Although I posted this in answer to a question in the regular Muzzleloading Forum, I'll copy it here for future reference for builders.
There may be things I left out so if anyone sees something missing, please feel free to add it.
Disassembly and reassembly of a sidelock isn't that hard.
I would strongly recommend that a mainspring vise be bought and used.
Also, be sure to have screw drivers that actually fit the screw slots. "Close" won't do.
My method of dissassembling a lock goes something like this:
1. Remove the lock from the stock.
2. Bring the hammer/cock to the full cocked position.
3. Place the mainspring vise on the mainspring and tighten the screw only enough to secure it firmly in place. (Do not try to compress the spring).
4. Push upward on the sear arm and lower the hammer/cock to the fired positon.
5. Carefully examine the mainspring to see if any screws are attaching it to the lock plate. If there is, remove the screw. If there is not, gently rotate the spring/vise clockwise to disengage the springs upper tang from the bolster.
Using gentle rotation, the spring should be pulled off of the lockplate. Put it in a safe place where it won't fall to the floor.
NOTE: If the lock is of the newer lock design (like some L&R locks) there is a small link which connects the mainspring to the tumbler. Use a piece of paper and draw which way the S bend in this link is going relative to the lock. This must be put back exactly as it is. If it is reversed, it will bind up and possibly break.
On my L&R Bedford, starting from the lower crosspin, it goes up and forward somewhat like a S, not a backward S.
6. Loosen the screw holding the sear spring in place at least 3 turns.
7. Using some needle nose pliers or a knife, disengage the sear spring tab from the sideplate by pulling it outward. When the tab has cleared the slot, rotate the spring counterclockwise away from the sear.
8. Remove the sear screw and the sear.
9. (for bridled locks) (The bridle is the part that fits over the tumbler. It has a hole in it for the axel on the inner side of the tumbler.) Find the location of the fly in the tumbler and keep an eye on it. It is easily lost.
10. Remove the bridle screws. Pay special attention to the shape of the fly and which direction it is installed.
With Silar locks it has a cast pin sticking out of the side of it so you can't put it together backwards. With the L&R locks the fly is a small irregularly shaped plate on a pin. It must be reassembled in exactly the same position that you find it. If it is assembled backwards it will not work correctly. (Draw a picture of it in the tumbler).
11. Remove the hammer/cock screw.
12. Support the lockplate on two blocks (or on the top of a vise with the jaws open about 3/4 inch). Do not be tempted to pry the hammer off of the tumbler with screw drivers or anything like that. Not only will you gouge the lockplate but there is a good chanch that you will break the very hard tumbler.
13. If you have a square punch that will fit inside the square hole in the hammer use it for removing the hammer/cock. If you don't, find a steel rod that will fit down inside the threads minor diameter and bottom out in the drilled hole.
This pin should not be dead soft like a nail and it should not be very hard like a drill bit. The soft steel can deform in the bottom of the hole and become locked in place by the threads. The very hard pin can shatter from hitting it).
Using a small hammer, tap the punch or pin down so it drives the tumbler out of the hammers square drive.
14. For a Flintlock frizzen, lightly compress the spring and remove the screw holding it in place.
This is one place that locking pliers will work. Use a piece of brass sheet metal to prevent the serrated teeth on the pliers from marring the spring. Lightly compress the spring so the frizzen is free to rotate a little.
NOTE: do not compress the spring any more than you have to to release the pressure on the frizzen. If you over compress it, you may break it.
15. With the frizzen spring removed, remove the screw securing the frizzen.
Do not try to remove the wheel if there is one on the spring or frizzen.
Re-assembly should be done in the reverse order.
You may lightly tap the hammer back onto the frizzen drive, but if you can get at least three threads of the hammer screw engaged, IMO it is best to just tighten the screw to seat out the hammer on the square drives shoulders.
When you replace the sear spring, (if you removed it from the lock plate) after the sear is in place, screw the spring back on the lockplate but do not tighten the screw.
Use your thumb to push the back of the spring downward and towards the lockplate. It will bend and the tab will engage the plate's slot almost automatically. Then tighten the screw.
Don't forget the fly when you reinstall the tumblers bridle.
If you are working on a late design lock with a mainspring link, double check your sketch to make sure it is installed correctly.
The bridle screw that goes thru the sear needs to be snug but not overly tightened. If you overtighten it, the sear will not rotate freely like it needs to do to do it's job.
To remove the mainspring vise, bring the lock to the full cock position, loosen the thumbscrew and remove the vise.
IMO: You can do this job without a mainspring vise but you risk breaking the mainspring with things like locking pliers.
.Locking pliers concentrate their load all in one place rather than spreading it out like the vise does. On locks like the Davis Jeager and French fusil locks it is almost sure to break the spring (don't ask how I know).
