Too strong a spring - how do you fix this

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I recently picked up an Intercontinental .36 Kentuckian Jaeger flintlock. Was a wall hanger, never shot. Have about 50 shots through it.
Not a bad little gun but the mainspring is far too strong, actually hard to cock and eats flints, had to replace the first one after 30 shots.

How to you go about lightening the tension ?

20220814_131649.jpg
 
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Here's a previous thread on a similar trouble.
 
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That thread mentioned is interesting. But, as an experienced screwer-upper of stuff I am not qualified to work on, I can tell you, using a belt sander/grinder to reduce the thickness of a mainspring is something that should not be attempted unless you are very good at working with machine tools, springs, etc. Before messing with it, I suggest you look for a replacement spring first. Hopefully, somebody expert here will chime in with other suggestions.
 
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I tried to post a catalog photo of one but it didn't work. It's $21.95. I have had one for years. You can also use a vise grip if you're careful and don't compress the spring too much. Just enough to remove it is all you need. That spring may be hard to grip.
 
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I recently picked up an Intercontinental .36 Kentuckian Jaeger flintlock. Was a wall hanger, never shot. Have about 50 shots through it.
Not a bad little gun but the mainspring is far too strong, actually hard to cock and eats flints, had to replace the first one after 30 shots.

How to you go about lightening the tension ?
What I do is grind or belt sand the long side (thinning not changing the width) trying to stay parallel to the original shape. I usually do nothing at the arch on the end. Go easy, keep your bare hands on the material. If it is too hot to hold, cool it down. Never let it get hotter than your hand can bear. You can ruin the temper. When grinding, go with the length, never across. Polish all grooves and scratches out. Any material that flexes is more susceptible to breakage if it is not smoothed back out. Since this sounds like your first attempt, don't take off too much and end up with a weak spring. You can always remove and take off more. Good luck!
Larry
 

Rock Home Isle

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That’s an interesting lock you have there…

You’ll want to get a spring vice…I have one in everyone of my shooting bags, and a couple extra in my gun cabinet. I have a replacement mainspring for each type of lock that I use in my guns…reminds me, I need to call Kibler…

It‘s a very simple process. Basically use the vice to remove the mainspring from the lock, then dip the spring in water, and use a belt sander to carefully thin the spring. I don’t wear gloves when doing this, I want to feel the temperature of the spring as I remove thickness. As the spring starts heating, dip it into the water, I use ice water.

Take off a little bit, dip the spring as it starts to warm up…put the spring back in the lock…check the force needed to cock the lock. If the lock is still too stiff, remove more…if it feels good…stop.

I try to be able to get at least 60 shots from a flint, at least that’s my goal. Most of my guns will go 80+ before the flint needs to be replaced. I say 80+, because at 80 shots, I stop counting.

I have one gun, my Early Plains Rifle, that I’m working on right now. With the spring, as is, from the builder, I’m getting 57 shots before I need to replace the flint…that’s a nebulous region, is it worth it to take the time.

I’m still deciding…
 
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There are youtube videos that will teach you how to knap your flint while it's on the rifle. After learning that, which is crazy simple, there's no reason to replace a flint unless it just doesn't reach the frizzen anymore.

Now, grinding that spring to reduce the force. All the advice above about keeping it cool is spot on. Too hot to handle is too hot.

Now, where to grind. There is a "width" and there is a "thickness", and how it translates can be confusing. Reducing the thickness, or grinding the top and bottom of the spring (in relation to how it sits installed on the lock) is the worst way to reduce one. Just a fraction of thickness removed will exponentially weaken the spring. It's never advisable to grind the thickness thinner.

The width of the spring, the sides (in relation to how it's installed in lock), is the best place to grind. It's usually evident on most locks anyway that's the area that's been ground some at the factory. It will be polished bright at least on one side.

The reduction in force will be much less dramatic by grinding the sides, and much less likely to go too far. This is understood by folks who tiller traditional bows depending on how much a bow limb needs to be reduced to match the other one. There's a mathematical equation that explains the difference but I can't recite it.

