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

A history of airguns

This report covers:

  • Michael’s Diana 27
  • Out of the box
  • Flat breech seal
  • No baseline test
  • Onward Through The Fog
  • Remove the action from the stock
  • Action into the compressor
  • Remove the piston
  • Disassembly complete
  • List of jobs
  • Summary

Michael’s Diana 27

Some time back, reader Michael mentioned some problems he was having with his new/old Winchester 427, which is a Diana 27 by another name. I offered to tune it for him because it’s been some time since I have been inside a 27. There are many new readers who are not aware of this wonderful air rifle, and I thought it was time they learned about it.

Diana made the model 27 for a great many years after WW II, and they made them for a number of other companies, as well. The guns were made in both .177 and .22, but Winchester and Hy Score only ordered them in .22 caliber, so a 427 and an 807 are always .22.

Out of the box

When I unpacked the gun it looked pretty good, so I gave it a thorough once-over. The metal was lightly speckled with surface rust, but it wasn’t too bad. The rear sight was missing its rear mounting screw, which allowed it to move side to side and also up and down, and the spring that keeps tension on the rear sight leaf is not installed properly. There is a spring stuffed inside the sight, but it isn’t in the right place and it doesn’t look like the correct spring. Also the windage adjustment screw that should be fastened to the sight by a small circlip is wired in place, instead.

The barrel pivot bolt should have a locking screw to hold it in place. Also the rubber bumper that goes on the butt to keep the rifle from slipping on the floor is missing.

Diana 27 rear sight
The rear sight is missing the back anchor screw (arrow) and a spring that goes up front is not mounted correctly. The barrel pivot locking screw is also missing (arrow).

Flat breech seal

The last thing I’ll note is the breech seal. On this rifle it’s quite flat and lifeless. It seems to be a synthetic seal and I will probably replace it with a leather seal that is longer-lasting and seals better.

Diana 27 breech seal
The breech seal is very flat and lifeless.

No baseline test

I had hoped to baseline the velocity of the rifle today before I started the work, but it detonated on the first shot and I decided not to. It vibrated and sounded like a dry-fire, which I will guess is mostly due to a large loss of air at the breech. I don’t want to shoot it in this condition.

Going inside

If you want to learn to rebuild spring piston airguns, this one isn’t the place to start. The trigger is quite complex until you understand how it works. When you do understand that the rifle becomes simple to work on. You will need a mainspring compressor for this gun.

Remove the action from the stock

The first step is to remove the barreled action from the stock. That’s two stock screws on the forearm and the forward triggerguard screw. Once the action is out, the sheetmetal end cap slides off and the rifle is ready for the compressor.

Action into the compressor

Now the action goes into the compressor and comes apart. The trigger parts are first to leave the spring tube.

Diana 27 end cap off
With the end cap off you can see the second cross pin that holds the action together. You can also see some of the 48-year-old oil that has hardened into varnish and is holding things together.

Diana 27 compressor ready
The action is ready to go into the compressor. When it does, the compressor will press on the inner trigger cage (two arrows) until the tension on the two pins is released. They will practically fall out of the gun at that point.

Diana 27 coming apart
The two pins have been pushed out and the tension of the compressor is relaxing. The black tube with the small coiled spring on top is the inner trigger cage. It’s where the ball bearings live.

Diana 27 tension off
Here, with all the tension off the mainspring, you see both the inner and outer cages of the trigger. The outer cage (silver) presses the balls back into the inner cage. When the gun is cocked, they are pushed into the groove at the base of the piston rod to grab the piston at the rear of its travel.

With the trigger parts out the mainspring and spring guide come next.

Diana 27 spring out
The mainspring and guide will slide out. This one is bone dry! You can also see the outer trigger cage (silver) and the inner cage (black) with one of the balls still inside. The outer cage has ramps on the inside that push the balls through the inner cage and into the groove at the base of the piston. The small coiled spring moves the outer cage when the trigger blade releases it, allowing the balls to move out of the groove in the piston rod base.

Diana 27 trigger parts
These are the key parts to the Diana ball bearing trigger. There are other external parts like the trigger blade, but these are the parts that make the trigger work so well.

Remove the piston



The next step is to remove the piston, but it is held captive by the cocking linkage, so the barrel has to come out of the action fork. The pivot bolt is removed for this. Now, the Diana has two asymmetrical washers — one on either side of the base block that holds the barrel breech. Make sure you get the order of these correct for when it’s assembly time. The dished washer goes on the left side, and what looks like a flat washer until you remove it, goes on the right.

Diana 27 barrel apart
When the barrel comes out of the action fork the washers can be see on each side. The cocking link will also disengage from the piston, so it can be removed.

The piston will slide out the rear of the spring tube now. You will have to pull the trigger to get the piston past it.

This piston was horribly rusty! It’s the worst I have seen. The inside of the spring tube is also rusty. That will have to be dealt with!

Diana 27 piston out
The piston is the rustiest one I have ever seen!

Disassembly complete

This is as far as we need to go, but I need to do something about that rust! I put the piston into a container and poured in Kroil (penetrating oil) to soak into the piston seal screw for a week or so. I need to say something to you now. Some people use WD-40 as a penetrating oil. It works okay on car parts but isn’t so good on the smaller parts that are found in an airgun. It dries and leaves a yellow varnish on everything it touches. So, don’t use it for jobs like these.

Diana 27 piston marinating
The piston seal is marinating in Kroil for a while. This box has a top to keep the oil from evaporating.

I also put Ballistol liberally on both the barrel and spring tube, where they will marinate for a week or more. And I hosed down the inside of the spring tube with it, as well. In a week or so I hope to tackle all that rust with steel wool, and the Ballistol should have loosened it.

List of jobs

There are a lot of little jobs in this overhaul. Michael sent a new mainspring and piston seal with the gun.

Remove the rust
The parts like the piston and compression chamber need to be cleaned, inside and out.

Remove the old piston seal
The seal is held on with a screw that hasn’t been moved in 48 years. With the rust that’s on the other parts we know this screw is rusted tight.

Install new mainspring and piston seal

Make and install new leather breech seal

Replace pivot bolt locking screw

Repair rear sight
Get a new screw for the rear base of the sight and install the spring so the sight leaf doesn’t flop. Replace the small circlip on the windage screw if I can find the right one.

Lubricate rifle
The new piston seal needs to be soaked in oil for several days before installation. I will use Tune in a Tube (TIAT) grease on the mainspring as well as the spring guide and piston rod — sparingly because this isn’t a powerful gun. I will also used it in the trigger assembly to hold the balls in place while I assemble the rifle. The cocking link will get moly where it connects to the piston and also on the pivot bolt and both side of both washers.


This overhaul will be more involved than I have done in the past on a 27 because there are a number of things to address. Cleaning the parts will be a major task, but once that is done this will be a Diana 27 once again.