Here is a link to a schematic diagram which gives all the part names CLICK
I got into this to help a friend with his family history shotgun. It was in rough shape. I had never had one of these apart but this one definitely needed complete disassembly, thorough cleaning and some parts. I searched in vain for disassembly instructions. Then I was pointed to a single post on another site that gave a few hints that got me going. Biggest concern was dealing with some fairly heavy duty action springs. Here is my experience. If you ever do get into a complete tear-down of one of these guns the first piece of advice is WEAR EYE PROTECTION
. Once you have your eyes protected, here is what you will need to do.
1. Remove forend
2. Turn top lever to unhook barrel
3. Remove barrel
4. Remove butt stock
a. remove butt plate or recoil pad
b. remove butt stock bolt 7/16" hex head
c. separate butt stock from frame
5. Remove trigger guard
Here is what you see now
6. Remove the hammer spring assembly (bushing, spring, plunger). EYE PROTECTION This is easier than it looks. Pull the hammer completely back. The small hole in the bushing will align with a hole through the plunger. Insert a small pin punch or a small finishing nail completely through. Now slowly release pressure on the hammer. The hammer spring assembly will be compressed and free. It may just fall out under gravity or you can gently wiggle it free. If you need to tear down this assembly, place it lengthwise in a vise and gently squeeze until you can remove the pin punch then slowly reduce the vise pressure until the spring is unloaded. The three parts (bushing, plunger and spring) can now be separated, cleaned, lubed and reassembled, again in the vise.
7. Remove the top lever plunger and spring. This can be done by grasping the flat plunger with needle nose pliers and forcing a little more spring compression to clear the locking bolt connecting rod and then pulling out sideways. Good to keep the frame wrapped with a cloth in case you lose grip and the spring flies. EYE PROTECTION
8. Remove the locking bolt plunger and locking bolt plunger spring. The pointed plunger is held by spring tension into a small dimple in the locking bolt connecting rod. Keep the spring covered with a cloth or your hand while gently forcing the tip of the plunger out of the dimple (surprisingly difficult for the short distance involved). A small flat screwdriver can be used to leverage it out. This can fly if you are not careful. EYE PROTECTION
9. Remove the hammer. Punch out the hammer pin. Note that one end is fluted. Punch this pin out from the other side, as the fluted portion will not pass through the hammer
10. Remove the top lever. Rotate and pull. Comes out easily.
11. Remove the locking bolt/locking bolt connecting rod assembly. Punch out the locking bolt pin. Note that one end is fluted. Punch this pin out from the other side, as the fluted portion will not pass through the locking bolt. The pin holding these two parts together is peened over and not meant to be removed.
12. Remove the firing pin. Remove the firing pin retaining screw. The firing pin will drop free. There is no spring involved. The screw simply stops the pin from falling into the action.
13. Remove the trigger. Punch out the small pin. Be careful not to lose the small spring.
I would like to say reassembly is the reverse but that is a bit of a cop out. Yes, it is the reverse order but the top lever spring/plunger and the locking bolt connecting rod spring/plunger are a real pain. The top lever spring is prone to flying away on you, so care required. EYE PROTECTION
14. Replace trigger. Set the spring (short side) on the trigger. Holding it in place with your fingers, place the two into the trigger cut out of the frame. This can be done easiest with the trigger approximately 90 degrees off its normal position. Insert pin. Move the trigger into the correct position.
15. Replace the firing pin and screw
16. Position the locking bolt and locking bolt connecting rod and top lever. Replace the top lever so that it fits between the two sides of the connecting rod and the end of the connecting rod rests inside the cut-out of the top lever. Now insert the bolt pin paying attention to the fluted end. You can insert by hand and ensure that the fluted end engages the fluted frame hole by rotating it until it grabs and won't move further. Tap into place with a brass hammer. Final placement requires a punch. Both ends of the pin will be proud of the frame metal. It will not have a flush side.
17. Drop the hammer from the top into place between the two sides of the locking bolt connecting rod. Align the hammer pin hole to the holes in the frame. Insert the fluted pin by hand and rotate until it engages the fluted frame hole. Tap into place with a brass hammer. Final placement requires a punch. Both ends of the pin will be proud of the frame metal. It will not be flush on either side.
Now we tackle the springs
18. Not quite reverse order but I found it easier to place the hammer spring assembly in position first. The hammer is loose now. Place the assembly with the plunger head engaging the hammer and the other end aligned with the hole in the frame. Pull the hammer back to hold the assembly loosely in place, ensuring proper engagement at both ends. Now pull the hammer all the way back and feel the spring tension. You will see pin punch or the small nail become loose. You can now remove the punch by hand or the nail with needle nose pliers. Allow the hammer to move forward under spring tension. While holding the hammer, pull the trigger to allow the hammer to rest in the full forward position. That one is done and relatively easy compared to the next two.
19. Replace the connecting rod spring and plunger. Note the two holes in the back of the frame. This group uses the top hole. The plunger has a very pointed end that engages into a small dimple on the locking bolt connecting rod. This now sounds easy but it is a real pain. EYE PROTECTION Place the spring on the plunger. This assembly is longer than the space available, so you need to align the rear of the assembly with the hole in the rear of the frame. Now you need to compress the spring and then push the pointed end of the plunger onto the connecting rod and move it into the dimple. There will be a satisfying click when that happens. The devil is in aligning the plunger while simultaneously compressing the spring. I found a small screwdriver blade inserted between coils of the spring could then be pulled back to compress the spring enough. Be prepared to be pissed off more than once while doing this.
20. Replace the top lever spring and plunger. This is the worst and most prone to flying away from you. Take a look at the back of the connecting rod. You will see a slot that is the width of the plunger. The plunger has to get into that slot. All you have to do is align the rear of the plunger with the remaining hole in the frame while simultaneously compressing the spring and pushing the front of the plunger into the slot between the connecting rod and the top lever. Easy right? NOT ? EYE PROTECTION.
You have now successfully disassembled and reassembled the components held within the frame of the 840. If you are an experienced gunsmith with tools I'm not aware of, you won't be reading this. If you are a tinkerer, you might find this useful. I could not find anything like this even with all those search engines out there so I'm paying back for all the times others have come to my aid.
Oh yeah, after you are done you can celebrate with beverage of choice - but not before. Be prepared to be annoyed and then immensely satisfied when you win.