The Diana two forty.
This report covers:
- Pyramyd Air
- Tissue paper test
- Removing the breech seal
- The depth of the two forty breech seal cutout
- What now?
- Let’s test it
- Install the factory seal
- The trigger
- Spring dry
- Things we now know
Today we take a look at shimming that breech seal in the Diana two forty. If you don’t remember the issue we had I recommend you read Part 2 again.
Before we begin I want to say a word about Pyramyd AIR and this rifle. I chastised them in Part 1 for calling it a Diana 240. Now, I’m not so sure that’s what they did.
When they stopped carrying the Diana 240 several years ago they sold the final rifles at $99.95. Coincidentally that’s what the two forty retails for today. It may just be that they left up the web page for the 240 and haven’t yet updated it to say ‘no longer available’. Diana no longer offers the 240; the two forty has taken its place. So Pyramyd needs to update their web page, and I hope they decide to carry the two forty, because it is a worthy airgun, as you shall see today.
Tissue paper test
Before I did anything else I did the tissue paper test that was recommended by reader Yogi. I used a genuine bit of tissue from a paper handkerchief and it was a very small piece. If there is a leak, it could be a small one and I don’t want anything to hinder the test.
The piece of paper handkerchief is just behind the breech, which is the slot beneath the rear sight. I did that so you could see the breech. To run the test push the paper over the breech that’s to the left in this picture. Then load a pellet and discharge the gun safely while closely watching the paper.
When I tested the two forty I noticed a very small puff of air on the right side of the breech, which is where the breech seal stood proud. The puff was very small and if I hadn’t been watching the paper I would have missed it, because the paper stayed in place where I put it. When a breech really leaks a small piece of tissue like this would be blown off the gun.
The leak was too small to do anything about I would have left it as it was. But so many of you wanted me to show replacingf a breech seal, so I removed this one anyway.
Removing the breech seal
Removing a breech seal from a breakbarrel breech is a risky thing. Some come out easily. Others won’t come out without damage. This time I got caught!
To remove the seal I use a tool that looks like a dental pick but it’s actually a tool that’s specifically made for purposes just like this one.
This is the pick tool I use to remove breech seals.
Breech seals come in all different materials and the one in the two forty is not conducive to removal without damage. Instead of an o-ring made from a Buna-N compound that works well with picks, this seal seems to be made of either a nylon or a PTFE compound. That makes it harder and more likely to incur damage when removed this way. When I removed it I put a mark on the outside that I think has ruined the seal.
The pick damaged the two forty breech seal.
Normally when you remove a breech seal you intend replacing it with a new seal, so any damage doesn’t really matter. But this seal wasn’t damaged until I removed it. You wanted to see how this is done and I’m showing you.
Under the breech seal is a clear flat washer that I also picked out. This washer is very much like the shim I planned to put into the breech cutout.
This clear washer is under the two forty breech seal.
The depth of the two forty breech seal cutout
Reader RidgeRunner asked me to measure the depth of the cutout that the breech seal fits in, so I did. He wondered if it was cut to a uniform depth all around. Yes, RidgeRunner, it is. The breech seal cutout measures 0.0975-inches deep all around.
If the Diana two forty was a German-made airgun I would just buy a new breech seal and replace it. But it isn’t. It’s made in China and I can’t find any place that sells replacement parts. There may be one, I just never found it. What to do, what to do?
Well, I know that many Diana rifles use plain o-rings for breech seals and, as shallow as the place on this rifle is, an o-ring might work. I have an assortment of metric o-rings and as it turned out, a number 11 o-ring from my kit is a perfect fit!
My metric o-ring kit.
I first put the transparent washer back in the o-ring hole, followed by the o-ring, and voila! I think I’m back in business!
The o-rings fits the breech seal place perfectly. Don’t be confused by the dark shadow at the bottom of the seal. The fit is perfect.
Let’s test it
Okay, the fit looks perfect. What does the tissue test say? Well, believe it or not, the tissue moved even less when I shot the rifle with this seal. It barely raised up when the rifle fired.
Now, that isn’t the final word, because I do have a chronograph. We baselined the two forty with the Air Arms Falcon pellet that I’m using for all the tests today. Before removing the factory breech seal the Falcon pellet averaged 561 f.p.s. for 10 shots with a spread of 4 f.p.s., from 559 to 563 f.p.s. With the new o-ring breech seal in place the same pellet averaged 474 f.p.s. The spread was 7 f.p.s., from 470 to 477 f.p.s.
Obviously this o-ring wasn’t cutting it, so I added a paper shim that is 0.010-inches thick on top of the transparent washer. Then I popped the o-ring back in place and the average with the same pellet went to 492 f.p.s. The spread was 8 f.p.s., from 488 to 496 f.p.s. This was better but not good enough.
Install the factory seal
I decided to install the factory seal next. I left the paper shim in place but turned the seal around so the nick on the top and side didn’t show. The average for 10 more pellets was now 577 f.p.s. The low was 576 and the high was 578 for a 3 f.p.s. spread. That’s a little faster than the original 561 average with the factory seal and I have to attribute that to the paper shim.
You also wanted to see the trigger, to see if anything might be done to lighten the pull. Here it is.
The two forty trigger is plain and has no easy possibilities for adjustment.
Here’s the trigger with the plastic end cap off. You can see that disassembly of this rifle requires removal of the threaded boss in front of the trigger and drifting out the single pin in the back of the spring tube.
The mainspring is dry.
With the stock off I also had the opportunity to see the mainspring. It was dry. So before I put the stock on I squirted a couple small dollops of Tune in a Tube on the spring. I’ll let that spread around for a bit.
I shot the rifle a few more times and it is now dead calm. The little bit of vibration I mentioned in Part 2 is now gone. That’s what I expected. I will chronograph it again, but not until I test it for accuracy at least one time.
Things we now know
We now know that the Diana two forty is assembled in a straightforward way. That means disassembly will be pretty easy.
Once the trigger is outside the spring tube there may be better opportunities to lubricate it. We will all see together when that time comes.
The Diana two forty is turning out to be a surprisingly nice and well-designed breakbarrel springer. I’m surprised by how well the Chinese manufacturer was able to control quality.