Here is a picture of how the internals work on the Diana modular trigger units. I have removed the guts and placed them back on the pins, except on the outside of the unit.
In the top of the picture you'll see a slieve with a red arrow that points to the right. This slieve is inside of the housing and it holds the three balls that grab the piston stem. As you cock the gun, the piston itself forces this slieve to the rear, As this slieve moves to the rear, the three internal balls move into their detent holes ( not shown ).
Please note the part to the left with the only left facing red arrow. If you look at the top of this part, just above the "hook slot", you'll see that there is a vertical surface, about 3/32" tall. The internal slieve moves backward during cocking and pushes rearward on this surface to get the catch out of the way of the "Slieve Release". Let's call this part the trigger sear, as it releases the Slieve Release.
The the slieve release is the part where the small downward pointing red arrow is. The part rotates around the pin on the parts right end. If you look carefully, just above the stem of the short red arrow, you'll see a small bump on the top of the Slieve Release. THis bump is what stops the intrenal sliding slieve from moving forward and releasing the three balls so the piston gets released and the gun fires. The Slieve Release is spring loaded to return to the up ( cocked) position. The picture below shows the Slieve Release and the internal sliding slieve. Note the bevel cut in the slot. This bevel pushes the Slieve Release out of the way during firing.
IN the photo below all the parts of the modular trigger are shown loose. They have been polished as part of the trigger tune I did.
IN the center you can see the Slieve Release with its return spring. ON the right is the internal slieve with it's spring and guide. You can also see the Trigger Sear with it's return spring. And lastly the plastic trigger with it's return spring.
Let's talk about what the springs do for a minute. It's very important to understand that the piston pressure acts on none of the internal parts except the three little steel balls. All the mainspring pressure has been removed by design from the trigger workings. It's a very smart design.
The internal slieve spring and guide does two things. It returns the slieve during firing so you can cock the gun again and have it postioned so the piston can push the slieve rearward etc. It also propells the internal slieve forward during firing. THis spring is responsible for forcing the Slieve Release out of the way as well as propelling the mass of the slieve and overcoming friction.
The small coil spring on the trigger sear ( The part with the hook that grabs the Slieve Release) forces the hook under the Slieve Release and sets the trigger in the cocked position.
The wire lever spring on the Slieve Release simply holds the Slieve release up until the Trigger Sear can grab it.
The thin spring on the black plastic trigger holds the trigger forward so it doesn't rattle around.
Now, let me get to the heart of what I want to talk about, springs. I've seen a lot of guys make assertions about how to tune this trigger by managing the springs. Some of these assertions are counter intuitive to what I see inside this trigger. I'm not saying they haven't experienced improvements. Read on, then you be the judge.
1.) The internal slieve spring and guide. If you're going to make a case for doctoring this spring, I say beef it up, don't cut it or lighten it. It has asolutley no affect on trigger pressure. It doesn effect lock time of the trigger. If this trigger is stronger it will force the internal slieve forward faster and this will cause a quicker release of the three balls that release the piston.
2.) The wire lever spring on the Slieve Release. DFWI. No gains here.
3.) The small coil return spring on the trigger sear part. I don't know for sure. A fellow could argue the case that you have to overcome this spring when you pull the trigger and you should lighten this spring to lighten the trigger. Personally, I don't buy it. When I put these parts into the housing on their own and pull against them without the other parts in there, there's just nothing there that amounts to much pull weight. However, I have not replaced this spring with a lighter one and seen what it does. This spring does need to be strong enough to positively return the sear when you're cocking the gun in a hurry. You go too light on it and you may get a fussy cocker. I have no idea wher tah boundary is.
4.) The thin spring on the plastic trigger. Pull against it, feel the weight, and you decide. I left mine alone.
This trigger unit was at least 10 lbs and probably near 15 lbs when I started. It was very crisp though and I have shot 5/16" groups at 10 M using the iron sights with the trigger as it was. Probably a better 10M average was 1/2". These are standing offhand groups. My point is that you can get used to almost any trigger.
I found that polishing the internals and properly adjusting the set screws led to a trigger that has a very crisp, perhaps 1 lb letoff. It does have a long first stage travel. That's OK with me, because it makes the gun safe.
Mu gut feeling is that all you need to do on this trigger is polish the bottom flat of the trigger sear and polish the mating surfaces of the Slieve Release, combine that with a proper adjustment, and you've got 95% of the gains. I'm so happy with mine I'm leaving it the way it is and not touching any of the internal springs.
Damn, that's a lot of typing for a two finger typist!
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