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CETME - all kinds of mayhem

Discussion in 'Rifle Country' started by ziadel, Apr 25, 2007.

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  1. ziadel

    ziadel Member

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    Well, years ago, I bought a CETME, first rifle, never quite worked right.

    Now that I am out here in Montana, I've been trying to fix it, I fixed the ejection problems (trigger pack not installed properly) and put in an unground bolt, but with +4 rolllers, we're not registering any bolt gap.


    So, I guess now new locking peice? How do I know the new locking peice will work?


    thanks in advance

    -adam
     
  2. Drakejake

    Drakejake Member

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    If you have a rifle that has a proper bolt gap with a ground bolt head and the rifle works fine, I believe you are good to go. (Do not fire a rifle which does not have proper bolt gap.) I am well aware that there are a number of uncredentialed Cetme "experts" who say that you should never use a ground bolt head, even if the rifle is functioning properly, but they have never produced any support for this position. They tell people who cannot get good bolt gap, except with a Century Arms ground bolt head, to rebuild the rifle ("repress the barrel"). But I do not think it is necessary to do so. Many of these commentators believe that when you check bolt gap you are checking head space, but this is not so. Cetme bolt gap relates primarily to proper functioning (effective operation of the roller-delayed blow back system), not to safety.

    Drakejake
     
  3. ziadel

    ziadel Member

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    Drakejake, I do not sharte that opinion, I'd like to get the rifle back into spec, with the fresh bolt, if I have to repress the barrel, then I'll just have to repress the barrel.


    I am trying to remember if I ever ran the bigger rollers in the ground bolt, I don't think I did, mebbe I'll try it for $hits an giggles, but even if that does solve all my problems, I'm still going to get this thing uip and running with an unground bolt.

    Hasn't HK publicly shook their finger at the whole bolt grinding thing?


    Honestly, I've had this rifle forever, my first rifle actually, never worked quite right, but now I've got the trigger pack mounted up properly and we're ejecting 100% of the time, now I just gotta take care of the bolt gap thing and I should be good to go.
     
  4. Drakejake

    Drakejake Member

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    My first Cetme had a ground bolt and a bolt gap of about .008. I replaced the ground bolt head with a new one, replaced the locking piece, and put in plus four rollers. No bolt gap. (Bigger rollers can increase the gap a good bit, an unworn locking piece can increase the gap a small amount.) I did extensive research and decided that there was no reason not to use the ground bolt head with a good bolt gap. And I have done so. You will make your choice but I do not see the point of spending $300 or more to tear up the rifle, rebuild it, refinish it, etc., when it is working fine and no one can prove that using a ground bolt head is unsafe.

    Drakejake
     
  5. rbernie
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    rbernie Member

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    Well, that's not quite true, Drakejake. Nobody can prove it to YOU, but most folks, when presented with the mechanics and dynamics of the locking system, will conclude otherwise. We've had this battle several times here and on other forums. You're very invested in your opinions, but (with all due respect) that doesn't mean that you're correct - just very convinced of yourself.

    Ziadel - do a search here for CETME and BOLT GAP and see what you drag up. It'll be entertaining. If nothing else, there'll be good pictures of the relationship between the pieces. :)

    In my opinion - a simple glance at the mechanics of the locking system clearly indicates that zero bolt gap is a sign of trunnion, locking piece, or roller wear. Some wear is normal and expected, and HK, for example, produced slightly oversized rollers to deal with this. If oversize rollers didn't help, then replacing the locking piece is the next step. If that doesn't do the trick, the recesses in the trunnion are likely worn sufficiently that the trunnion is past its servicable life. Drakejake or anyone else is welcome to shoot a rifle with a ground bolt head and a worn trunnion, but I would prefer a less-worn trunnion sitting inches in front of my face.

    That's just me.
     
  6. Sunray

    Sunray Member

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    "...a CETME...never quite worked right..." From Century? It'd be their usual total lack of QC.
     
