Author Topic: Bolt lug broken off... again.  (Read 390 times)

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Offline TJNewton

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Bolt lug broken off... again.
« on: October 08, 2019, 02:39:00 AM »
I was at the range doing slow fire at 100 yards and my rifle jammed.  It's never jammed before.  The carrier was open about 1.5" and the round was stuck cockeyed with the bullet half in the chamber.  I cleared it, chambered another round, and it happened again.  On inspection, I saw that the narrow, left-sided lug on the bot had broken off.. just like it had before described in this thread:

https://czfirearms.us/index.php?topic=96542.0

The thing is, this is a different rifle from the one in the thread above.  Different carrier, different locking piece, different bolt.  This rifle has about 2,500 rounds on it.  It's never malfunctioned or acted up before.  While it tosses spent cases wildly, it's no different from any of my other VZ2008's, or my SKS's for that matter.  Based on my own experiences with multiple VZ2008's and other 7.62x39 rifles, I don't think it's overgassed.  The piston is dished, not flat.  The spent cases looked normal.  I guess it's possible that all my VZ2008's and all my SKS's are overgassed, but I doubt it.  They match every report I've read of spent case trajectories and patterns.  I was shooting Golden Tiger, about 200 rounds into a new case.

Below are pics.  The lug snapped clean off.  What is interesting about the marks in the receiver is that while the wider lug does drag along the right side of the ejector, the area to the left of the ejector where the narrow lug travels is completely untouched. 

Very strange.  Two broken bolts on two different rifles.  Anyone have any thoughts or insights? 

https://imgur.com/a/Iy2MfVz#MMlvdfm

https://imgur.com/kKmrLKd

https://imgur.com/0qXQLJr

And by the way, it's been a while since I posted pics here.  I'm using Imgur and can't seem to embed the pics.  Any advice?

Offline OldGringo

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Re: Bolt lug broken off... again.
« Reply #1 on: October 09, 2019, 10:37:26 AM »
Very strange.  Two broken bolts on two different rifles.  Anyone have any thoughts or insights?

In your first post the issue was with a Century VZ 2008.
Is the rifle that is currently giving you problems a D-Technik/CSA Vz58 or Century VZ 2008?

Offline TJNewton

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Re: Bolt lug broken off... again.
« Reply #2 on: October 09, 2019, 11:34:11 PM »
Very strange.  Two broken bolts on two different rifles.  Anyone have any thoughts or insights?

In your first post the issue was with a Century VZ 2008.
Is the rifle that is currently giving you problems a D-Technik/CSA Vz58 or Century VZ 2008?

Still a VZ2008.  In searching for causes, I've come across only three other reports of the narrow lug breaking.  Two were D-Technik/CSA rifles, and one was Canadian, maybe also made by CSA.  I don't think the issue is with Century builds.  There are several members here who have fired thousands of rounds through their tabbed VZ2008's without any problems.  Back in 2014-2015 when VZ2008's were being sold by PSA, J&G Sales, and Atlantic Firearms, they would come in waves.  I remember there were about 2,000 in stock with each vendor each time.  Without going back over previous threads, I estimate that at least 20,000 VZ2008's were sold from 2008-2016.  Maybe more. 

The most common failure, although rare and also relevant to CSA and original Czechoslovakian rifles, seems to be broken firing pins, then after that, broken strikers and disconnector springs.  I've seen very few accounts or videos regarding failures of the VZ58 platform, maybe 5% as many as I've seen about AR15's.  I've seen about as many failure videos regarding AK's and SKS's, but as both those platforms were produced 100-to-1 over VZ58's, I have to admit that VZ58's probably aren't quite as reliable.

This bolt breaking problem seems to be very rare and not limited to VZ2008's.  I'm thinking it may be just bad luck with surplus parts of unknown wear.  Working on old cars, I've seen similarly unusual streaks of failures with used parts.  If you have any insights or thoughts, however, I'm definitely interested.

Offline RSR

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Re: Bolt lug broken off... again.
« Reply #3 on: October 11, 2019, 10:22:06 PM »
Search the threads -- I think it was CitizenPete but might have been someone else posting regarding some issues with the last round of VZ58 receivers rails sitting a little low and resulting in some issues. 

Also, possible you were using the same mag when both breakages occurred?  That it's a mag issue?

Offline TJNewton

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Re: Bolt lug broken off... again.
« Reply #4 on: October 12, 2019, 12:33:51 PM »
Search the threads -- I think it was CitizenPete but might have been someone else posting regarding some issues with the last round of VZ58 receivers rails sitting a little low and resulting in some issues. 

Also, possible you were using the same mag when both breakages occurred?  That it's a mag issue?

Thanks for the brainstorming.  I don't know if it's due to the receivers made by Century.  If it would be, I would think this problem would be much more common.  I don't think CitizenPete had any broken bolts, but I'll have to check those threads.  If it were due to a shallow receiver, then I'd think the bottom of the receiver would show evidence of the narrower lug scraping, but with each rifle the receiver to the left of the ejector was untouched by the bolt.  The right side -- the wider lug -- has heavy marks on all my VZ2008's.

I don't think it's the magazine either, although I wish it were something that easy to take out of the picture.  The first bolt broke with no magazine inserted and no round being chambered; I was testing the tactical bolt release.  I think I did it three or four times before the bolt broke.  The second time was during firing at the range.  It is a good point, though, and I'll examine the magazine and its interaction closely. 

