Narrowing lock mainspring?

I am finishing up inletting my lock internals on my first build and need to narrow the nose of mainspring slightly to keep it out of the barrel channel. My lock is a Deluxe Siler. Looks like less than a 16th inch reduction of the spring will let the lock fit the mortise correctly.

Is it best to grind the spring in or out of the lock plate?

I did a little measuring and need about .006 more to have my bolster against the barrel. I suspect about .020 reduction on the spring will have the lock in place and the spring not touching wood anywhere. When I used a forsner bit to clean out a spot for the spring the barrel side of the bit broke through the barrel channel very slightly (pin head size holes) so I know I don't have much wood to work with.

YOu should be able to file the side of that spring out of the lock. Remove it, and hold the lock in the jaws of a good bench vise. Mark how much metal you want to remove, and then use a good, new file to do the work. If you are removing as little metal as you have indicated, I would not bother trying to use any kind of grinder. You will not reduce the spring tension of that mainspring much at all with that little amount of stock removal. Most springs are about 3/8-1/2 inch wide. There is plenty of metal there for removing some of it. You will get better ignition, and better life of your flints, or nipples, if that mainspring is no more than 15 lbs. in strength. put the lock in the stock, and put the butt of the stock on a bathroom scale. Note the gun's weight. Then watch the scale as you slowly cock the hammer back to full cock. subtract the high weight figure from the weight of the gun, and you have the tension of the spring. Its usual today to find springs that have more than 25 lbs of tension in them, which will eat flints, and peen down nipples so that you have to replace them often. On flintlocks, the frizzen spring also has to be controlled, and 3 lbs. is more than enough tension in that spring for it to do its job. I often see frizzen springs that are more than 10 lbs.

Thanks Paul, that is what I needed to know.

Well, I get to put in my two cents.
If you are going to grind or file on your spring, you need to cut in the 'long' direction of the spring. NEVER cut or file perpendicular to the leaf. If you do, you are making little scratches which are called 'stress risers'. These can turn into cracks when the spring is under tension resulting in a broken spring.
Just remember to do all of your work parallel with the leaf and it is a good idea to use a fine silicone carbide sandpaper (black) to reduce the depth of any scratch as much as possible.
This is especially important where the spring bends back on itself (at the forward end) because it is vertually impossible to file parallel with the material when it turns a corner and reverses direction.

zonie

Zonie said:

Well, I get to put in my two cents.
If you are going to grind or file on your spring, you need to cut in the 'long' direction of the spring. NEVER cut or file perpendicular to the leaf. If you do, you are making little scratches which are called 'stress risers'. These can turn into cracks when the spring is under tension resulting in a broken spring.
Just remember to do all of your work parallel with the leaf and it is a good idea to use a fine silicone carbide sandpaper (black) to reduce the depth of any scratch as much as possible.
This is especially important where the spring bends back on itself (at the forward end) because it is vertually impossible to file parallel with the material when it turns a corner and reverses direction.

zonie

Click to expand...

Sounds like to me someone needs to do a pictorial tutorial on spring adjustment & proper thinning! :hatsoff:

Davy

Eric Krewson said:

I am finishing up inletting my lock internals on my first build and need to narrow the nose of mainspring slightly to keep it out of the barrel channel. My lock is a Deluxe Siler. Looks like less than a 16th inch reduction of the spring will let the lock fit the mortise correctly.

Is it best to grind the spring in or out of the lock plate?

Click to expand...

OK....I'm confused......is the edge of the spring bumping into the barrel?

Instead of reducing your driving force (spring) could you file a bit off the flat, sorta knock off the edge where two sides meet to make the angle? Make it somewhat round, not to much, just a bit.

Yeah, you can just knock the corner off the top arm of the spring to clear the barrel if you want. I have narrowed a Siler mainspring quite a bit before...it does make for a nicer feeling lock, and while I do not like springs as feather light as Paul does, I can certainly stand them to sometimes be a bit lighter than they come.

Of course, if the spring does break into the barrel channel...big deal. Mine almost always do, and it certainly isn't uncommon to see this on old guns...along with a large notch filed into the barrel to clear the spring!