Mars Cetme bolt gap [Archive]

mojojojo

02-21-2008, 02:27 PM

Does anybody know what bolt gap settings the original Mars import Cetmes were set up with? I have 2 examples and their bolt gaps measure
.08" and .12". They're not near the max setting, but all the internals on both guns look near new.

I know that HK specs are anywhere between .04" to .2", but I was wondering if anyone here knows for sure where Santa Barbara timed their Spanish beauties out at?

Thanks!

rustypirate

02-21-2008, 02:46 PM

Mojojojo,

As far as I know there is no documentation on bolt gap settings for the CETME available.

The spec settings for HK is between.004" and .020", I think you missed a 0 in your measurements above.

mojojojo

02-21-2008, 03:00 PM

You're right... my bad.

rustypirate

02-21-2008, 03:07 PM

No problem, just don't want to confuse any newbies.

SteelCore

02-21-2008, 03:45 PM

the .008 is a bit on the low side, it may shrink after 100-300 rds thru it.

The .012 should be low-median, may shrink to .011 or .010 after 100-300 rds.

Since it is a mars, it may be made with all matching parts, and the bolt gap may not drop as fast as the CAI 'parts gun' CETMEs.

We use the HK bolt gap specs for the CETMEs.

If you're uncomfortable with the bolt gaps, get a set of +2 and +4 rollers from Robert@RTG. They'll be handy in the future if you do shoot these.

It has always been a Daydream of mine to get a MARS from someon who doesn;t know what it is worth, and they charge me normal CAI CETME price ...

Hey... A guy can dream, rite?

jfowl31

02-21-2008, 06:48 PM

SteelCore

02-22-2008, 11:35 AM

100-300 rounds shrinking 2 thousandths??? Ive put over a thousand rounds through mine and it hasnt moved a bit!

-->Well, I guess you're just lucky that way. When I got my CETME, it was gapping at .011, and went from that to .010 in about 200 rds. Dropped to .009 in another 400rds or so.

Now I've replace bolt heads, and it gap .017, and that must be the sweet spot for this CETME, because it it lie shooting an AK now. It used to take .75 sec for the thing to settle back on target, now, there is no muzzle rise at all.

I've got over a thou rds thru my CETME since 2002.

jfowl31

02-22-2008, 04:55 PM

Not saying you're lying or anything, but if these are military rifles (which they originally were), we're talking about these rifle going from top performing brand new to out of commission/out of spec in a mere few thousand rounds. I would bet that CAI did a sloppy pin job or something to let it "settle" that much.

rustypirate

02-22-2008, 10:49 PM

No, they just did not use matching parts.

When the rifles were new, they also had a "break-in" period where the bolt gap dropped slightly, then settled in.

After many thousands of rounds of use, the bolt gap gradually dropped more, but not quickly.

The problem with the Century rifles is that they are putting bolt groups into trunions that they did not "break-in" to, so the rifle goes through yet another break-in period where the bolt gap drops until the moving parts settle, then the bolt gap stabilizes, and you are good for many thousands of rounds down the barrel.

tomoshenko

02-23-2008, 07:45 AM

[QUOTE=SteelCore;63485]the .008 is a bit on the low side, it may shrink after 100-300 rds thru it.

The .012 should be low-median, may shrink to .011 or .010 after 100-300 rds.
[QUOTE]

100-300 rounds shrinking 2 thousandths??? Ive put over a thousand rounds through mine and it hasnt moved a bit!

I've got y gap set to .014 wth +2's and it runs smoother than it did at .018 at +4s and .008 with the regulars. My "experiment" tells me that 10-15 is the "sweetspot" but YMMV as always.

I'm always having a hard time conceptulizing why a low range(.004-.006) bolt gap gives more recoil. My mind wants to believe that it unlocks farther down the pressure curve when pressure drops more and therefore sends back the bolt with less violence. I thought the main disadvantage of a low range bolt gap was incomplete cycling due to the less crisp bolt action. This would be a real failing for a battle rifle that has a FA setting. Maybe someone can explain this better to me. What would be really cool is if someone could empiricaly gather data from a rifle shot over time and the life of the bolt gap in a mechanical recoil measuring device eliminating human error.

I am willing to attempt such a project if the fine gentlemen of this forum are willing to donate their extra battlepacks to me for the sake of science.
Hey guys..where did you go?..anbody there???

jfowl31

02-23-2008, 03:59 PM

I'm always having a hard time conceptulizing why a low range(.004-.006) bolt gap gives more recoil. My mind wants to believe that it unlocks farther down the pressure curve when pressure drops more and therefore sends back the bolt with less violence. I thought the main disadvantage of a low range bolt gap was incomplete cycling due to the less crisp bolt action. This would be a real failing for a battle rifle that has a FA setting. Maybe someone can explain this better to me. What would be really cool is if someone could empiricaly gather data from a rifle shot over time and the life of the bolt gap in a mechanical recoil measuring device eliminating human error.

I am willing to attempt such a project if the fine gentlemen of this forum are willing to donate their extra battlepacks to me for the sake of science.
Hey guys..where did you go?..anbody there???

I didn't notice any significant difference in recoil at any of the different gap settings. I've always speculated like you are saying though that recoil "should" be lower actually with a lower gap, but maybe its the recoil of the actual "shot" that is felt more, and not the recoil of the bolt. With it taking longer to unlock, more of the shock of the round being fired is absorbed by your shoulder before the carrier and bolt come flying back.

But like I said, I'm not recoil shy, and have not noticed a difference in recoil at any of the mentioned gaps.