    Last edited: Apr 26, 2007
  7. 49north

    49north Member

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    A new locking piece will probably not give you a proper bolt gap if you now have none with an unground bolt. It is my understanding that factory maintence of the cetme rifle did not include grinding off the bolt to achieve a proper bolt gap. Larger rollers were used, bolts and locking pieces were replaced and then the barrel was repressed to achieve a bolt gap. I think drakejake had a death wish to fire a bolt that is improperly assembled and does not follow the manuals, for I have never seen info stating that grinding the bolt is normal service for a roller lock type rifle. But I am SURE that drakjak know best...NOT.
     
  8. ziadel

    ziadel Member

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    anyone know where I can find a new trunnion?

    how much do they cost?

    and any instructions on how to deal with this?

    thanks
     
  9. chestnut ridge

    chestnut ridge Member

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    This is very interesting; and since I have a CIA cetme; very important.
    Could you please perhaps put up some pictures that show a ground bolt and an unground bolt. also where to check bolt gap and headspace; and the methods to do so. thanks.
     
  10. ziadel

    ziadel Member

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  11. ziadel

    ziadel Member

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    ok, now with the ground bolt and the +4 rollers my bolt gap is .25mm.... but thats WITH the ground bolt....

    any hope that the new locking peice will be able to pull me back to specs?

    I looked at the one in there now and if its hard hard steel like I figure it prolly would be, then its seen some wear, nice bright mirror smoothe....
     
  12. Trebor

    Trebor Member

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    Tag because I need to follow that link later and figure out if my CETME has a ground bolt or not.

    Did *all* Century CETME's have a ground bolt head? Or just some rifles? Were they typically earlier or later production?
     
  13. ziadel

    ziadel Member

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    just some of them did, I have no idea if they were more abundant in the earlier runs or the later ones...


    anywhom did a search of the bolt gap thing, i just wanna know if theres a chance in hell that if I put a new locking peice in the bolt, I can make specs....

    otherwise we need to talk about wether or not I can handle a repress by myself..
     
  14. ceetee

    ceetee Member

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    Well... I'm no expert, so take this for what you paid for it... Adding a new locking piece might help you increase your gap. If not, the only other alternative I know of is to have the barrel repressed. AFAIK, that's not a job for someone that's never done one before. One other thing to check out; somewhere online is posted the proper measurement of an unground bolt. You can use that to make sure your unground bolt is the proper length. You can also see how much your old bolt has had ground off of it. It's possible that if you have .25 mm gap with your ground bolt, but it's had maybe .12 mm ground off of it, then you're true bolt gap would be .13 mm... See?
     
  15. Drakejake

    Drakejake Member

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    I think some people have reversed the true significance of bolt gap and worn parts (rollers, trunnion, bolt face). Inadequate bolt gap can be caused by part wear, but the point is that the rifle needs adequate bolt gap to function. Bolt gap is not simply an indicator of parts which need to be replaced. Inadequate bolt gap means you need more bolt gap. If parts are worn, but you still have adequate bolt gap, the rifle is safe and functional. If adequate bolt gap is obtained by grinding the back of the bolt head, you still have bolt gap, even with a worn trunnion, for example. The mechanism automatically compensates for some wear so there is no danger from excessive head space, as with bolt action rifles, for example. Some wearing of the parts does not increase head space because of the nature of the roller-delayed blow back system.

    Proponents of the opposite position on these issues have not been able to point to a single blow up or head separation caused by a ground bolt head. Problems of this kind in the Cetme are caused by excessively hot ammo, use of commercial .308 instead of military 7.62 by 51, bullets or cases lodged in the barrel, etc., not by worn parts or a ground bolt head.

