Kit Lock Geometry Riddle

[JBulitz]

I recently got a deal on an incomplete trade gun lock from a fellow at a show, which appears to be an old Upper Missour River Trading Co kit that hasn't seen daylight in a decade or three.
I'm piecing it together and trying to figure where the mainspring screwhole and pinhole need to be drilled, and I've run into this geometry problem.  I'll preface this by noting that everything is annealed, and that the sear, cock and tumbler are as-cast, including the notches in the tumbler.  I've omitted the bridle here so the tumbler is fully visible.
Everything appears to line up reasonably at half-cock, but at full-cock, there is no room for the "hook" of the mainspring between the tumbler claw and bolster (if the spring could even bend that far), and the cock appears to be WAY too far back - the neck is actually touching the lock plate.
Adjusting the cut of the tumbler notches seems to be one likely option, but before I do any more drilling and/or filing, I'd greatly appreciate any advice!
Better yet, has anyone else assembled one of these?
Thanks very much in advance.

[rich pierce]

I’m sure others with more experience will help but let me ask about the position of the cock at rest. Does the lower jaw rest close to the fence?

I note there is no fly. So you will need to avoid a light trigger pull.

In my thinking, several factors could cause your situation.
1) Poor design
2) Sear pivot hole slightly too far forward causes the tumbler to have to rotate back too far
3) Full cock notch set back too far (see #1 above)
4) Cock is not indexed properly on the tumbler axle. But you can check by the “fired” position.

[JBulitz]

I’m sure others with more experience will help but let me ask about the position of the cock at rest. Does the lower jaw rest close to the fence?

I note there is no fly. So you will need to avoid a light trigger pull.

In my thinking, several factors could cause your situation.
1) Poor design
2) Sear pivot hole slightly too far forward causes the tumbler to have to rotate back too far
3) Full cock notch set back too far (see #1 above)
4) Cock is not indexed properly on the tumbler axle. But you can check by the “fired” position.

Thanks Rich- I believe that of what you suggested, #1 is a general contributor, as the cock falls right to the edge of the fence as it should, the "lip" on the neck resting properly on the bolster.  The full-cock position poses the real dilemma.
As for the rest of your notes, the hole for the tumbler shank was located as part of the cast plate and thus determined the position of the bridle (in which holes were also pre-existing as cast) and sear screw.  And, the shank was cast square so the index of the cock was pre-determined.

[rich pierce]

I’m not sure how to salvage it. A very small shortening of the sear nose or bringing the full cock notch closer to the half cock notch could alleviate the severe back bend of the cock. Problems like this remind me that the geometry and function of a good lock requires quite a bit of fiddling if scratch built. I’m only guessing that although patterns were used, small adjustments were required in final fitting of lock parts when they were forged and filed.

[Curtis]

I have an upper Missouri Trading Co lock in the shop right now, just finished the basic assembly.  Not sure it is the same lock, mine is an large early Ketland I believe?  I purchased the lock at the CLA show 8 or 10 years back...  I will try to remember to take some photos in the next day or two and compare the locks.  I had a couple of issues with the lock but not any problems with the mainspring/bolster interface. 


Curtis

[smart dog]

Hi,
I've built quite a few locks.

1. The full cock notch is too far back on the tumbler and needs to be cut closer to the half cock notch.  The cock does not need to rotate that far back.
2. Without a fly detent you are going to have real problems with this lock because it will catch in half cock when fired with a light trigger pull.  That is because it appears the radial distance from the center of the tumbler spindle to the lip of the half cock notch is greater than that distance for the full cock notch.  The radial distance to the lip of the full cock notch should be greater or at least equal to the radial distance of the half cock notch. That way, when you pull the trigger back and release the tumbler, the sear is kept far enough away from the tumbler so it does not engage the half cock when fired.
3. The toe or foot of the tumbler probably can be filed a little shorter.  You want to position the mainspring such that the hook is fully on the toe of the tumbler when at rest and then slides all the way back to the corner where the tumbler curves up around the spindle. Ideally, the end of the hook tucks into that corner when you go from half to full cock and the tumbler mostly rotates around the end of the hook rather than lifting it up very much further.  That will give you considerable mechanical advantage and allow the lock to let off a little in force as you bring it to full.
4. Position the mainspring as above but also make sure the lower leaf is as straight as possible when the lock is at full cock with no upward bend.

Here is a good example for you.  It is a superb French musket lock for a Miroku Charleville that I worked over recently.
   


Note at rest the mainspring is well up the tumbler foot not resting on the very end.



At half cock, the hook of the spring slides farr back on the foot, almost to the corner.  The black tape shows where the spring leaf was at rest and how far it moved upward when brought to half cock.



Now I bring the lock back to full cock.  Notice the hook is bedded in the corner of the tumbler (instep of the foot I guess you could call it).  Note how little further the spring moves from half to full cock.
 







Also note how straight the lower leaf of the mainspring is at half cock.  This lock really works well with all the sparks landing in the pan.



dave

[Bob Roller]

That lock was a poor idea to begin with but you MIGHT start by lowering the "cock"to to the plate and then position of the main spring so
it is on the "toe"of the tumbler and then locate and drill to pin hole and the hold down screw will be in place.Now see if it will reach fully cocked.If it does without hitting the bolster consider yourself lucky.I abandoned this style of mechanism in 1970 and haven't made one since.Good luck with this project.One more thing.MAYBE a relief cut for the sharply curved mainspring can be added by using a milling machine.
Bob Roller

[Curtis]

Here are some photos of the one I have... still several problems that I need to address with this lock.  Some of the problems I dealt with so far:  Had to weld up a bridle screw hole and reposition; Shoulder on cock was way high so I braised in a piece to make it stop the cock on the lock plate when the tumbler hit the bridle stop;  File the spring toe on tumbler shorter for clearance; greatly increase the arch on the mainspring foot by filing the underside of the arch.

I agree with Dave (Smart Dog) that  your full cock notch it too far back from the half cock notch and needs to be moved forward before it will ever work.  Also a photo of your mainspring would be nice to see.

Also just an FYI - Apparently the frizzens on these locks are mild steel and need to be DEEPLY case hardened or faced with some high carbon steel














Curtis

[JBulitz]

Thanks all for great photos and so much good advice. Sounds like I've got my work cut out for myself (glad I got somewhat of a deal on it), but I'm heartened by the 1st set of adjustments.  Curtis, my next question was going to be about frizzen metallurgy, so thanks for that heads-up.
Here is everything assembled.

I brought the full-cock notch forward about 1/16", and I adjusted the sear nose for contact, which removed about 1/32."  I had to slightly deepen the half-cock notch maybe 1/32" after bringing the radius down to match full-cock.  Amazing how drastically such small amounts of metal removal changed the cock's positions.

Everything looks much better now and it appears there will be room for the mainspring- if not, shortening the tumbler claw a little ought to work.  The one small problem in the aftermath is that a short flint would be necessary, because the cock now sits about 1/8" further forward at half-cock. This might just have to be a concession unless I make a new tumbler, which may be a possibility in the future.
I'll get the mainspring in soon, and we'll see where that puts me.
 

[rich pierce]

It looks like you’ll be fine with your flint strike. I agree with you; it’s amazing how small adjustments in the location of the sear nose, for example, can result in big changes.