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

A history of airguns

This report covers:

  • The piston seal
  • Tractors
  • Hierarchy of removing a stuck piston seal screw
  • Time to move on
  • Remove rust
  • What’s next?
  • Have to make a breech seal
  • How to make the seal
  • Summary

This is a continuation of the report on overhauling reader Michael’s Winchester 427 that is a Diana 27. Today I’m showing you the details of working on a spring gun.

The piston seal

When Diana chose a threaded screw to attach the piston seal to the piston body, they couldn’t have selected a worse method of fastening. A screw was a common way to attach spring-gun piston seals in the 1950s and ’60s, but it wasn’t a good way, because over time the screw threads corrode and cement the screw in place. In the case of Michael’s rifle, the corrosion is particularly bad, so that screw wants to stay put.


Those who have worked on older tractors know what I am about to say. When you have a stuck fastener, don’t force it. You work on it again and again over time until what was stuck no longer is. If you force it you will have the even worse problem of removing a broken fastener. So you come at it again and again and again, and time is usually on your side.

Hierarchy of removing a stuck piston seal screw

1. Try to unscrew with a screwdriver. Be careful to not deform the screw slot.

2. Soak in penetrating oil and then try to turn the screw.

3. Use an impact driver with a bit that fits the screw slot precisely. In the 1960s, Honda motorcycles had aluminum heads that deformed when heated and cooled repeatedly. The fasteners on those heads were Phillips screws — a bad combination. What bike owners who did their own maintenance did was purchase impact drivers to get the screws out the first time. Then they replaced all the screws with fasteners that had better heads for maintenance — Allen capscrews, for instance.

The impact driver works by inserting the correct bit in the screw you wish to remove, then turn the driver in the direction of loosening as far as it will go (usually righty-tighty, lefty-loosey).Then, smack the end of the impact driver with a hammer. That drives the bit into the fastener head tightly and also imparts a slight turn in the direction you wish to go.

Diana 27 impact driver
An impact driver is for freeing stuck screws.

4. Apply heat to the screw. I used a soldering gun to keep the heat confined to the screw and not the leather seal or fiber spacers, because I didn’t want to destroy them — yet. Now, immediately follow the heat with the impact driver.

5. Squeeze the piston seal assembly with a large set of pliers and attempt to tun the screw that way. I tried that but all it did was mark up the spacer behind the piston seal.

6. Cut away the seal and both spacers, then put the head of the screw in a vice and turn the piston body. This should free the screw. If it breaks you will have to drill out the stub and either use an Easy-Out or just drill and tap a new hole for a new larger screw. Use a stainless steel screw if you go this way.

Diana 27 piston seal
Michael’s old piston seal is held on by a screw that’s rusted in place. In this picture you can see the fiber spacer inside the seal as well as the spacer behind the seal. The back spacer was crushed by pliers when I tried to unscrew the entire seal assembly, but it should straighten out if I can get the piston seal off. Before I used the pliers the inner spacer was flush with the top of the seal.

Time to move on

I spent about two hours getting the bit for the driver ground and then working on the screw. I didn’t grind the bit myself. Cory at AirForce Airguns ground it for me. Not only did he have access to much better equipment for this; he also did excellent work. This bit now fits the screw slot exactly!

Remove rust

I am faced with removing a lot of rust from the inner parts of this rifle. To take the picture of the piston rod end that I showed in Part 2 I worked for 15 minutes on the tip of the rod with stainless steel wool. It was actually a pot scrubber pad that has now moved into my toolbox.

While I was doing that I scraped the outside of the piston body with a knife blade. That removes the rust and scale better than anything I know of. The piston body still looks discolored, but it’s free of rust on the outside. The rod and inside of the piston are another thing. They will take a lot of time scraping, and there will be some rust I can’t get. I might try a rust remover once I get the piston seal off.

Diana 27 piston body and rod
Here you see the outer piston body that I have scraped smooth and the inner piston rod that is still rusty.

What’s next?

I have spent a lot of time already and not accomplished much. What more can I do? Well, there is that breech seal that needs replacing.

Diana 27 breech seal out
You can see how flat the rear seal was. It is also dead, as in not resilient. The groove in the breech on the 27 is shallow, so a leather seal should do well. Whatever doesn’t fit inside the groove will squash out to the sides. Keep it oiled and it will last for decades!

Have to make a breech seal

I looked through my airgun repair box for a breech seal that might work, but they were all too small. So the next step is to get out the raw material!

Diana 27 leather belt
I have been making breech seals from this old leather belt for several decades!

How to make the seal

You can make the new seal with a sharp razor hobby knife by cutting around the old seal placed on top of leather and then cutting out the center for the breech. But several years ago I found a better way. A set of cheap hole punches from China now help me make my seals.

12-24-18-06-Diana-27-sizing-the-hole-punch sizing the hole punch
The old Diana breech seal fits into the hole punch perfectly.

12-24-18-06-Diana-27-sizing-the-hole-punch sizing the breech hole
This punch is just right to make the breech hole.

You can make either hole first. I use the largest punch first and it doesn’t matter where I put it on the leather — also long as there is leather all around it. The smaller punch can then be carefully positioned and turned several times to make a mark on the leather. That will tell you if it is close to being centered. Do your best, but also bear in mind that leather is a natural product. With use it will squash out in all directions and fix any slight misalignments.

I have a 3-inch square wooden post that I put under the leather for this. I use the end grain of the wood to absorb the punch’s blow. That keeps the punch from dulling.

 Diana 27 breech seal
Two hole punches cut the breech seal from the leather belt. It isn’t perfect — just close as I can get it.

When I put the breech seal into the groove I tapped it in and started squashing it with a rubber hammer. It squashed out and looked larger on one side. It was also standing way too tall. So I trimmed it down with a razor knife.

Diana 27 breech seal in gun
Even after some trimming the seal looks too wide on one side. I kept trimming it carefully and then I oiled it in preparation for assembly.

Diana 27 breech seal oiled
Here is the new breech seal, nice and juicy and ready for assembly.


I have accomplished a lot in this report. I will continue to work on the piston seal screw over the next several days. If I get it — fine. If not I will order the other parts (the fiber spacers and a new screw) and just cut the old seal assembly off the piston.