The only other critical thing is doing your grinding down the length of the spring and not "cross grain". And it's a really good idea to file out and polish all your sanding/grinding marks as that will stave off future breakage of the spring prematurely. Bright and shiny with no visible scratches is a good idea. I do it with a dremel and polish wheel and polishing compound. Keep in mind that filing and polishing after grinding will ALSO contribute slightly to reducing the force of the spring, but it usually isn't a huge concern if you grind it properly to start with and don't have to polish it much.

It's not a hard job, but there are rules to follow.......

Picture added: the red line is the side or width of the spring, and where you want to grind it. The yellow is the direction you want your grinding marks to go, as much parallel with the spring limbs as possible.
Screenshot_20220814-231833_Gallery.jpg
 
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M. De Land

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There are youtube videos that will teach you how to knap your flint while it's on the rifle. After learning that, which is crazy simple, there's no reason to replace a flint unless it just doesn't reach the frizzen anymore.

Now, grinding that spring to reduce the force. All the advice above about keeping it cool is spot on. Too hot to handle is too hot.

Now, where to grind. There is a "width" and there is a "thickness", and how it translates can be confusing. Reducing the thickness, or grinding the top and bottom of the spring (in relation to how it sits installed on the lock) is the worst way to reduce one. Just a fraction of thickness removed will exponentially weaken the spring. It's never advisable to grind the thickness thinner.

The width of the spring, the sides (in relation to how it's installed in lock), is the best place to grind. It's usually evident on most locks anyway that's the area that's been ground some at the factory. It will be polished bright at least on one side.

The reduction in force will be much less dramatic by grinding the sides, and much less likely to go too far. This is understood by folks who tiller traditional bows depending on how much a bow limb needs to be reduced to match the other one. There's a mathematical equation that explains the difference but I can't recite it.

The only other critical thing is doing your grinding down the length of the spring and not "cross grain". And it's a really good idea to file out and polish all your sanding/grinding marks as that will stave off future breakage of the spring prematurely. Bright and shiny with no visible scratches is a good idea. I do it with a dremel and polish wheel and polishing compound. Keep in mind that filing and polishing after grinding will ALSO contribute slightly to reducing the force of the spring, but it usually isn't a huge concern if you grind it properly to start with and don't have to polish it much.

It's not a hard job, but there are rules to follow.......

Picture added: the red line is the side or width of the spring, and where you want to grind it. The yellow is the direction you want your grinding marks to go, as much parallel with the spring limbs as possible.View attachment 155867
I like to avoid grinding through the turn back which puts grind marks across the edge grain in the turn. Instead use about a three inch diameter grind wheel mounted in a lathe chuck or even a hand drill and reduce both the upper and lower leg widths along the red line tapered into the turn back and sear end leaving them full width. It can be done well with a Dremel tool as well if one is careful.
 
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I like to avoid grinding through the turn back which puts grind marks across the edge grain in the turn. Instead use about a three inch diameter grind wheel mounted in a lathe chuck or even a hand drill and reduce both the upper and lower leg widths along the red line tapered into the turn back and sear end leaving them full width. It can be done well with a Dremel tool as well if one is careful.
Meh, judicious filing and polishing takes care of the bend. Belt sand it, polish it up, be done. No need to over think it.
 

LRB

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What I do is grind or belt sand the long side (thinning not changing the width) trying to stay parallel to the original shape. I usually do nothing at the arch on the end. Go easy, keep your bare hands on the material. If it is too hot to hold, cool it down. Never let it get hotter than your hand can bear. You can ruin the temper. When grinding, go with the length, never across. Polish all grooves and scratches out. Any material that flexes is more susceptible to breakage if it is not smoothed back out. Since this sounds like your first attempt, don't take off too much and end up with a weak spring. You can always remove and take off more. Good luck!
Larry
The width can also be changed along with the thickness if needed.
 

M. De Land

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Meh, judicious filing and polishing takes care of the bend. Belt sand it, polish it up, be done. No need to over think it.
I make these springs from spring stock and have found that grind , file marks or edge micro corner cracks, , in or near the turn back on the side edge and corner is the main reason they fail. Also the method I described allows width reduction from both sides of the upper and lower legs while maintaining it in the middle (turn back) and both ends which centers spring profile laterally in the respective seats. Its not overthinking to address the whole spring function.
 
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