Offline ObiWanBonJovi

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Re: Bolt lug broken off... again.
« Reply #5 on: October 14, 2019, 03:39:14 PM »
This makes me curious as to whether you frequently let bolt drop home with no round to feed.  I wouldve never expected a problem from that, but can now see how it could be.  I find that I generally close bolt slowly if im not feeding a round, and Im curious as to whether this issue could be caused by slamming it home without a round to slow it

Offline TJNewton

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Re: Bolt lug broken off... again.
« Reply #6 on: October 15, 2019, 12:00:40 AM »
This makes me curious as to whether you frequently let bolt drop home with no round to feed.  I wouldve never expected a problem from that, but can now see how it could be.  I find that I generally close bolt slowly if im not feeding a round, and Im curious as to whether this issue could be caused by slamming it home without a round to slow it

That's a good point and I've thought of it as well, although I wouldn't expect a problem either, considering the force and violence that happens with the chambering, firing, and ejection of each round.  The first time the bolt broke, I was testing the TBR.  The second bolt broke during live fire at the range -- however -- the magazine I was using was inconsistent with holding the bolt open after the last shot, in effect letting the bolt slam home without feeding a round.  This magazine holds the bolt open only about 50% of the time.  Same effect as racking the bolt without a round.

Another consistency is that both rifles had the same TBR installed, switched from the first rifle to the second.  I wonder if the use of the TBR somehow makes the bolt slam forward with more force.  Instead of the bolt being pulled back slightly and allowing the bolt hold open to lower, the TBR allows the bolt to go forward immediately.  I'm not sure if or how that would make a difference.  Maybe it gives it more "snap", although I would think that pulling the bolt back in the conventional way would load the spring slightly more than using a TBR.  It is a common factor with both bolt failures, but could very well be coincidence.  Again, I remember this feeling from years of trying to pinpoint failures and causes when working on classic cars.  I guess that combloc rifles are kind of the classic cars of firearms.

Offline RSR

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Re: Bolt lug broken off... again.
« Reply #7 on: October 15, 2019, 04:46:40 AM »
TBR should result in less spring tension/force upon returning to battery, not more.  More compressed spring = greater force; less = less force.

If you're having one side of bolt get off parallel, it could result in awkward forces being applied to the other side.  Sort of like how ARs with upper receivers whose upper receiver faces are square/true result in barrel/barrel extension being slightly off kilter and break bolts at greater rates, and how removing material from bolt lug opposite extractors so that it doesn't interface w/ barrel extensions on ARs leads to longer bolt life as it allows remaining lugs to more equally distribute loads, etc.

Offline TJNewton

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Re: Bolt lug broken off... again.
« Reply #8 on: October 15, 2019, 06:06:47 PM »
TBR should result in less spring tension/force upon returning to battery, not more.  More compressed spring = greater force; less = less force.

If you're having one side of bolt get off parallel, it could result in awkward forces being applied to the other side.  Sort of like how ARs with upper receivers whose upper receiver faces are square/true result in barrel/barrel extension being slightly off kilter and break bolts at greater rates, and how removing material from bolt lug opposite extractors so that it doesn't interface w/ barrel extensions on ARs leads to longer bolt life as it allows remaining lugs to more equally distribute loads, etc.

It makes sense that pulling the bolt back by hand compresses the spring slightly more and creates more force.  As for off-parallel, I wonder if the TBR, just before it "lets go", holds onto the right-side wider lug just a little longer, causing the bolt to yaw, and pushing the left-side narrow lug slightly forward. 

Regardless, I did remove the TBR and reinstalled the factory bolt release, even though it may be irrelevant.  Was the TBR really responsible?  Was it the coincidence of both bolts being heavily worn and fatigued?  Was it another common factor?  Was it two other factors unique to each breakage?  Impossible to tell and it would require an unrealistically huge supply of bolts, ammunition, and time to scientifically eliminate all the variables.  I've headspaced a replacement bolt and just need to go shooting, taking care and inspecting and observing as I go.

RSR, as you mentioned with removing the material from AR bolt lugs, I was thinking of filing back just slightly the narrow lug on the VZ bolt.  Maybe by just a few thousandth.  Do you think it's a good idea?  I would hate to meddle unnecessarily, possibly creating a problem where none exists.  Would the heat/friction caused by filing weaken the metal and cause the same result? 

Thanks for the insights, everyone.  Let me know if you think of any other possibilities.

Offline RSR

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Re: Bolt lug broken off... again.
« Reply #9 on: October 16, 2019, 01:33:59 AM »
Hand filing shouldn't create enough heat to affect tempering.  Remember that the VZ58 uses a double-stack mag, so that lower lug does strip rounds so doing so might affect angles.  There *shouldn't* be a need to modify the bolt -- unless CitizenPete recommended it/found it to remedy.  I haven't gone back to look at his posts regarding the clearance issues.

Another thought is that some have found that VZ2008 and czech vz58 barrels are different diameters at the barrel shank/chamber, so maybe the VZ2008's wider shank plays a role.

With the bolt, my thought is just that the bolt has the ability for play in all directions including rotationally, so any clearance/interference issues could be moving in any direction and causing add'l clearance issues.  Maybe degrease your rifle and chalk up your bolt, etc., and see any/all interference issues in your hand cycling.  For instance, hitting that extractor enough times during recoil might cause fractures that eventually breaks the bolt once it returns to battery -- just a possibility, not saying this is what happens...

Just speculating on possible causes, but might give you some add'l angles to pursue.

 

anything