Cetme Barrel Depth Question

[Shovelheadfxwg]

I'm new to building Cetmes, but I've built a dozen or so AK kits. What I have is a Cetme kit from Centerfire Systems. It came with the new barrel, which is already drilled where the pin goes through the trunnion. I pressed the barrel in the trunnion until the drilled groove lined up with trunnion hole. and the barrel seems to be below the trunnion at the breach face. I check the bolt gap, and it is about .009 inch. Is this acceptable? It looked a little strange to me that the back of the barrel wasn't flush with top of the bore in the trunnion. I was surprised when the bolt gap was so close.

[jfowl31]

The barrels that came new with the Centerfire kits are cut in the wrong place... thats why they came with the kit for free. Rotate the barrel 180 degrees, and press it in from the back checking the gap until it drops to .020" and redrill and pin it there.

I dont know how you are getting .009" of gap at that cut spot because the pictures Ive seen of them, they are something like 1/8" off where they are supposed to be. Maybe your trunnion is tight up that far or something, but if you press it from the rear and check the gap often, you shouldnt have problems.

[Smokehouse69]

The barrels that came new with the Centerfire kits are cut in the wrong place... thats why they came with the kit for free.....
... they are something like 1/8" off where they are supposed to be.

No sheit? I remember Perro commenting on turning the barrel 180 and redrilling it, but I didn't know he was talking about the Centerfire kits... Seems like they should have said something about that, the bastiches!!!:1106:

[jfowl31]

well.... thats the pictures Ive seen at least. pull out the old barrel and compare it I guess would be the easiest way to tell, but Im purty sure that they are a bust for using the pre-drilled spot.

[Shovelheadfxwg]

Thanks for the info, jfowl. I'll take a measurement before I remove the barrel and again after I get it set correctly. I'll let you guys know how far off the mark it is. I wonder if the Century kits with new barrels are the same way?

[Shovelheadfxwg]

A couple of more questions. Are you guys drilling these with a high speed drill bit, on a drill press? Are you drilling with the correct size drill bit, or drilling undersize and reaming, or filing it to fit? I noticed that several threads say press the barrel in from the rear, is there a reason for not pressing from the front? I also noticed that everyone seems to say weld the trunnion before pressing the barrel, is there a reason for doing this as opposed to pressing the barrel first? Sorry for the interrogation, just trying to learn the do's and don'ts from the ones that have already done it.

[Big Steve]

When your trunnion is already drilled and you try to drill through the trunnion hole and into the barrel. The drill bit will want to "walk" towards the outside of the barrel because you are trying to drill into a 45 degree surface. This will egg shape your trunnion hole and the bit will not drill a straight hole. The only correct way to do it is to start the hole with a 3/16 center cutting end mill cutter. That way your drill bit has a flat surface to start on. You may get away with using one in a drill press, but a Bridgeport type mill is best. Measure your pin and order a chucking reamer .0005 smaller than your pin. Mc Master Carr has them in 1/2 thou. increments. I think a #9 is close but it depends on your pin as I have seen them differ in size. You don't have to weld it first as long as you slip it into the receiver when you put it all together to check the gap. That keeps the bolt carrier straight. Take the cocking handle and support out so it does not give you a false reading.
Steve :machinegun:

[Seattlefungus]

Steve, every try removing the extractor, putting in a go gauge, then locking the bolt in while pressing the barrel? Just curious.

[rustypirate]

Steve, every try removing the extractor, putting in a go gauge, then locking the bolt in while pressing the barrel? Just curious.


DANGER WILL ROBINSON DANGER!

This is not the factory recomended method for correctly judging the barrel depth and you will NOT want to follow this method.

[bullseye]

From what I remember if the barrel was already drilled(not by you) you will want to rotate it 180 degrees and re-drill. You do not want to re-use the original hole.

[Perro]

1/8 inch off?? no way dude - not even close - he couldnt get .009 gap with it 1/8 off. ive pressed one of my barrels in, and the slot cut into it gave me a .014 true gap on one of my rifles - each gun is different cause each trunnion and set of parts are worn differently, so it goes to different spots on different guns. Im not about to say ALL of them are good to go, but i can tell you the one ive done so far has been spot on perfect. If it had 1/8 inch gap, there would be zero bolt gap - none whatsoever

the barrel is supposed to be slightly recessed from the trunnion. it even is on a factory built mars cetme, or HK91

the reason you flip the barrel over 180 degrees is so you can set bolt gap, and no other reason.
put the barrel in there with the groove facing up
press it in to gap of choice
drill it
drive the pin in

thats it


big steve is correct, you need to mill a flat into the barrel first, or your drill bit will want to walk on you. You can flip the thing around and drill it halfway from each side, and then finish ream to make it even more accurate of a hole.

that hole does not need to be dead nuts accurate though. When you shoot the gun the barrel wants to move forward. When the thing settles in after the first shooting session, the barrel moves forward until it comes to complete rest against that pin, and then its locked in there - it aint moving after that. Your only concern is that the pin hole not be too lose so that the pin works itself out of the hole

a regular 5mm drill bit works just dandy at doing this - might want to get a carbide drill to do it though.

[jfowl31]

http://www.gunboards.com/forums/uploaded/drine/2007228112930_DSC00918.JPG

[jfowl31]

I had seen a couple or few pics that looked like that... thats why I assumed the new cetme kits that come with the free barrel are drilled incorrectly.

I figured that the guy who pressed it in still way recessed, and got .009 was somehow measuring the gap incorrectly or something after seeing that picture.

maybe most of them are closer than that, and that one is a major exception.

[Perro]

A couple of more questions. Are you guys drilling these with a high speed drill bit, on a drill press?
carbide is nice



Are you drilling with the correct size drill bit, or drilling undersize and reaming, or filing it to fit?
5mm


I noticed that several threads say press the barrel in from the rear, is there a reason for not pressing from the front?
cause the barrel is tapered which allows it to self align itself into the trunnion. pressing the barrel in from the front would require a special jig to be made - pressing it in from the back requires a mandrel of some sort, and a press plate - much easier to do it from the back - its possible to do it from the front, but easier to do it from the back cause of the taper on the barrel.



I also noticed that everyone seems to say weld the trunnion before pressing the barrel, is there a reason for doing this as opposed to pressing the barrel first?
thats just the way they do it at the factory. For years, you could get a brand new FMP receiver with the trunnion already installed to the receiver. it really doesnt matter how you do it though - it obviously can be done out of the receiver. One other thing - it will be easier to check gap with it already installed into the receiver. You can check gap with a loose barreled trunnion, but then you have the problem of getting the bolt set unlocked from the trunnion (screwdriver and pry in the area you check gap)

[Perro]

well, like i said, i cant say all of them are good to go
the one i bought off a guy on the cetme forum (forgive me, cant remember who i got it from) gapped naturally at .014 on the hole that was already in it - i had to chase the hole with a drill bit, but it gapped perfectly (or well enough anyway)

WHICH BARREL IN THAT PHOTO IS THE NEW BARREL?

[Perro]

the barrel on top with the black paint on it has the flaming bomb - can i assume that that barrel is the new one?? if thats the case, then that barrel wouldnt be recessed as much as the one on bottom

but the bottom barrel looks incorrect for sure - mine wasnt that centered.

ill have to pull my parts kits out i have stashed to look at the other barrels

[drine]

The drill rod is a permanent piece of that barrel now. I used that to press it out. The bottom is one from a partial kit I bought. New old stock. Any how, I ruined it during the press so both are crapola now.
My understanding from the guy I bought it from is the same as the rumor here. They slotted them wrong so they give them away in kits now.
I have a new G3 barrel coming now from Numrich. I haven't heard any reviews of that barrel but hopefully it will function good. :thumbup:

[jfowl31]

That is Drine's picture, so perhaps he can enlighten us as to which barrel is what. I thought the one on the bottom was the new one, since it hasnt been painted yet. And the fact that the pin slot doesnt look like its ever had a pin through it.

I think (just speculation) that normally the pre-drilled barrels are fairly good to go like you said Perro... but these new kits with the free barrel have good reason for the barrel to be free... they are drilled wrong.

Everything about that would make sense except that if they are all drilled that far off, one shouldnt be getting a reading of .009" with it that far off... thats why I was thinking perhaps the trunniong starts to tighten up if its that far off, and the bolt gap starts to rise... As in maybe (I havent inspected an unistalled trunnion, so this is just a wild guess on my part) the recesses in the trunnion start to taper back out after 1/8" or so, so the bolt gap would start to rise back up since the rollers cant press out very far. Follow me?

Could be completely wrong, and the new barrels could be good and that one wa sjust a fluke, but Ive heard several reports of the new barrels in the Centerfire kits being the same way from a few different members.

[jfowl31]

well he beat me to the post...

[Perro]

make your mandrel out of a piece of 1 inch aluminum rod - turn the end down to fit inside of the chamber about 1/2 inch
the part that fits inside of the chamber shouldnt be load bearing in anyway, it is only meant to keep the mandrel from cocking side to side.

ill get you pictures of mine, but i have to dig it out, its buried somewhere. until then, see the attached image to get an understanding of what it looks like

why is your new barrel no good?? If its just cause of that pin hole, flip the barrel around 180 and redrill into fresh meat - just cause that slot is cut incorrectly, doesnt mean the barrel is ruined - or maybe its ruined because of something else im unaware of??

the barrel i bought in the white in the spanish vci paper with the spanish writing on it, pressed in and gapped perfectly - maybe the barrel i got isnt the same as the ones they are giving with the kits? That is a good possibility. I have 2 of the centerfire kits in storage, ill have to dig them out to check those barrels - i didnt look much when i got them, i just packed them with the heat sealer, and away to storage they went.

[Perro]

try this pic instead, the software resized it

http://www.cetmerifles.com/mandrel.jpg

[Shovelheadfxwg]

Update
I rotated the barrel 180 degrees and pressed to the proper depth. It appears that the measure difference is .119 inch. Jfowl hit it pretty close when he said 1/8" off. I'll run a 3/16 end mill in the trunnion hole and spot it before I drill it. I measured the pin and it measures .196 inch. I thought about drilling it with a 3/16 drill bit and filing the last .009 or so out with a round needle file.

