other way around I just don’t get?


  3. B.B.,

    “Threefer”,….. good one. 😉 Interesting how the breech seal fixed it all. Makes sense though.

    On the smoke/chrony issue,… I have had a springer crack and then blow smoke (after) the shot. So I take it that some smoke was (still) in the barrel and ahead of the newly loaded pellet? If so, blowing through the breech end would evacuate it. I am not convinced that smoke was the cause. Not sure how you could test that theory further, but it would be an interesting test.

    Good Day to you and to all,….. Chris

    • Chris,

      As I was telling Yogi, my Webley smokes real bad. So bad that you cannot see through the bore. He had the muzzle right at the start screen. Talk about muzzle velocity.

  4. B.B.,
    That’s some great info here, especially about the breech seal!
    And I think it’s really cool how “The Guru of Airguns” is still learning things…as are we right along with him. =>
    Thanking you for the interesting report,

  5. B.B.,

    So Krytox works best as a lubricant for precision fitted parts. Where there had been wear or some sort of slop between fitment of parts it would be best to use some form of grease as lubricant. This is similar to using cyanoacrylate based glue for parts that precisely fit and an epoxy glue for parts with some gaps between.

    Breech seals seem to have an effect more than we think regarding how an airgun behaves when shot.


    PS: Section Tuning time first paragraph first sentence: “We learned in Part 2 that the Kryrox (Krytox) grease that many people tout is no good for dampening vibration.

    I had always thought it was Serendip until further research showed that it could also refer to Serendib.

  6. Once again I learned something from the master. I knew that that the breach seal affected speed, but I didn’t imagine that it would affect the dynamics so much. In hindsight, it kind of makes sense but, then again, most things seem obvious after we understand them.

    Thanks B.B. for taking us on the ride!

    PS. It’s time to go back and check the breach seal of my old and abandoned Mahely rifle.

  7. B.B.,

    Great report today! We all got to learn something new I believe. Just a thought, could you put some TIAT on the spring without disassembly, thus possibly making the gun less prone to vibrate?

    This shows that we need to be more cognizant of breech seals and their condition. Keep them lubed with silicone grease as well.

  8. B.B.,

    Do you suppose the new, rubber breech seal makes the shot cycle smoother because it provides a bit of shock-absorption? I have a couple air rifles with flattened leather seals that seem to have a sharp recoil, and I’m wondering if that might be the reason.


    • Michael
      A bad breech seal is like dry firing a springer in a sense.

      The piston slams forward because there is no cushion of air to slow the piston down. Or not enough cushion.

      Kind of like using a light pellet that fits slightly loose in the barrel. No time for the piston to slow down. It just slams forward.

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