Help with Mainspring Replacement

[Steve Tobler]

Some of you may recall me writting about a Centermark Fusil-de-Chasse I bought in a pawn shop that had a broken mainspring. Well, as it turned out, the lock was by R.E. Davis and TOTW carried a replacement and it recently came in, so I'm ready to install it. But I only have one problem .... I don't know what the heck I'm doing !!!

Ha, ha, ha, now that is out of the way, of course the replacement part or the mainspring vice I bought didn't come with any instructions, so I am seriously interested in any assistance with the procedure or process to safely install it in the lock without breaking anything (me included).

The top of the mainspring fits into a notch in the lock and there is a guidepin on the spring near the "V" that fits a recess hole in the lock.

I've searched around the forum for existing threads, but didn't come up with much. Seems I read somewhere to put the **** in the half-**** position to install a mainspring, but I can't find that reference again.

The vice is put on the way I think it should logically be, without any pressure applied. Is this correct ?

How much force do I use on the vice to compress enough for install ?

Looks like it requires quite a bit of compression, then hopefully I could slip the end of the spring above the sear (?) and then slide the pin and top into place and then release the vice pressure.
Does this sound right ?

I know it can't be that easy.
Thanks guys ....
:bow:

 

[Shine]

It is, you got it don't crank it down to much :thumbsup:

 

[paulvallandigham]

To slit that top tab into the notch under the bolster, while fitting that pin into the hole in the stock up front, you will have to squeeze that lower, working arm ENOUGH so that you can slide it just into place on that horn on the face of the frizzen. POLISH the horn's inside curve to a mirror finish, and put a drop of oil on it, before you slide the nose of the HOOK on the end of the lower working arm of the spring onto the horn. Also, while its out of the lock plate, polish the heck out of that NOSE on the hook, where its going to rub against the inside curve of the horn on the tumbler. You want ZERO friction between these two surfaces to get the most work out of that spring, and tumbler. Polish and oil the surface of the lock plate, and of the inside of the tumbler where it may rub against some part of the plate. Again, you want that tumbler to move freely, without DRAG.

As already indicated, don't over squeeze that spring arm with the vise. That is how springs BREAK. You want it to be squeezed together just enough to slide into( or out of ) place, so that you can clean the parts, or work on them. Try not to squeeze the two arms together so that they are parallel to each other. Few leaf springs can take that kind of tension. If you have to squeeze the spring arms together that much or MORE, STOP. You may need to remove some metal from the nose of that hook before mounting that spring in your lock.

The other place where you can remove metal to help a spring fit a lock better, without being compressed so much is that tab on the upper arm. If that tab is shortened, the upper arm goes further upward in the lock. That means that the lower arm can be mounted with the hook on the horn and Not compress the two arms so much together, or stack the spring, the lower over the top one.

That Tab on the upper arm only needs to fit in that slot- nothing more. Most such tabs are too long, IMHO. Its only there to establish a footing for one leg of the triangle that is the spring.Ifyou have any questions send me a PT, and I will send you my phone number. You can call me and i can walk you through working on that spring, and getting into your lock. OK? :thumbsup:

 

[tg]

If I recall one wants the lock to be in the fired position which has the least amount of pressure on the spring, then only depress the tool just enough to allow the new spring to go into place, some veteran gunbuilders will likly confirm or advise to any changes in what I posted so give them a bit of time to respond, it has been ten years since I changed a main spring and may be a bit fizzy on the details.

 

[Lorne Johnson]

Paul's advise is very good (as it generally is). Main springs are intended to be fitted, on the theory that one can file a little off the tab and the hook, but one can't file it on. Just take it slow and be patient.

To get the idea of using the vise, it is handy to have taken a mainspring off. To do so, one cocks the lock then puts the vise on with NO PRESSURE. Then uncock the lock, and the spring can be removed. (That's the most amount of compression the spring should get.)

 

[Rifleman1776]

Yes, Paul's directions are good.
EXCEPT, he had a typo that could be confusing.
His second word "slit" really should be slip. Yes, do some filing and stoning to avoid friction. Do take some time. When you are finished you will be an expert. Glad you got the vice, it distributes stress and will 99% guarantee you won't break your spring.

 

[paulvallandigham]

That raises another thought. I generally don't get the opportunity to work with mainsprings that have never BEEN in a lock before, so "breaking a mainspring in" becomes another technique. With today's steels, its probably not necessary, but with old springs, and springs that come from other countries, where we have NO clue as to what kind of metal is used to make the spring, it can make the difference between a working spring, and a broken one.

I like to take any new spring, in that spring vise, and slowly turn the screw to tighten the vise, and close the two arms of the spring together. But, when I close the spring a few turns, I back the screw out to rest the spring, and give it time to adjust to the new "Tension".

I do this repeatedly in stages, until it slips into the lock. I don't want to over compress any of these springs, as that is when they DO break! By tightening and relaxing that spring arm( the lower arm of a mainspring is the one that does all the work) The spring gets used to being exercised, and put under tension, then released.

I have seen enough springs brought to me after they broke when the gun was fired and the tension was suddenly released, to understand that BOTH putting tension on the springs, and that sudden release cause metal fatigue in some springs.

as you tighten that spring vise screw, always be checking that working arm to make sure that the entire ARM is moving between the jaws of the vise, and that there is not a "kink" or sharp bend occurring in the arm as tension is increased. In theory, at least, that working arm should bend as a unit, remaining flat along its entire length as tension is applied. :thumbsup: