|First, let’s look at how the roller locked system works, and how it relates to headspace. Headspace is set by the distance from the front of the chamber to the recess in the bolt face. That has nothing (well, little) to do with bolt gap. Measuring headspace in these things is a tedious process, since the bolt can "adjust" itself to case length. But note that further below in the H&K manual, it does state bolt gap is an indicator of headspace.
Unlock timing of the roller-locked, blowback system has to do with how and where the rollers are in the trunnion recess. Since this is the only lock for the bolt, it is critical that the bolt unlocks correctly, both for operation and safety. Timing in the CETME is slightly different from the G3/HK91.
Here is a link to an in-depth description on the H&K roller locked, delayed blowback system (for you technical types):
As you can see, the only thing that locks the bolt in battery is the force of the rollers against the trunnion. Yes, there is a recoil spring. Its only function is to slow down the bolt, and return it forward after firing.
The only simple indicator of bolt position and parts wear, as recommended by HK, is bolt gap.
The Cetme/G3 manuals say that bolt gap should be .004 to .020. This distance is between the rear of the bolt head, and the front of the bolt carrier. Note that the locking piece is different in the CETME and HK, as is the bolt head, and cannot be interchanged. The rollers do interchange.
Bolt head OAL (overall length) on a CETME is 1.835", on a HK it is 1.845". Use this measurement to determine if your bolt is ground or worn. The rear of the bolt head should also have a bevel. Many ground bolts have sharp corners on the rear of the bolt; the grinding has removed the bevel.
Let’s step back and take a look of the operation of the bolt, and how it is locked into the trunnion. Unlike most weapons, which use a lug system, the CETME/HK have rollers, which “pop” out into recesses in the trunnion, as the bolt slams forward into battery.
Here is a diagram of the CETME chamber:
(Just to save confusion, in this diagram the trunnion is labeled "locking piece", and the locking piece is labeled "firing pin holder". This was from a Spanish manual and translated, so I apologize for any confusion. Locking piece is the correct name for the "firing pin holder")
This is in the locked, ready to fire position. Note the relationship of the various parts. The face of the bolt is in contact with the chamber end of the barrel, the rollers are forced out into the trunnion, by the locking piece (arrow shaped part inside the bolt, with firing pin running thru it). The locking piece is pressed forward by the carrier. Note the shell in the chamber, and the recess on the bolt face. Wear on the bolt face or rear of barrel could affect headspace, and would be indicated by a decrease in bolt gap. But the gap does not measure headspace, only wear. Wear in other parts is also indicated by changes in the gap.
Remember that: bolt gap is an indicator of wear, assuming proper dimensions of bolt parts.
In an in-spec weapon, there is a gap between the rear of the bolt head and the bolt carrier, when the bolt is closed and locked, and the trigger is dropped.
Now, take at look at this diagram (this is an HK, and has some slightly different angles in the trunnion recesses). This shows how the relationship of the rollers, LP and trunnion change as the barrel moves back during pressing, or the LP moves forward as parts wear. You can see how the forces and angles change, as well as the timing of the unlocking phase.
As parts wear, the locking piece will go farther forward inside the bolt head, and the bolt gap will decrease. The gap decreases because the LP is held in the bolt carrier, so as the LP moves forward, it takes the carrier forward with it, closing the space between it and the rear of the bolt head. The gap is nothing more than a way to measure the wear of the critical parts in your weapons bolt locking system. This is critical, as it is the ONLY thing that contains the 50,000 PSI chamber pressure when you fire.
Stop and think about that. We are talking .001 changes affecting the entire system. The ratio of the CETME is 1:4; in other words for each change in the face of the barrel or bolt face, the rollers and angles change the gap by four. For example, press the barrel back .001, you gain .004 in gap.
At some point, the carrier will bottom on the rear of the bolt head. This is zero gap. If the weapon is fired past that, the rollers will start to get loose in their recesses, since the carrier can no longer push the locking piece far enough forward to press the rollers out correctly. This is an extremely unsafe condition, as there is no positive lock to contain chamber pressures. On a blowback only weapon (like a Thompson SMG) the bolt is made large and heavy to tame the recoil forces. On the CETME/HK, it can launch the lighter bolt assembly back with extreme force.
