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How is M3 bolt door locked ?


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The M3 has an issue -- the bolt door does not lock when the bolt is in the forward position. It opens and closes freely.

 

Question: How is the door locked in this bolt forward position? No explicit locker that I can see... totally depends on the attrition from the hinge?

 

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The back edge of that tab on the ejection port cover is supposed to push back on the bolt slightly when it is closed and the tab is resting in the recess in the bolt. The ejection port cover may be bent or the tab may be worn.

Edited by Big Al
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Tab on inside of the cover is a safety. It fits into the cocking hole when cover is closed to prevent the bolt from backing up sufficiently to chamber and fire a round if the gun is dropped with a loaded mag inserted. Many smg's have been outfitted with such devices due to accidental discharges from dropped weapons with loaded mags.

There is no door "lock" but there is a flat spring riveted on the offside of the door hinge that exerts friction pressure against the door hinge to resist its opening and closing.

FWIW

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The back edge of that tab on the ejection port cover is supposed to push back on the bolt slightly when it is closed and the tab is resting in the recess in the bolt. The ejection port cover may be bent or the tab may be worn.

 

When bolt is cocked (open position), that's the case -- close the door and tab pushes the bolt back a little bit so (1) the tab blocks the forward path of the bolt and (2) the bolt is moved away from the sear. For safety measure, both are necessary.

 

But when the bolt is in the forward position... close the door.. does that tab pushes the bolt back a little bit too? On the sample in my hands, it does not. And, pushing back bolt in this bolt position is not necessary, due to the sear does not touch the bolt at bolt's forward position. In theory, that step is unnecessary.

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Tab on inside of the cover is a safety. It fits into the cocking hole when cover is closed to prevent the bolt from backing up sufficiently to chamber and fire a round if the gun is dropped with a loaded mag inserted. Many smg's have been outfitted with such devices due to accidental discharges from dropped weapons with loaded mags.

There is no door "lock" but there is a flat spring riveted on the offside of the door hinge that exerts friction pressure against the door hinge to resist its opening and closing.

FWIW

 

I inspected the other side of the door, and felt you are correct -- there is a flat spring against the door hinge. The friction between the spring and hinge must be the source of the force to prevent door from open/close freely.

 

On this gun, that flat spring must be worn, so it does not exert any friction on the hinge anymore. I did not see obvious gap between the spring and the hinge either. I tried to insert a thin piece of paper between them and failed -- this worn spring probably just missed a few mils... thinner than paper.

 

Since the spring is riveted on the frame, it's not removable. Any thought on how to fix this issue?

 

 

 

 

post-261917-0-69919900-1572032094_thumb.jpg

Edited by yohuang
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This design has inherent disadvantage -- it depends on friction, and the designer obviously did not think about worn parts issue... or, he thought the whole thing could be thrown away if parts is worn. I could not threw it away though :) It's too late to complaint about the design.

 

One path to fix it,, that I can think of at this moment,,,, is to enlarge the door hinge hole a little bit, that would increase the outer diameter of the door hinge. A little bit, maybe 0.1 millimeter, or 0.15 millimeter,,, that might resolve the lose of friction issue.

 

I am not a mechanical guy, not sure that's possible or not.

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Or as they say-if it aint broke, dont fix it.....

 

Great guideline! However, this loose door does affect (1) safety and (2) shooting. At range, I have to use a rubber band to tie the open door in place to prevent it from jumping up and down freely. The gun shoots great, but this rubber band workaround make me uncomfortable.

 

Temporarily, I plan to apply a little bit eproxy on the hinge to make its diameter a few mils larger. If that does solve the problem, I will consider to enlarge the hinge slightly via professional service. If that does not solve the problem... I have to find other ways. Fixing these minor issues bring in some fun in the gun hobby.

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The eproxy coating cured, and I installed the door back to the frame. It works, the spring tension came back. However, it worked only for a few open door & close door operation -- that flat spring in the back acted as a sharp knife and peels off the coating quickly.

 

But this proved the concept -- increase the outer diameter of the hinge a little bit will resolve the issue. How to achieve that takes more thinking. Yesterday, I thought about increase the hinge hole size a little bit. But I negated that approach -- although increase the hole size could increase the hinge's outer diameter, it also makes the hinge movable on the axis. That approach won't work.

