Cetme Ground Bolt Heads and Safety Issues

[Drakejake]

Over at two Cetme discussion groups (Cetme and FR-8 Forum) and (Cetmerifle.com) I have been carrying on a discussion of ground bolt heads and safety issues for several months. Some Cetme users have been maintaining that a Cetme should not be fired if the bolt head has been ground. Century International Arms, the primary assembler of semi-automatic Cetme Sporters in this country, has sometimes ground bolt heads in order to get the proper bolt gap (look elsewhere for detailed discussions of the importance of bolt gap in the functioning of these rifles). Bolt gap is required with these roller-delayed blowback rifles to get the locking piece to fully extend the rollers and lock-up the bolt assembly during firing. As the rifle is fired, parts wear until it is necessary to replace the standard rollers with larger ones and the worn locking piece with an unworn one. The problem arises when these parts are replaced and there still is no or insufficient bolt gap. Apparently Century dealt with this problem by taking some metal off the rear of the bolt head, thus bringing the bolt gap up to spec. Century has claimed that this is a legitimate procedure and that they do the grinding in the proper way.

Assume that you buy a Cetme and examination indicates that the bolt head has been ground and that if you replace the ground BH with an unground one, and install bigger rollers and an unworn locking piece, there is insufficient bolt gap. A coterie of Cetme fanciers has claimed that you cannot use the ground bolt to get bolt gap and that you must "repress the barrel," basically reconstructing the rifle. In other words, according to these commentators, the only valid alternative to junking the rifle is spending several hundred dollars to have the rifle rebuit.

One might think that those offering such radical and expensive advice would be able to produce clear and convincing proof of the need. Quite the contrary, these people cannot produce ANY evidence to support their claims. They are not gunsmiths, have no official credentials in working on HK type rifles, cannot cite any studies or examples of ground bolts causing blow-ups, nor can they quote any authoritative firearms manuals to support their claims.

I will go even further and state that the ringleader of this coterie and his allies do not even understand how the roller-delayed blowback action works. They do not understand why bolt gap is required, do not understand the concept of headspace applicable here, do not understand the relationship between headspace and bolt gap, and basically are trying to give advice above their level of expertise and knowledge.

The problem becomes even move severe because the leader of this coterie of experts without portfolio owns one of the Cetme sites and is a moderator on the other. And he is very intolerant of dissent. If you disagree with him on this subject, you are threatened with expulsion or even "banned for life."

Since these forums do not allow full discussion of ground bolt issues, these questions must be addressed in more general firearms discussion groups. I am therefore asking members of this forum for their opinions and experiences relevant to the following questions:

Is it unsafe to fire a Cetme which has a ground bolt head but proper bolt gap? If so, why?

Thanks,

Drakejake

 

[rbernie]

Is it unsafe to fire a Cetme which has a ground bolt head but proper bolt gap? If so, why?

I've been told that the bolt gap accomplishes two things: it headspaces the cartridge and it interrelates with the timing for the unlocking delay. From what I can tell, the push-back that you're getting on other forums is based upon concerns over the latter issue. Even if the headspace is OK, it is postulated that the ground (shortened) bolt will unlock too early, causing a potentially dangerous condition whereby the bolt head loses support while high chamber pressures are still present.

I don't have a freakin' clue as to HOW the timing of the unlocking is affected by the presence/absence of a ground bolt. I have no dog in this fight. Your questions are reasonable, but as far as I can tell there is no way to definitively address them using words alone.

I believe that the only to resolve this would be to demo it dynamically. You would need two rifles; a control rifle with proper headspace and an unground bolt, and your 'ground bolt with proper headspace' CETME. You would first check the headspace on both to establish a baseline between your rifle and a 'proper' rifle. You would then need to check the timing on the unlocking in both rifles, to see what differences in unlocking timimg exist as a function of having a ground bolt.

Does this make sense?

A coterie of Cetme fanciers has claimed that you cannot use the ground bolt to get bolt gap and that you must "repress the barrel," basically reconstructing the rifle. In other words, according to these commentators, the only valid alternative to junking the rifle is spending several hundred dollars to have the rifle rebuit.

This is the crux of the issue. The position of these folks is that if the barrel sits too inboard (requiring a ground bolt to headspace properly) then the delay between unlocked-but-moving-back and fully-unlocked will be wrong.

