Sear and Tumbler notch from a lock in question

[Darkhorse]

These are the best I can do with close ups. These are from a post I made a few days ago about a lock no longer holding when cocked. To my untrained eye I can't tell if this wear is normal or not. Or perhaps the geometry is a little off and combined with wear it won''t hold full cock anymore.

[rich pierce]

Normal. I can see where the fly is tracking. I forgot the original discussion but does it not hold when lock is out of the gun?  Do you not want to send the lock back to the maker?

If bad out of the gun, and I wanted to fix it myself:  my process would be to vewwy cawefuwwey stone the sear nose so it is square,  not left as cast. Remove almost no steel. Angle must be on a radius from tumbler axle. Of course first I’d check hardness with a full then a new needle file to get a sense of whether it’s going to hold up or not.

[WKevinD]

I know this is simplistic but it is how I try to solve problems, I try to figure out what I'm doing wrong before messing with materials. Sometimes it works.
If you have a couple of similar locks.
Is this a pin mounted fly? Is the fly in backwards? Is it the correct fly? Is it the correct sear?

Kevin

[retired fella]

a couple of years ago I experienced a similar problem with a bud siler lock.  turns out that after 25 years the sear spring had weakened.  bought a new one from jim chambers and cured the problem.  no honing or filing was done.

[Darkhorse]

The lock was bought new and has been in a rifle I built some 20 years ago. This rifle has been shot a lot over the years. Single trigger. I can't cut it with a file. I have tried it with a brand new full strength sear spring with the same results. In or out of the rifle it does the same thing. Sometimes it will cock but a slight bump of the hammer and it fires.
I am going to buy a new sear and tumbler but I want to try and fix the old ones for the experience. To me, there doesn't seem to be enough wear to cause it not to hold at full cock. That's why I'm wondering if the geometry is a little off. Due to the age and service until now is why I hesitate to send it back to Jim Chambers.
This lock is off my deer rifle and has been trouble free until now. I took a sear and tumbler out of another lock to get my deer rifle ready for the season and now it works fine.
I've probably forgot and left something out here so I'll try and copy the link to the original post.

http://americanlongrifles.org/forum/index.php?topic=55722.0

[EC121]

Try stoning the top of the sear nose to give it a sharper edge and let it go into the full cock notch a little deeper.  Also polish the side of the sear so it won't bind on the lock plate or bridle.  Back the sear screw off a quarter turn and try it.  If all is right, the sear spring shouldn't be needed to hold at full cock.

[D. Taylor Sapergia]

This issue is not difficult to repair, but it may take you some time and disassemble and re-assemble the lock several time during the fix.

The angle of the full cock notch should be a straight line through the axis of the tumbler, and the nose of the sear must match that.  From your lovely balck and white photo of the sear, I see that it's sharp upper edge is polished round.  That leaves only the bottom part of the sear's nose to engage the notch in the tumbler, which looks adequate.  Very careful work with a three sided stone in the tumbler notch will return it to being square with a tangent drawn across the tumbler's circumference.  Lacking a stone, use 320 or 400 grit abrasive paper or cloth backed with a safe sided three square file.  Use a very light touch, and examine the progress under magnification frequently.  Do not deepen the full cock notch.  Once you're satisfied with the tumbler, work on the sear's nose to make it match.  Install the tumbler and sear without the bridle to ensure those angles are exactly the same.  I think you'll be surprised how satisfying it will be to have remedied this yourself.

[Darkhorse]

Many thanks to all who responded. And thank you Taylor for the detailed description of how to fix the problem. I know how satisfying it will be to fix this myself. My goal is and has been for several years to learn all I can about rifle locks, problems and remedies. I have polished and tuned my locks and they have worked great for several years but I've never had a need to work on the sear and tumbler notch. Well, I've got the opportunity now.

[Jim Chambers]

You can follow Taylor's advice which is excellent, or send the lock back to me.  It would take me less than one minute to adjust the full cock notch and check the sear and have the lock working properly again.

[Jerry]

Had similar problem last week. Squared up the tumbler full cock notch and sear nose. Re-assembled
paying close attention to the tightness of the sear screw. Oiled internals and everything worked great.
After several shooting sessions I’m at the opinion that the sear screw had worked loose. By the way,
adjust the sear screw without sear spring.

[Blacksmoke]

Drakhorse:  One further comment -  First I endorse Taylor's advice and then I would add that a lock with the correct geometry does not need a sear spring to keep the sear engaged in the full cock notch.  The sear spring is a kind of safety device to make sure that the sear stays in place un till you pull the trigger.  The angles on the tumbler/sear nose need to mesh in relation to each other.  Hope that helps,  Hugh Toenjes

[Bob Roller]

Drakhorse:  One further comment -  First I endorse Taylor's advice and then I would add that a lock with the correct geometry does not need a sear spring to keep the sear engaged in the full cock notch.  The sear spring is a kind of safety device to make sure that the sear stays in place un till you pull the trigger.  The angles on the tumbler/sear nose need to mesh in relation to each other.  Hope that helps,  Hugh Toenjes

The sear spring also makes sure the sear get to where it's supposed to go to begin with.

Bob Roller

[D. Taylor Sapergia]

On many commercial locks, the sear screw doesn't bottom against the threads without binding the sear between the plate and the bridle.  So sometimes this can be cured by chucking up the screw in the drill press, and with a safe sided flat file, dress the inside shoulder of the screw so that it can go deeper without pressing the bridle against the sear.  The sear screw should bottom against the plate without biding the bridle against the tumbler.
The sear spring is necessary to kick the sear up into the notches as the tumbler rotates.  But it should not be relied upon to hold the sear nose in the full cock notch.  that is accomplished with the geometry of those faces.

[Daryl]

That is accomplished with the geometry of those faces.

90 degree engagement!

[Darkhorse]

I am going to accept Mr. Chambers offer, that way I'll know it's done right the first time. I have sent him a PM and I'm just waiting for a reply.
Again, thanks to all who responded, your replies have added a great deal to my understanding of the sear and tumbler notch relationship, and how to correct problems. I really understand this more than most of you realize as I retired as a civilian Master Toolmaker from the Air Force and before that I worked several years in a Tool & Die shop where we manufactured school bus bodies.
My first flintlock was a GPR over 20 years ago. Somehow this same thing happened to the sear and tumbler on that rifle and it gave no warning until one quiet afternoon a buck approached and I cocked the rifle in anticipation of the shot. As soon as I released the hammer it fell and the rifle went off. Scaring the "you know what" out of me and the deer. Even though I was 16' off the ground and alone on a large tract of land I was obsessed with the fear I might have shot someone. I could not get the rifle to hold at cock again so  I just ordered new parts from Lyman.
So that's to explain why I'm so concerned about a lock that's doing the same thing. And why I want it done right.
As an aside my shop was damaged by hurricane Micheal, along with other things, and I haven't quite got it back usable. It's cleaned out just waiting on the new ceiling. And all my tools are boxed up. It does have a new steel roof finally.
I have fine stones, medium stones and coarse stones. Both square and slips. But I don't have a triangular stone. I do have a triangular file with a safe side I use for cutting dovetails so I guess I could use that and some sandpaper to work on the tumbler notch if it came to that. But I still don't think I would trust my work at this point.
When I install the sear I always check to see if anything is binding. Usually the sear screw is bottomed out then backed off just enough for the sear to move freely.