by Tom Gaylord
Writing as B.B. Pelletier

Diana 27
Michael’s Winchester 427 is a Diana model 27 by another name. The rifle pictured is my Hy Score 807/Diana 27

Part 1
Part 2
Part 3

A history of airguns

This report covers:

  • Reader MarklinJHawkland
  • The blind pin
  • Sheared the screw
  • The lesson
  • Cleaning the spring tube
  • Other cleaning
  • The assembly
  • Summary

I’m tuning and overhauling reader Michael’s .22 caliber Diana 27, which is actually a Winchester 427. In Part one I disassembled it and in Part two I showed you how the ball bearing trigger works. In Part three I talked about removing the old piston seal that had a screw that was stuck. I have a lot more to say about that today.

Reader MarklinJHawkland
This is the reader who solved the puzzle. I knew there is a pin in the head of the piston, because when I disassembled the rifle someone had tapped that pin in far enough for me to see it. Unfortunately it was probably that and not the rust that had jammed the screw in place.

And BB Pelletier added insult to injury by trying to drift this pin on through the piston! I figured I was just unable to see the other side of the pin on the head of the piston, because who in their right mind would ever use a blind pin to secure a screw?

The blind pin
A blind pin only goes into something halfway. Think of it like a nail. You can’t push it all the way through because it doesn’t go all the way through, so there is no way to push it back out from the other side. I never thought anyone would use such a thing on a part that is meant to be disassembled periodically.

Well, Diana of the 1970s did! I don’t think it’s there to secure the screw; I think Diana used the blind pin to fasten the piston head to the sheet metal piston body. And, when it is tapped in farther, it impacts the screw threads from one side, jamming them, in place. Then, add 40 years of rust to the problem and that screw isn’t going anywhere!


This is the head of the original pistol in Michael’s rifle. That blind pin shouldn’t be tapped in like this.

Was the pin jammed in that way at the factory? I don’t want to think that it was, but when I opened the gun the parts looked like they had never been apart, yet the pin was definitely driven into the piston head. And my tapping it even more with a pin punch certainly didn’t help. Probably somebody had the gun open years ago, tapped the pin in and then couldn’t get the piston seal screw loose and just left it that way.

Sheared the screw
I spent about an hour trying to coax the screw free with the impact driver. All the while I was pouring Kroil penetrating oil on the screw head in the hopes that some would get down into the threads.

Anyway, the piston seal screw was jammed so tight that the impact driver finally sheared it off at the piston head. I knew something had to be done quickly. I ordered a new Diana 27 piston with the seal attached from T. W. Chambers & Co. in the UK. I also ordered a second piston seal set for the piston with the sheared screw. The order went in on December 22 and both items arrived at my mailbox on January 3. That’s great service!

sheared screw
The impact driver eventually sheared the shank of the pistol seal screw, leaving a good stub for removal.

old seal assembly
The old piston seal assembly has 4 pieces — the seal, the inner spacer that keeps the soft leather from collapsing in on itself, a fiber spacer that was behind the seal and the screw that held the seal assembly to the piston body. Notice how crushed the spacer that goes behind the piston seal is (bottom right). It should be flat.

new seal assembly
The new seal assembly appears very similar to the original, except for the color of the parts.

new piston
The new piston looks like new old stock from Diana, which it may be. I can’t detect any difference in manufacture. It may look corroded in the picture but that is just patina. There isn’t a spec of rust anywhere.

new pin detail
This is how the blind pin is supposed to look. This seal could be disassembled. The leather piston seal has already been soaking in oil for 12 hours.

My plan is to use the new piston and seal to finish the tuneup on Michael’s rifle. I hope to eventually repair his old piston and keep it for a spare, and I will write that up when it happens. You never know when another Diana 27 will come along!

The lesson
Learn from this! If you are working on a Diana 27 or any air rifle that has a pin through the piston head, determine whether it is a through pin (some are) or a blind pin like this one. Don’t tap a blind pin in or you will jam the screw that holds the seal. I think the screw was jammed more from the pin impacting it than from the rust.

Cleaning the spring tube
All the parts that go back into the gun must be cleaned, but the spring tube is the most critical, because that’s where the rifle’s power is generated. We want to remove all the rust and leave as smooth a surface as possible on the inside surface.

I had “marinated” the spring tube inside and out for about 10 days with lots of Ballistol, which removes rust. It looked horrible inside, but I knew that was the Ballistol making the rust bubble up to the surface.

spring tube Ballistol
This shot of the inside of the spring tube, looking through the cocking slot, looks horrible, but I knew that was just the crud the Ballistol had lifted off the metal.

To clean the inside of the spring tube I used a shotgun cleaning rod with a tip that was looped. I shoved 0000 steel wool through the loop. Then I chucked the rod in my drill and turned it as fast as it would turn. Running the rod in and out of the spring tube for two minutes loosened everything!

rod with steel wool
A large shotgun cleaning rod spun by an electric drill is perfect for cleaning the inside of the 27 spring tube.

The steel wool and Ballistol made short work of the rust. After cleaning I ran in some rags followed by paper towels to get everything out. What was left won’t show very well, as it is a smooth blued steel surface.

spring tube after cleaningn
What you are looking at, the bright part, is the same place you saw in the photo above. It is the inside surface of the spring tube and is now perfectly smooth and free from rust. You can see that the bluing (yes, the inside of the spring tube is blued) has been removed by the piston moving back and forth and that is the only wear that’s there. The factory cross-hatching is still sharp, though you can’t see it in this picture!

Other cleaning
The surface of all the the metal parts was cleaned with steel wool and Ballistol. Since I had coated every external surface with the stuff while waiting for the parts to arrive, the rust came off immediately.

The smaller internal parts were also given the steel wool and Ballistol treatment. This took very little time, because most parts weren’t rusted that bad.

The assembly
I’m not going to assemble the rifle in this report, but I wanted you to know that Michael bought a new mainspring for his rifle. The combination of that new spring, the fresh breech seal I made and a new piston seal will put the power this 1970 rifle back to like new.

Summary
I will assemble the rifle next. In that report I will show how to assemble the ball bearing trigger that can be a little tricky. Then I will chronograph the rifle and see what the tune has done for it. I haven’t decided whether I’ll test the rifle for accuracy or not. We shall see.