 

[ceetee]

In theory, the rollers pressing into their recesses provide tension holding the bolthead in battery (along with the locking arm, etc). When you fire, the acton of the bolt coming out of battery is delayed by the time it takes for those rollers to retract into the bolthead, and the bolt to unlock from the trunnion. The bolt needs to be delayed coming out of battery so the pressure created by firing the round is contained by the chamber. If everything is in spec, the rollers apply just the right amount of pressure, the bolt unlocks at just the right time, and the system works slicker than a Swiss watch.

If the rollers wear down, they won't be fitting properly into the recesses into the trunnion. Because of this, there won't be enough tension holding the bolt in place, Because of this, the bolt will unlock too soon. If the bolt unlocks too soon, the case will begin to be extracted before the pressure inside has dropped to safe levels. If that happens, it's possible that the case will not be able to contain the pressure, and the brass will rupture. Not good. Not only that, but if the bolt starts coming out of battery too forcefully, it will cause undue wear on the rifle.

I've seen photos of a CETME where the rifle's recoil was so harsh that the welds around the takedown pin holes broke out. It's possible that damage was caused by an out-of-spec bolt group. I've also seen photos of KB'd CETMEs. Most of those were attributed to bad ammo. It's possible, though, that out-of-spec bolt groups helped those rifles KB. Personally, I don't know. My CETME has a ground bolt, but with +4 rollers (and adding on a pinch to make up for how long the bolt should have been) it seems to be in spec. I think this issue is a sore spot with the CETME forum guys because if Century had spent a tad more time getting the barrels pressed right in the first place this issue never would have happened. As it is, you should know what your bolt gap is, know what it's supposed to be, and if yours is questionable, at least be alert for signs that your bolt may be unlocking early.

 

[rbernie]

When you fire, the acton of the bolt coming out of battery is delayed by the time it takes for those rollers to retract into the bolthead, and the bolt to unlock from the trunnion.

My impression was that the rollers are ramped/pressed into the bolthead when they reach the end of their locking recess. Is this an accurate impression?

 

[ceetee]

The bolt is like...

Imagine a wedge inside a square tube. At the end of that tube are two slots. When the wedge presses forward, two rollers are pressed out (by the angles of the wedge) so that they extend out from those two slots. The rollers mate exactly into two recesses in the trunnion.

When you fire, the impulse presses against the bolt head, trying to force those two rollers out of their notches. They press against that wedge (the locking piece). As they force the wedge back out of the way, they retract themselves into the bolt head. Once they have retracted into the bolt head far enough, the bolt can start to move backward. The fired case is extracted, and as the bolt reaches it's most rearward position, the fired case is ejected.

The bolt is spring-driven back forward, stripping a new round out of the magazine. As the new round reaches the chamber, the wedge (the locking piece) continues forward, driving the rollers into the recesses of the trunnion. Once the wedge reaches it's most forward position, the rollers are as far into the trunnion as they can go, and the bolt is locked up.

There are more parts involved, but that's a simple version. The actual rate of fire is determined by things like angle of the ramps on the locking piece, how well the locking arm mates into it's recess, the size of the rollers, how well the rollers mate into the trunnion, etc. The most convenient way to measure the wear on all these parts is by building a small gap into the system. Once that gap has reached a pre-determined wear point, then new parts should be added to bring it back into spec.

The bolt gap itself does nothing except be an indicator of how worn all the other parts have become.

 

[rbernie]

As they force the wedge back out of the way, they retract themselves into the bolt head. Once they have retracted into the bolt head far enough, the bolt can start to move backward.

So there is ZERO rearward movement until the rollers retract?

I would have thought that SOME rearward movement would have to occur to allow the rollers to push the ramp out of the way.....

 

[jford1]

Ask an Expert!

Give investment grade firearms a call. They’d be able to answer your question and if needed fix up your rifle. I have a PTR-91 and understand how the action works but wouldn’t want to guess if what Century is doing is safe. If you have malfunctions Century will make it shoot again but probably not fix it the correct way.

www.investmentgradefirearms.com

 

[BigG]

I would guess that the recoil would start the locking piece moving (bouncing back since it can't move any further forward) and everything else would depend on the inertia of that initial shove.

 

[jklinstein]

Drakejake,

Did you ever respond to my comments on those other forums? I noted how a ground bolt head can allow the locking piece to rest against the bolt head instead of applying pressure to the rollers. No pressure on the rollers and there is no lock-up.

It is that simple - a ground bolt can make the rifle dangerous to the user. Will it go kaboom every time a bolt is ground? No. It just removes the ability to have confidence that the correct forces and lock-up are taking place.

I'm a degreed mechanical engineer with 13 years of experience in making mechanical systems. I modeled the CETME trunnion, bolt head, locking pieces, rollers, etc, in a CAD system to better understand the issue. When I saw the issue with the locking piece coming to rest against the bolt head instead of against the rollers I came to see exactly why grinding the bolt head can create a problem.

Jacob

 

[ceetee]

So there is ZERO rearward movement until the rollers retract?

I would have thought that SOME rearward movement would have to occur to allow the rollers to push the ramp out of the way.....

I'm not an engineer, nor did I stay at a Holiday Inn last night, but...

As far as I know, the only rearward movement upon firing is of the locking piece being pushed back by the rollers. The bolt head itself doesn't move back away from the chamber until the rollers have retreated back inside the bolt head enough to clear the recesses in the trunnion. Since the rollers are round, this wil happen on a curve, not like tripping a sear. That's how the delay is built in.

 

[Drakejake]

Jacob,

I examined my ground bolt head and locking piece and saw that there was no way that the locking piece could hit the inside of the locking piece. The locking piece was being stopped by the full extension of the rollers, not by anything else. You should keep in mind that the amount of grinding that has been done by Century is less than two hundredths of an inch. So long as the locking piece can go far enough into the bolt head to push the rollers all the way into the trunnion recesses, there has been no change in headspace or in timing, as far as I can tell. Some Cetme owners are confusing bolt gap with headspace and do not understand that grinding the bolt head has no effect on headspace or the functioning of the rifle, provided bolt gap is in spec. Provided there is adequate bolt gap, changes in headspace in these rifles involve wear at the other end--on the bolt face and the chamber face, as my research indicates.

____________________________

People have stated--and I think this is reasonable--that Century does not repress the barrels in their Cetmes and G3s. They remove the barrel and trunnion as a unit from the old receiver and then set it in the new receiver. Since headspace is determined by the relationship of the chamber to the trunnion recesses, Century does not change the headspace. Whatever headspace the old rifle had, is transferred exactly into the new receiver. Apparently some of these old rifles were out of spec due to wear and Century had to grind the bolt heads to produce an in spec bolt gap. It would be too expensive for Century to repress the barrels, and if they did, do you think they could do so with the requisite precision? I doubt it.

Drakejake

 

[jklinstein]

I examined my ground bolt head and locking piece and saw that there was no way that the locking piece could hit the inside of the locking piece. The locking piece was being stopped by the full extension of the rollers, not by anything else.
How did you inspect the gap between the end of the locking piece and the bolt head while the rollers are locked in the trunnion? But I only noted that grinding the bolt head makes improper contact a dangerous possibility, and you have not shown how this is untrue. In my CETME there was physical evidence of repeated contact between the locking piece and the bolt head.

You should keep in mind that the amount of grinding that has been done by Century is less than two hundredths of an inch. So long as the locking piece can go far enough into the bolt head to push the rollers all the way into the trunnion recesses, there has been no change in headspace or in timing, as far as I can tell.
Great. Mine was ground about .010 inches and that was enough for the locking piece to bottom out against the bolt head. How was it that you have come to be convinced that the rollers are FULLY supporting the locking piece?

Some Cetme owners are confusing bolt gap with headspace and do not understand that grinding the bolt head has no effect on headspace or the functioning of the rifle, provided bolt gap is in spec.
I recognize that this is you opinion. I disagree, as does everyone else I've talked to who has any clue about how a CETME or H&K rifle works.

Provided there is adequate bolt gap, changes in headspace in these rifles involve wear at the other end--on the bolt face and the chamber face, as my research indicates.
Great. Your thorough research of one aspect of the functioning of these rifles does not translate into understanding the critical issue of the locking piece bottoming out against the bolt head. This proves to me that you do not know what you don't know.