 

Looks like the right way to fix this issue is laying down a thin layer of steel on the hinge. There are professionals in another city having experience and sophisticated equipment doing that, I have not visited them for a few years. Hopefully, they are still in business.. Will find out when I get time towards the year end.

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So you're going to modify the door by having a welder butter an layer of steel on it and grind it to contour? That will make it no longer original which is something you should care about in a gun with collector value. It will also require re-finishing the door and it may not match the gun all that well when you're done, further decreasing the value.

 

Here's a novel idea: fix it by replacing the part that's defective - the spring. And before you ask, you cannot build up the spring with weld. The heat will destroy whatever temper is left.

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Since you seem opposed to fixing it properly-heres an idea-swag the where the pin goes through and increase tension on the pin....then it will take more to rotate it and not be welding on anything but then again the better idea is still-have the spring replaced but what would I know.
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Since you seem opposed to fixing it properly-heres an idea-swag the where the pin goes through and increase tension on the pin....then it will take more to rotate it and not be welding on anything but then again the better idea is still-have the spring replaced but what would I know.

 

I am open minded. Regarding "increase the tension on the pin" -- that's possible, if I tighten the hinge. But I did not see why that's a proper solution, due to the flat spring is still not functional, the dependence on flat spring friction is moved to dependence on hinge axis friction. Not a good way IMO. It's also possible to insert a thin steel chip under the flat spring to solve the issue. There are a few ways to make it work.

 

[Edit] I just ordered an M3 door. A spare door would give me room for making mistakes.

Edited by yohuang
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Everything is fine other than the assembly of the gun. Whomever put the cover on allowed the flat spring to rise up on top of the cover instead of under it. You need to take the pin most of the way out, hold the spring down, and shove the pin back in. When the cover is closed the spring will fall into the detent slot in the cover and hold it closed and with the spring underneath it will also hold it open. HTH

 

For the record, covers never matched exactly since they were made separately and finished separately and then the guns were assembled. I have a new gun and the cover does not match, nor does the stock.

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Everything is fine other than the assembly of the gun. Whomever put the cover on allowed the flat spring to rise up on top of the cover instead of under it. You need to take the pin most of the way out, hold the spring down, and shove the pin back in. When the cover is closed the spring will fall into the detent slot in the cover and hold it closed and with the spring underneath it will also hold it open. HTH

 

For the record, covers never matched exactly since they were made separately and finished separately and then the guns were assembled. I have a new gun and the cover does not match, nor does the stock.

 

Thanks for the information!! I saw that slot on the cover, but I did not think its function. Now, I understand that slot offers the locking edge function when the door is closed. The flat spring is very stiff, I could not push it down by fingers. Probably special tools is needed.

 

The marking on the door is "G L B 301469", which looks being a part number, not a serial number. The wire stock has no marking.

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Hmmm.. even a simple piece of steel, its shape could have some reasons behind that. The only photo that I can find on hinge is this one, not for M3, but for M3A1:

 

https://www.apexgunparts.com/m3-grease-gun-cover-hinge-wa.html

 

So, M3A1's flat spring has a small bent at its top end, forming a "L" shape. It should be the turning edge of the spring falling into the cover slot when the door is close. And, that short leg of the spring could prevent it slipping into the other side of the door hinge (unless it's bent hardly, which is unlikely due to the spring's stiffness). This shape makes lots of sense.

 

But that's M3A1, not M3.. On M3, I did not see that L.. It's pretty much straight. Was this L flat spring one of enhancements on M3A1?

Edited by yohuang
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  • 3 weeks later...
  • 3 weeks later...

How about a follow up post from Yohuang, did you ever get this fixed?

 

It was sent to a local professional (pullmanarms.com) in October. They told me that they were a month behind work schedule so I had to wait.. Now it's December...

 

I called Dan. He told me it's fixed and I could pick it up next week. Will post more story on this when I get it back.

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  • 2 weeks later...

Test fired it again after door repairing. Everything works fine now. I got a magazine loader this time -- without loader, I could not load more than 10 rounds with fingers in the past. Even at its low firing rate (far away from other guns' burrrrrrp sound, M3 sounds more like drum beat), I still fired 150 rounds out quickly from one magazine .... paid $18 for one-hour in-door range use, but runs out of ammo quickly, so I left early.

 

 

 

 

 

 

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