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  #1  
Old 09-13-2021, 04:04 PM
Sed_Of_TLC

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MU-12 bolt mainspring



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Hi All

I have an MU-12-2 where the bolt is playing up a little. I think it's the spring as it has a tendency to be a little stiff to open the bolt. I have another MU which I destroyed the bold using a lot if ignornace and more than a little brute force! I did post about it several years ago, under a different username!

Anyway....was getting misfires which required multiple lifting and lowering, of the bolt, so I swapped the firing pin out and the problem went away.

Now it's very hard to cock the bolt or adjust the spring correctly so I think the mainspring may be on its way out.

So does anyone know the specs for the mainspring or could I just go to a spring company and say "Give me a spring that looks like this!"?

My other option was take it to a gunsmith but it would probably be cheaper to buy a Match 54,

Thanks in advance

Jim
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Old 09-14-2021, 01:09 AM
tim slater is online now

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Jim,

A weak spring often does cause light strikes, but why would it also make the bolt harder to operate? If anything, the bolt should take less force, as there is less resistance from the spring.

When was the last time that you stripped the bolt to clean and lubricate it? If you are wary of taking the bolt apart, having damaged one before, an ultrasonic cleaner would be non-invasive, so would a bucket of paraffin. Disassembly would let you inspect the internals for wear, which can bind the bolt, and easily lubricate bearing bearing surfaces like the cocking cam.
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Old 09-14-2021, 01:17 AM
Sed_Of_TLC

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I've had it apart several times to change the firing pin and adjust the tension of the spring. Haven't lubricated it.

It's either too light a strike on the cartridge or it's an absolute swine to cock the bolt to when putting it in the rifle.

Also, depending on the tension of the spring it won;t let the bolt go forward enough to close....and no idea why!
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Old 09-14-2021, 07:55 AM
tim slater is online now

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Quote:
Originally Posted by Sed_Of_TLC View Post
I've had it apart several times to change the firing pin and adjust the tension of the spring. Haven't lubricated it.

It's either too light a strike on the cartridge or it's an absolute swine to cock the bolt to when putting it in the rifle.

Also, depending on the tension of the spring it won;t let the bolt go forward enough to close....and no idea why!
I wonder if the spring is binding inside the bolt? As I wrote before, a weak spring should make the bolt easier to cock. If you can adjust the compression, perhaps its too compressed and sticking out and catching.

Try a little grease on the camming surface.
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Old 09-14-2021, 12:06 PM
aslap

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Send me a pm

Jim,

I think I know the problem. Send me an email with your phone number to:

aslap AT Comcast DOT net.

Itíll be easier discussing it rather than writing about it. I have two of them and wound up knowing a lot more about the whole firing mechanism than I ever wanted to.

Tony
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Old 09-15-2021, 01:15 AM
Sed_Of_TLC

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Sorry Tony but I'm in the UK.

Steve Maly and Tim Slater have given me some advice so will investigate these ideas.

J
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Old 09-15-2021, 09:56 AM
aslap

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OK

It has everything to do with the screwing in of the cocking piece onto the firing pin itself. I completely rebuilt an MTs-12 from a spare receiver and a mismatched bolt. It took quite a while to figure things out.

You will note that as you screw the FP into the cocking piece, the relationship of the sear to the FP changes. If the FP is screwed in too far, the bolts gets very hard to work and sometimes it wont even close on a round. Not screwed far enough and the gun will fire when the bolt is closed.

I also believe I have a weak spring in the rebuild MTs-12. My other one is OK. It looks like one way to get more spring tension is to simply crank down on the cocking piece but that leads to the problems outlined above. My guess is that each one of these rifles was hand-fitted to get the right FP fall and bolt open/closure. The ring that sits between the front and rear of the bolt bodies is used to set the final headspace. At least that's what I think. My two rings are slightly different in thickness on the two rifles.

To get it to run, I started with the cocking piece screwed down further that necessary. When you close the bolt, you can hear and feel the FP hitting the sear. I adjusted the cocking piece out little by little until you can no longer hear or feel the FP hit onto the sear. You will also be feeling the bolt getting easier to open and close. If the spring is weak, when you get to this point, you'll be getting light strikes. To correct that, I made a spacer out of SS tubing that fit between the lip on the FP that stops the front of the spring and the spring itself. That allows for a bit more pop on the spring without messing with FP/sear relationship.

I check the pin fall using a depth micrometer on the cocking piece and measuring the FP position through the end of the cocking piece. IIRC, you'll get a FP fall of right around 0.120" or so with everything set up right. That will keep the FP around 0.015" off the stop.

Here is a pic of the rebuilt one. It's painted blue now. That's a Rock Creek barrel on it. I had access to a CNC lathe when I was working so I did all of the machine to get the barrel to fit. I made an adapter to go from the big MTs metric thread to the more common 3/4-16 thread used in the rimfire bench rest guns. It shoots pretty good.


Good luck and I hope you get things sorted. The guns look like farm tools but they can shoot!

Tony
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Old 09-16-2021, 12:30 PM
tim slater is online now

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Tony,

Could you post a picture of the bolt disassembled please? I think I can picture what you mean; the bolt is timed to the trigger, rather than the trigger to the bolt.

The Mts12 wasn't the only rifle to have a separate headspacing part: the British Centre-10 had a headspacing ring between the rotating handle and body. I think the uber-expensive Grunig comes with inserts for adjusting headspace.
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Old 09-17-2021, 01:50 PM
aslap

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Thatís it

Tim,

That is a perfect explanation. I donít have any pix of the bolt opened up but if I do have to get back into it, Iíll post them here.

Iím assuming that every one of these rifles was hand fitted in terms of the bolt/sear action and headspace. They probably had boxes of firing pins; springs; and spacers to get things right. So changing anything out would probably require a trip back to the armory to get tuned back up.

In my case, I was working with a receiver and bolt that were not matched. I bought the receiver from a member here to have if I ever needed spare trigger parts. I got the bolt, from all places, eBay! I had a lot of work to do to get the pieces to work together. The method of creeping up on how deep to set the FP into the cocking piece was the only thing that worked. I took measurements from my complete rifle and transferred them over to the new build and it didnít work.

I have a manual for the rifle that has a schematic of the rifle. PM or email me and I can send you a copy if you need it.

Tony
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Old 09-17-2021, 01:54 PM
aslap

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Interesting thing

When I measured the headspace on my complete rifle, it came out at 0.050Ē which you would think would be bad. But the thing really shoots. On the build with the mismatched parts, I get 0.047Ē on the headspace. Again, bigger than the common 0.043Ē that is the norm for the benchrest guns. I think I read something once that said the ammo the Russkies used in their team competition guns had a thicker rim than something like Eley at the time.

Tony
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