 

[Drakejake]

Hey, Jacob,

I answered your question. You do not like the answer. OK. I have cited several sources in support of my positions. I have not seen any responses. "Huntingguide" in particular exposed Perro's ignorance. If your locking piece is hitting the inside of your bolt head, something which I have never seen mentioned by anyone else, by all means replace that bolt head, or rebuild your rifle, if you think that is necessary.

Drakejake

 

[Drakejake]

Cetme "Experts"

Jacob, you refer to those people who have "any clue" about Cetmes. Who are those people, how do you know what they know, what is the basis of their knowledge? The fact that someone thinks he knows something and continually implies that he is an expert, does not fly with me. The fact that an individual such as Mike Perro has created a forum for discussing Cetmes and has posted hundreds of messages on the subject doesn't make him an authority. He is an electrician, not a gunsmith, as you know. He has not been trained at the H&K factory. To take another example, the well-known JFowl has just now stated once more that when you are determining proper bolt gap in a rifle that has a ground bolt head you must deduct the amount that has been ground from the bolt head from the actual bolt gap. In other words, he is claiming that the stated specs for bolt gap do not apply to rifles which have shorter than standard bolt heads. How does he know this? Where did he get this idea? Well, he got it from someone who MADE IT UP, but you take his statement as knowledge. This is nothing but the copying and repetition of totally baseless assertions. The fact that JFowl makes this statement without giving any reason or authority tells me HE doesn't understand the roller-delayed blowback action but for some reason wants others to accept his statements as gospel. Is JFowl a gunsmith? Has he been trained at the H&K or Cetme factories?

My authorities on this question are the various Cetme and HK manuals you can read on line. They tell us what the bolt gap should be. They don't say anything about measuring the bolt head or what you should do if the bolt head is shorter than standard. You will not find a statement in any of the HK or Cetme manuals that if the bolt head has been ground, the bolt gap should be increased to, say, .025 or .035 inches, instead of .004 to .020. When JFowl and others go beyond the manuals, they are simply making it up. If I am wrong, cite your authority.

Drakejake

 

[jklinstein]

My authorities on this question are the various Cetme and HK manuals you can read on line. They tell us what the bolt gap should be. They don't say anything about measuring the bolt head or what you should do if the bolt head is shorter than standard. You will not find a statement in any of the HK or Cetme manuals that if the bolt head has been ground...

So the manuals you're referring to have not, by your admission, addressed the issue of having a shorter than spec bolt head. I've never seen it mentioned in any automobile manual that having some salt (or sand, or Pepsi) in your car's oil is significant, so I guess it's OK... by your logic.

I'm not saying that the guys on the other forums have it figured out. I'm saying that you have not proven your case.

Jacob

 

[ceetee]

Not to repeat myself here, but I still think the main issue is the fact that that Century could've built these rifles correctly in the first place for just a few cents more each, but chose not to. Kind of like if they imported a whole bunch of surplus Mausers, and purposely swapped parts around so none would match. Done the right way, you could have had a rifle much more valuable, but by not performing due diligence, you have a rifle much less valuable.

Whether it's a safety issue or not... Like I said: As long as you, the shooter, are aware of the question, you can be alert to signs that the rifle may be functioning improperly, and hopefully you won't have an out-of-battery experience...

 

[Drakejake]

Jacob, I HAVE made my case over and over in a number of messages. But you have chosen to ignore my arguments. I am relying on the bolt gap specs stated in the official manuals issued by the manufacturers of these firearms. Purchasers of these firearms are entitled to rely on these official statements. Those of you who are claiming that the manuals are wrong or incomplete have the burden of proof in this issue. By the way, the claim that ground bolts are unsafe and never should be used contradicts assertions that the amount that was ground off must be added to the bolt gap specs. If you should never use a ground bolt, you do not have to revise the specs. Which of these conflicting positions do YOU take?

You have failed to bring forward any relevant evidence, authorities, or well-founded arguments to support your "experts." You are merely repeating the unsupported claims of others. Jacob, I think your mind is closed on these questions and so I will not reply to your posts any more. Open-minded readers can study all of the messages and make up their own minds.

Drakejake

 

[Ian Sean]

Which of these conflicting positions do YOU take?

I have a CETME, it came with a ground bolt, it looked like someone held it on a bench grinder.