If the bolt lock timing is off, either way, you can have case separations, excessive recoil, and extraction failures. In extreme cases it can cause catastrophic failure to the weapon.
Here is a video of a cut way bolt, showing it’s operation:
It is interesting to note that in the H&K manual, in the section on how to check bolt gap, it mentions that correct gap assures proper headspace:
Now, would a gap derived from a ground bolt give you assurance that your head space was correct?
The H&K manual goes on to recommend optimum gap, as well as warn about malfunctions from improper gap (notice the wording):
So if you grind the back of the bolt to provide a false indicator of wear, what do you have? A gun that maybe works, or maybe doesn't. It is also a gun that is in a condition that CETME and HK would never recommend.
But, why does a ground bolt induce a false gap? The Century builders ground the rear of the bolt, to give an acceptable measured gap. Why would they need to do this? Because the rifles were assembled from used parts, and had an unacceptable bolt gap when done. This was due either to worn trunnions, improperly pressed barrels, or excessively worn parts that were used to assemble the guns. Study the above diagrams for a few minutes, and think about that. While you are doing that, consider all the wear points we are dealing with: Barrel chamber end, bolt face, trunnion, locking piece, rollers,
Now, ask yourself what was changed in the system when the back of the bolt was ground? What critical wear points were addressed and fixed by grinding the rear of the bolt head? Look at the wear points that are indicated in the H&K manual on the page above. What was addressed by grinding down the rear of the bolt?
The answer is: nothing. Absolutely nothing in the locking system was changed. The critical relationships of the parts in the locking system are exactly the same as they were, before the bolt was ground. The fact that there was little or no gap prior to the bolt grinding, was an indicator that something was wrong in the bolt locking system of the rifle.
There is one case in which a ground bolt could restore function to an out of spec weapon. If you were at zero gap, and the rollers were loose in the trunnion, a ground bolt could restore enough gap to allow the locking piece to press the rollers out far enough to lock. This is not the correct way to repair, in this instance.
Further, in an extreme case, with a ground bolt it could get bad enough that the front of the locking piece bottoms on the bolt head, and the bolt is not locked. The rollers would be loose, and you would have a “good to go” gap reading. Granted, you would have to grind a bunch to reach that point, but why take the chance?
By grinding the back of the bolt, you are gaining a false reading, and a false sense of security. Since the only way to measure wear in the bolt system is to measure gap, and Century knew that, they gave you an apparent “good” gap.
Now, what can we do to verify what the actual gap is? Basically there are two options:
Install a known, good OAL bolt head. Measure gap.
Measure your ground bolt, and subtract the difference between your bolt and a new one, from the measured gap. Why would this work? Because in order to get a factual measurement of your gap, you have to take into account the material removed from the rear of the bolt. The gap measurement is based on having correctly dimensioned parts. Without taking into account the material removed from the rear of the bolt, you are not getting a true indication of the current condition of the rifle. Can I repeat that enough?
“So now I find my ground bolt rifle has little or no bolt gap, how can I fix it?”
According to the HK manual, you can replace the bolt head, locking piece and rollers. Larger rollers reduce the distance the LP has to travel; they reach the sides of the trunnion cut-outs sooner. A new LP serves the same purpose, reducing the space between the rollers and trunnion. The bolt head can wear on the face, or the inside of the windows for the rollers.
If that doesn’t get you into spec, you are looking at pressing the barrel and/or replacing the trunnion. The barrel press is not mentioned in the manual, but you have to remember we are dealing with used parts, that are reassembled into working rifles. And finding a new trunnion will be difficult at best.
“But my CETME with a ground bolt and “good” gap runs fine, cycles like it should, why spend more money on it??"
That answer is up to you. If you are comfortable with a weapon that is out of spec, (according to CETME and HK), and aren’t worried about 50,000 psi (or more with commercial loads) next to your face that may or may not be contained, then by all means shoot to your hearts content. The purpose of this is to explain why grinding a bolt does not “fix” your rifle in any way, shape, or form.
Here is the bottom line.
If you have a new CETME or HK, assembled correctly with new parts, and are starting with "x amount" of bolt gap, monitoring that gap can tell you when something changes, or wears. Then you can get in there and see what has changed. It is nothing more than a "health" indicator for your rifles critical operating parts.
Starting with a ground bolt, your entire frame of reference is skewed from the get go.