    Drakejake
     
  16. Onmilo

    Onmilo Member

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    I haven't seen it yet because our shop doesn't deal with these rifles but I would think that grinding any heat treated part will cause the part to fail way before it is supposed to namely because you are grinding through the heat treated surface or you are altering the heat treated surface structure through changes caused by the grinding.
    Maybe the company who built these rifles re-heat treated the bolts, I don't know.
    I have inspected several of these CETME and G3 copy rifles during the time I was deciding on exactly what copy of this rifle design I was going to buy and all but the expensive H&K 91s and the PTR-91s exhibited some minor to major issues such as misalignment, shoddy barrel installations and bent sheetmetal receivers.
    Anytime an assembler has to grind an already tolerenced part to make it fit it indicates some major issues.
    I haven't heard very many positive things from people who have bought these assembled from parts kit rifles, that is all I can offer up.
     
  17. rbernie
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    rbernie Member

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    Nobody on this forum has introduced the notion of headspace in to the discussion - that's a coloration of an argument from another forum. Headspace in set by the relationship of the barrel to the trunnion and the trunnion's location in the receiver, as best I can tell. Certainly, it has nothing to do with bolt gap presuming that the relationship between the locking parts allows the bolt head to properly go into battery.

    When you repress the barrel, what you're really doing is removing the trunnion from the receiver (grinding the spot welds off the cross pin and removing the pin), removing the barrel/trunnion assembly from the receiver, pressing the barrel out of the old trunnion and pressing it into a new one, and then resetting the trunnion into the receiver, pinning it, rewelding the pin, and then refinishing the receiver to repair the damaged finish from the welding operation. This is not a trivial process. If you find someone who offers this service, balance your impression of the cost associated with that work against the actual work that needs to be done.

    http://www.militaryfirearm.com/Forum/showthread.php?t=848

    It's a shame that Century built rifles from worn-out parts kits, but they did. I have two CETMEs that are perfect, but I shopped around and was very careful in what I bought.

    Last I heard, inexpensive CETME parts kits ($100) were still available from places like Centerfire Systems. That's one way to get a potential barrel/trunnion donor for the repair, if you choose to do the repair yourself.
     
  18. glimmerman

    glimmerman Member

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    Drake jake has been told by every board, and I mean EVERY board that he is full of **** and has no clue as to what he is talking about concerning roller locking actions. He just cant get it into his thick skull that a ground bolt will give a false bolt gap reading. If anyone takes his advise on anything to do with roller locking actions please send your attorney to his house when your rifle blows up in your face.:banghead:
     
  19. Daniel T

    Daniel T Member

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    So then you can post a link to evidence that a ground bolt head has caused a KB? A lawsuit against the CAI monkeys over damages caused by a ground bolt head?

    I'm not taking any sides here, but it sounds pretty reasonable to ask for some evidence of a problem before assuming there is one.
     
  20. Drakejake

    Drakejake Member

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    Glimmerman, you are a liar and a fool. As such, you are representative of many of those who pose as Cetme experts but really don't understand the rifle. If you knew what you claim to know, you would be able to cite some evidence or proof. Since you can't do that, you call names like school children on the playground. NO ONE has given any proof for the claim that using a Century ground bolt head is unsafe. Why?--- because there isn't any such proof.

    Drakejake
     
  21. ziadel

    ziadel Member

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    Hey, Boys, I did'nt ask if running an unground bolt was ok, in fact, I clearly stated that I did NOT think it was ok at all.

    Why are we discussing this crap again, all I need to know is if anyone think the new locking peice could take me from no bolt gap to ok bolt gap.
     
  22. rbernie
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    rbernie Member

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    Drakejake - you're looking at this backwards.

    I'm an engineer, and I've been doing engineering work for better than twenty five years now. The things that I've learned include the notions that static views never convey the dynamics of a system, and that I'm probably not as experienced in understanding a system as the folk who designed the product (since they most certainly went thru many physical prototypes with differing design parameters, and certainly DID perform test-to-destruction and other measures of robustness).