I have since purchased an unground bolt and installed oversize locking rollers. I myself tend to agree that Century screwed up and there may be an issue and I am not taking any chance with a 50,000 psi controlled explosion 1 foot from my head.

Now, with all he conflicting data and opinions on this technical problem...who knows...but bolts WERE all made a certain size to be in spec. The only true solution is to contact HK and see if they will weigh in on this issue themselves.

One thing I take from Drakejake's comments, If I am reading it correctly that ground bolts are OK, then as my rifle wears, and my gap decreases, I can continue to grind my bolt smaller and smaller to keep a proper bolt gap? Or am I missing something?

 

[ceetee]

If I am reading it correctly that ground bolts are OK, then as my rifle wears, and my gap decreases, I can continue to grind my bolt smaller and smaller to keep a proper bolt gap? Or am I missing something?

Fundamentally...


Remember: The "headspace" (for lack of a better term) is determined by the way the locking piece presses against the rollers, and the way the rollers fit into the trunnion. When these parts wear out, they have to be replaced. If the trunnion or locking piece were to never wear, you would just have to get new rollers of the same size (8.00 mm).

Since the trunnion and locking piece DO wear over time, you have to get over-sized rollers (+.02 or +.04 mm) to compensate. As your parts wear, the bolt gap decreases. The ONLY thing the bolt gap does is show you when your parts are worn and need attention. Just grinding the bolt more would be like riding on your rims after your tires have gone flat.

 

[davek]

Those of you who are claiming that the manuals are wrong or incomplete have the burden of proof in this issue.

I don't think anyone is suggesting that the manuals are wrong or incomplete because they don't say not to grind the bolt. Isn't it possible that it never occurred to the designers that anyone would try to grind the bolt, and that's why they didn't specifically say not to do it?

Let me ask you this. Does it say anywhere in CETME or HK manuals that grinding the bolt in order to get the bolt gap right is acceptable?

 

[Drakejake]

Century has claimed that a Spanish armorer's manual states that using a shorter bolt is an acceptable way to get proper bolt gap in the Cetme, but they have not produced proof of this.

We should remember that the Century Cetme and G3 Sporters are not the same rifle as the originals. The originals, made in Spain and Germany, were new military rifles produced to exacting standards. The Century rifles are in effect reproductions assembled from used parts and replacement receivers of dubious quality. There are hundreds of reports that Century sometimes does a horrible job of assembling their rifles. Many of them do not work properly, but, even so, I have seen no evidence that these rifles are dangerous or prone to blow-up. I do not think a company could stay in business selling hundreds or thousands of time bombs to the public.

Several people who really know these rifles (not shooters who like to tinker) have pointed out that some of the statements which apply to the original rifles do not necessarily apply to the sloppily assembled Frankenrifles put out by Century. Of course, Cetme and HK would not grind bolt heads to get head space. And of course there is nothing in their manuals about doing so. They would more likely destroy these worn out rifles or repress the barrels. Century was buying these worn military surplus rifles, putting in American parts, and making them compliant with American gun laws. One may assume that Century could not replace trunnions, repress barrels, etc., and still put out a product that sold wholesale for a couple of hundred dollars.

But the fact that Century ground less than .02 inches from some bolt heads does not imply any of the things that the amateur experts have made up:

that it is unsafe to shoot a rifle with a ground bolt,

that bolt gap with a ground bolt is "false,"

that you must have a larger than spec bolt gap to compensate for a ground bolt.

My point is that until these proponents produce some evidence or convincing logical arguments, Century rifle owners are reasonable in ignoring their statements and in enjoying their rifles. At any rate, that is what I am doing. Others can and should decide for themselves.

Drakejake

 

[jklinstein]

One person says:
One thing I take from Drakejake's comments, If I am reading it correctly that ground bolts are OK, then as my rifle wears, and my gap decreases, I can continue to grind my bolt smaller and smaller to keep a proper bolt gap? Or am I missing something?
Another one says:
Of course, Cetme and HK would not grind bolt heads to get head space. And of course there is nothing in their manuals about doing so. They would more likely destroy these worn out rifles or repress the barrels.

 

[jklinstein]

You have failed to bring forward any relevant evidence, authorities, or well-founded arguments to support your "experts." You are merely repeating the unsupported claims of others. Jacob, I think your mind is closed on these questions and so I will not reply to your posts any more. Open-minded readers can study all of the messages and make up their own minds.

