Help troubleshooting a Davis Twigg lock please

[Goo]

I bought two of the Davis Twigg locks to build matching fowlers for a customer.  There were a number of issues.  The lock would barely stay in full cock position to point where if you breathed on it it would snap.  I fixed this by adjusting the the nose of the sear and the full cock notch. The next issue was the cock was too thick at the tumbler axle. This did not allow the shoulder on the cock to strike the lock plate correctly which can allow damage to the main spring. I adjusted this. Then I discovered the axle on the tumbler was sloppy through the lock plate.  The lock plate hole was not matched to the axle diameter it was left as cast because the axle had maybe been over machined? This was causing the tumbler to bind under the load of the main spring in two places. First spot was up against the lock plate and second between the pivot point of the sear between sear screw pivot and the belly of the tumbler.  This revealed burrs on the tumbler up against the lock plate.  This also caused the half cock notch to lip to snap off when it bound up on the sear pivot point.  This was corrected with a replacement tumbler.  I need to comment that IMHO the hardness of these tumblers has not been drawn back enough. 
After all this I finally had the lock working beautiful shower of sparks I sent the gun, then customer calls and says the lock won't trip. From what they describe it sounds like the main spring has failed. Any Ideas? Sorry for the rant.

[bama]

I think today's builders are more aware of lock quality due to the amount of good information that is available from many different sources. We have the CNC'd locks that are being produced that are being held to very fine standards and tolerance's. Once you use one of the CNC'd locks its hard to go back to the locks that are being asssembled using castings where the castings are not being properly cleaned up and assembled as is. I have had some that the tumblers were so soft it was no problem to cut them with a file.

There are only two or three lock makers that makes locks today that can be used as received. I won't mention their names as most experianced builders know who they are.

All the other locks require work to make them acceptable.

[smart dog]

Hi Gustav,
The Davis Twigg lock has a number of issues, some of which you discovered.  Here is an image of the inside of the stock lock.



The mainsprings are too weak, the flint cock does not over hang the pan enough at rest, the head of the sear screw fits into a hole of equal diameter without a shoulder so it does not press on the bridle at all.  That was a cheap way to eliminate having to thread the screw precisely so it does not pinch the sear. The tumbler post sticks out too far such that the shoulder of the flint cock barely overlaps the bolster on the lock plate.  Here is my rework of it.  I first made a new mainspring of a length that allowed me to take some off the shoulder of the flint cock so it over hangs he pan more but the stirrup does not drop below the lock plate. The mainspring is much stronger as well and has the right geometry at full cock. I replaced the bridle and sear screws with ones that have bigger heads and stronger slots as well as shoulders that fill the original counter bored holes in the bridle. The sear screw has a second shoulder at the end of the threads so it cannot pinch the sear.  The inside of the lock plate had to be flattened and then polished.  I annealed the tumbler and filed the square shoulder down a little so the flint cock sits closer to the plate.  I also ground down the hump on the feather spring a little and reshaped it to reduce the excessive tension on the battery and allow it to snap over at just the right time.



I will case harden the tumbler, sear, bridle, flint cock, top jaw, and lock plate when I am ready to case harden the battery and after cutting teeth in the jaws and engraving.  It should be a good lock in the end.

dave

[Bob Roller]

I used the EXTERNAL parts I bought for 12 of these and 2 more with customers parts.I made a mechanism that was like those seen in English match rifles and no reported problems.These locks have potential as do others but the LOCK IS INSIDE the plate,That style of mechanism was not intended for anything beyond an occasional shot  and not for today's competition shooters.The mechanisms I made were the same as those shown here recently with the "3 pin" bridle.I had the shooters of today in mind and copying a flimsy mechanism is not a good idea.I made locks until 2019 and then stopped because of lower back problems and then made only triggers on a "once in a while"basis.Whether or not I can restart is in the category of . In 1978 I was contacted by a man in what was then West Germany who offered cut white flints and wanted to know about my lock work and I sent him a lock made on the Chet Shoults external parts with my bench crafted mechanism and the day it arrived in Germany I got a call from Gunther Stifter and was told all the locks I wanted to make could be sent to him and there was no griping about price and later the shop of Helmut Mohr called and asked about a lock for a Boutet target pistol and I was able to send him a suitable lock that was very similar to the small L&R plus a cap lock for a Schuetzen rifle and triggers for it.It was a good run and and had Gunther Stifter not contacted me I would have closed the shop to all lock and trigger making in 1978 and concentrated on automobile parts like bronze bearings from one to a truck load and reconditioning stator supports for GM automatic transmisisions.One car job I enjoyed was given to me by a Canadian who was an executive for IBM and the job was to copy every nut,bolt screws and sjackle bolts wit bushings for a Duesenberg he was rebuilding.
Bob Roller

[Goo]

I think today's builders are more aware of lock quality due to the amount of good information that is available from many different sources. We have the CNC'd locks that are being produced that are being held to very fine standards and tolerance's. Once you use one of the CNC'd locks its hard to go back to the locks that are being asssembled using castings where the castings are not being properly cleaned up and assembled as is. I have had some that the tumblers were so soft it was no problem to cut them with a file.

There are only two or three lock makers that makes locks today that can be used as received. I won't mention their names as most experianced builders know who they are.

All the other locks require work to make them acceptable.

I didn't have such a great experience with the last super popular cnc brand lock I bought For another customer about 4 months ago either.     The trigger pull was so difficult almost impossible to to trip the lockout of the gun.  I managed to lighten it up slightly but never got it where I was completely pleased.   I reasoned if I had to go to a bunch of trouble might as well spend less money up front.