Pictured and written by Yar1182.
You will need a number of tools for this job. Philips screw drivers, a dental pick, Sheridan valve tool, and a valve punch. You will also need replacement parts such as o-rings, lead seals, and possibly a valve guide and chamber plug. Begin by removing the bolt, pump, trigger frame, and co2 plug. The pictures show the trigger frame still on to help illustrate the proper orientation of the gun during the disassembly and assembly. I suggest you remove the trigger frame so not to damage the sear.
Remove the rear frame screw. This will allow you to access the hammer and mainspring. Remove these from the gun.
To remove the jam nut you MUST use a Sheridan valve tool. It has a square end to remove the jam nut and a threaded portion for removing the valve.
Use the Sheridan valve tool to remove the jam nut. The jam nut is torqued in very tight. The thing to remember is that the threads are very fine and brass. It is very important not to damage the threads. Keep the tool strait and level while removing the jam nut.
Turn the tool counter clockwise to remove the nut. I suggest placing the gun in a vice if available. Make sure you use a towel in the vice so as not to mar the gun.
Here is the jam nut removed. This nut retains all the valving in the gun. Along with the jam nut is a lead seal. It looks like a lead 0-ring. Usually it pops out with the jam nut. If not fish for it with the dental pick.
Close up of the jam nut and the Sheridan tool. If you look carefully at the end of the tool you will see the square end for the jam nut and a circular opening. The circular opening is threaded for removing the valve.
Insert the tool again and thread it into the valve. One it is fully threaded on, gently pull the valve out.
Here is the valve guide and exhaust valve. Notice the pointed pin on the valve guide. This is the puncture pin for 12 gram c02’s. Sheridan guns without the 12 gram co2 will not
Now use a dental pick to remove the valve guide o-ring. You may have to fish around for it. Try not to damage it if possible.
For Sheridans with the 12 gram co2 in the front, you will need a valve punch tool. Push it down the end and “tap” the chamber plug out. An experienced airsmith knows how hard to strike the chamber plug to get it out with one shot. I best describe it as a medium strength tap. You must use the valve punch tool so as to not damage the internals of the gun.
Place the valve punch tool down the FRONT of the gun in the 12 gram co2 chamber. Use a ball pen hammer to strike the valve punch tool. Again I suggest you place the gun in a vice.
The valve plug will come out the rear of the gun. Along with it several more components will be freed..
Here’s a picture with all the internals lined up in proper order for reference. The internals dorm right to left – chamber plug o-ring, chamber plug, chamber plug sleeve, valve spring, valve guide, exhaust valve o-ring, exhaust valve, lead seal, jam nut, hammer, main spring, rear plug.
Now it’s time to reassemble the gun. Inspect the chamber plug face to see if it’s deformed. This is the area that causes a lot of leaks. If deformed replace the plug. Place a fresh o-ring on.
Next slide on the chamber plug sleeve.
Slide on the chamber shelve.
Insert the valve spring and valve guide. The exhaust valve has a face that connects to the exhaust valve. Make sure this face is clean and burr free. This is the second most likely component to cause a leak.
Place a fresh o-ring on the exhaust valve and place it on the valve guide.
Make sure all components are aligned properly. The valve guide puncture pin must align with the hole in the chamber plug
All the internals will go pack in the rear of the gun. Here is the tricky part. Stack and thread the internals on the valve tool (POINTING UP). Hold the gun with the muzzle pointed UP and insert all the internals.
The most dangerous step is next. Carefully thread all the components back in. It is very easy to cross thread (damage the threads). From experience I gently rattle the components, while gently threading it in 1/32 of a turn till I can “feel” it going in strait. If it feels like it is binding in the slightest back it out and start over. If you cross thread the gun, you will need a special thread tap to correct it. Thread the jam nut in completely, but do not over tighten.