DJ, I admit that I've stated that you alone are the only person I've heard who states definitively that grinding a bolt head can't cause a problem. That's as far as I've come to "repeating the unsupported claims of others" - I've pointed out that you're the only person I've come across who argues your position. So let's not bring "others" into the discussion.

On the other hand, you truly have chosen to ignore the issue I've presented. The locking piece possibly bottoming out on the bolt head is something I've never heard anyone else discuss. Your counter argument has consistently been either "prove it from an authoritative souce" (or the tried & true "you're just repeating others"). Before I sold it I had evidence of this problem. I'm not saying it will happen to every rifle with a gound bolt... but it happened to mine which had only (IIRC) 0.010 removed.

But like others have noted, you seem to be suggesting that one need only grind their bolt a little more if their bolt gap is low and everything is now okee-dokee. I've never found that removing material from my guage to be a good way to get a repeatable measurement.

jacob

 

[jklinstein]

But the fact that Century ground less than .02 inches from some bolt heads does not imply any of the things that the amateur experts have made up:

that it is unsafe to shoot a rifle with a ground bolt,

that bolt gap with a ground bolt is "false,"

that you must have a larger than spec bolt gap to compensate for a ground bolt.

So these are the big blunders you're arguing against.

How small is too small? in a CETME bolt head? in brake rotor thickness? If I ground your brake rotors some handy amount below spec. would you be confident that your brakes were "safe"?

The use of the word "false" for bolt gap with a ground bolt is not an indication that the measured gap is not truly there. It is meant to indicate that you're not getting the reading that an unmolested gauge (i.e. bolt head) would provide.

The last point follows on the previous.

jk

 

[Drakejake]

(This is not a reply to any prior message but another comment on the topic.)

Beware, those of you who own Cetmes or interested in them. A number of the people who are constantly lecturing others on these rifles do not understand them.

Example: Those who state that with the Cetme you measure headspace by measuring bolt gap. This is untrue. Headspace and bolt gap are two different things and largely independent of each other. I have already cited sources on this.

Example: Those who have repeatedly asserted that measuring bolt gap is the way that you measure wear on rollers, locking pieces, the trunnion recesses, etc. This statement reverses the true relationship between bolt gap and worn parts. You replace worn parts to get proper bolt gap. You do not measure bolt gap to see if parts are worn. The function of bolt gap is to insure that the locking piece can fully lock up the bolt head (put rifle into battery). If you have proper bolt gap, the locking piece will do its work. You measure bolt gap to make sure that the rifle will work properly, not to determine if parts need to be replaced. If bolt gap is out of spec, then you may replace parts to get it into spec (.004 to .020 inches). If bolt gap is out of spec, it doesn't tell you whether the rollers are worn or unworn, whether they need to be bigger or smaller. To determine these things, you LOOK AT THE ROLLERS AND MEASURE THEM. If your bolt gap is too small, you do not from that fact know the cause. You could have an unworn locking piece but badly worn rollers. You could have unused rollers and locking piece, but substantial wear in the trunnion recesses.

Example: Those who are constantly warning others of Cetme/G3 rifles which are unsafe. As others have pointed out, these rifles are designed to work and be safe even when worn or out of spec. Most of the people who are repeatedly talking about Cetme safety have no idea what they are talking about. Cetme blow-ups seem to be rare and usually related to overly hot ammo, bad ammo, or obstructed barrels, not to anything about the condition of the rifle itself. For example, the use of Indian ammo or commercial .308 instead of 7.62 NATO seems to be involved in most blow-ups or case separations in these rifles. If I am wrong about this, please cite some examples or evidence.

Example: Those who claim that ground or worn bolt heads give a "false" reading of bolt gap. There is no such thing as a "false" bolt gap. Bolt gap is bolt gap. PERIOD. If your bolt gap is within the proper limits, you are set, regardless of whether you have a shorter than standard bolt head. The only exception I can think of is if the bolt carrier is too long and is hitting against the cocking piece, thus pushing the cocking piece back. Assume, for example, that your bolt gap is too large. Does that tell you that rollers are worn? No. An excessively large bolt gap could be caused by the bolt carrier hitting the cocking piece. There are a number of discussions about this problem and what to do about it.

Drakejake