    The question isn't who HAS had an issue due to a worn and weakened trunnion - it's how much more likely is an issue to occur as a result of a worn trunnion. Since I do not have the means to test rifles to destruction to prove or disprove the designers or manufacturers maintenance recommendations, I have to presume that they established those parameters for a darn good reason.

    For example, do you have any insights into how the machined angles of the trunnion immediately fore and aft of the locking recesses affects the dwell time or acceleration curve for the bolt head? Can you quantify how much protrusion by the rollers into the locking recesses is required in order for the bolt to be considered 'in battery'? Do you know how deep the trunnion recesses can get before no amount of protrusion by the rollers will seat the rollers firmly against the outer walls of the locking recesses? What are the wear characteristics of the trunnion as the bolt seats further forward or back into the trunnion? How much fatigue cracking is likely to occur as material is abraded away from the locking surfaces, and where? What are the likely failure modes if the trunnion locking surfaces become worn past their design specifications?

    If you don't know this kind of stuff, how can you evaluate when the trunnion is worn to the point of danger?

    You can't.

    So if HK and others have a firm maintenance and R&R approach to managing wear in the roller locking system - in the absence of conclusive instrumented tests proving them WRONG, it would be folly to disregard their guidance based primarily upon the absence of Internet-based anecdotal KABOOM stories.

    You are certainly free to do whatever you want. But most 'accidents' are foreseeable, and caused by folk presuming to understand more about a given situation than they do. Staring at a static system - with no insight into the subtlies of the dynamics, knowledge of the metallurgy involved, and with no insight into prior testing that validated the design and provided operational parameter insights that can only be gained by stressing the system - doesn't really give you much opportunity to learn.

    It does increase the likelihood that something bad may happen.

    We're discussing it because this is a shared forum and some folks need to learn this stuff. :)

    I have no direct experience with bolt gap changes based upon locking piece replacement; my rifles have all had good gaps. Given the advice that Drakejake provided based upon his experiences, I'm presuming that the answer is no.
     
  23. Drakejake

    Drakejake Member

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    Rbernie, I think you have gotten off track here. Those commentators who oppose firing a Cetme with a ground bolt head are not saying that the trunnion is overly worn or needs replacing. They contend that the problem is lack of bolt gap, which they propose to remedy by repressing the barrel. They do not advocate replacing the trunnion. At any rate, I believe the Cetme action self-adjusts to compensate for wear in the trunnion and the bolt face. Thus, head space remains constant, despite wear. Proper bolt gap is required to make the rifle time and cycle properly. If the bolt gap is in spec, albeit with a ground bolt head, the rifle will function and be safe. That is my position, but if you or anyone else can produce evidence to the contrary, I will certainly consider it. I have asked for such proof many times and have only gotten personal attacks in response.


    Drakejake
     
  24. rbernie
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    rbernie Member

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    They sometimes do, and you've been too busy talking past them to hear it.

    The bolt gap is a means by which the wear in the locking mechanism (rollers, locking piece, and trunnion) is instrumented. When the gap cannot be corrected via R&R of the rollers and locking piece, the trunnion must by definition be out of spec.

    Pressing the barrel forward (putting the rollers further forward in the angled area ahead of the locking recesses) will certainly increase the bolt gap as a expedient repair. It will also increase the dwell time during which the bolt is 'locked', and that is not an inherently dangerous thing.

    But replacing the wearable parts (e.g. rollers, locking piece, trunnion) is, as far as I can tell, the only way to actually REPAIR the wear, as opposed to compensate for it.

    And this is always where we wind up. Absence of proof is not proof of absence.

    In the end, we wouldn't have these wearying discussions time and again if you simply would be content to hold your own opinions and be done with it. But you cannot help but to bring this up at every opportunity.

    Frankly, I think it's a disservice to the forum to provide gunsmithing advice that runs contrary to the accepted maintenance practices when the risk of injury is borne by others and not yourself.
     
    Last edited: Apr 27, 2007
  25. scottsw1

    scottsw1 Member

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