[drine]

Sharp edges left from years of use I suppose. When I pressed this barrel, slot side up, I missed the gap first go round. The thing was gauled severely and lots of metal shavings were found. After another try, it started getting loose around 25 gap so I figured I messed up. I used an arbor press so not much over force on my part. Now the "new" barrel can be pushed in by hand. Mic at @21.96mm so I've set it aside. Maybe it can be salvaged but I opted for another new barrel and count it as a harsh lesson I should have already known.
BTW, the barrel came from gunboards with the front half of an old receiver/trunnion, and a triple tree, FH,etc. All of it had been sprayed black, the barrel rerolled in the Spanish wrapper.

Not meaning to hijack your thread Shovelheadfxwg.

[Perro]

the barrel pin is 5mm just as a FYI

[jfowl31]

It would be nice to get a 100% confirmation that the predrilled slot on the Centerfire "in the white" barrel is no good so we can sticky it in the build section. It seems to be more and more aparent, and really makes sense... NOBODY like Century, Centerfire, and other big guns will give something like that away for free when they could charge a premium.

Glad to hear youre getting it sorted out shovelhead

Any idea why you were getting a .009" reading when it was so far off? Was it hitting the front side of the indention?

[Perro]

ahhh i see - i didnt know if you thought that since the slot was cut bad, that it was ruined, or what.

[Shovelheadfxwg]

Any idea why you were getting a .009" reading when it was so far off? Was it hitting the front side of the indention?

The best that I could tell, the rollers had passed the widest part of the trunnion and were trying to lock against the front side.

Thanks to everyone for their input !!!
I'm feeling alot better about it now. Actually I'll feel alot better when the hole is drilled and the pin is in. I wish I would have gotten in on the Cetmes when the kits were alot better and had the original barrels, but I was addicted to AK kits at the time. Actually I'm still addicted to the AK's. I do have another Cetme kit on the way though.

[jfowl31]

take some pictures of the intermediate phases as those are always fun to look at. What receiver are you using, and what finish will you be going with.

The next thing to consider and be careful about is making sure your cocking tube is gapped right.

[Shovelheadfxwg]

take some pictures of the intermediate phases as those are always fun to look at. What receiver are you using, and what finish will you be going with.

I'm using a stamped Century receiver, with new wood from Cope's on this one. I have a cast stainless G3 receiver from Century for the next one. I bought a G3 rear sight from RTG and have green G3 furniture to put on it. I wanted to get a G3 kit but waited too long, I guess they are pretty much all gone now.
I'll probably finish both with Gun Kote, I've used this with good success on most of the AK builds.
What dimensions are supposed to be off on the stamped receivers from Century? So far every thing seems to be spot on, except that I had to grind the excessive weld beads down and file out one of the pin holes where the stock mounts. It had a little bit of weld on the inside of the pin sleeve.

[tomoshenko]

+1 to all on this thread. This is great information. I was going to use my new barrel as a "template" to start doing exporatory barrel pin hole drilling on my CAI but as I learned here, since the barrel looks like the bottom one, I'd be way off. There use to be a post that showed where to locate the hole on a
undrilled receiver. Anybody save that?

[pjm204]

Wish I had read this earlier. I pressed my barrel in, pinned it, welded the trunnion in, and then welded over the pin holes to conceal them. I now either have a ton of work or a ruined barrel/trunnion. I also didn't read about rounding the sharp edges so I bet my barrel will be loose if I can repress it. Any ideas as to what I should do? am I screwed? also can someone give me some good measurements as to where I should start drilling to get through the pin?

[jfowl31]

whats your BG pjm204?

I dont know for a FACT that all the barrels were bad, just seen a few pictures that showed some bad ones....... figured they were all the same, and thats why they were free. As perro stated, he's had barrels that were cut in the right spot (or close enough), so maybe yours was fine.

[jfowl31]

shovelhead, the spots that were bad on the stamped receivers are the mag well (normally), and the scope mounting lugs are normally off. Other than that, sometimes the receivers are too wide and need to be "squished" inwards on the sides, and are generally just too soft IMO. I think that if Century used some high-grade steel, perhaps things would be better... that and taking some time on the magwell area.

[pjm204]

the bolt gap seems okay, right around .020" but the barrel doesn't come to the end of the trunnions, the bolt isn't even touching the barrel. I'm so pissed at myself. Oh well, at the very worst I'll need to drill out my welds and replace the barrel and trunnion. Hopefully I can drill out the barrel pin. Anyone know a cheap source for barrel/trunnions?

[jfowl31]

at least you didnt go fire it and have it go Kablow instead of Boom!

Drine just bought a new barrel... theres a thread on it a little down the page.

Dont know about a source for Cetme Trunnions.

[mprtech]

My take on pressing the Cetme barrel is this, (I only know 'cause I did it twice).

The proper bolt gap distance is less that .010 of barrel movement. (I know 'cause I measured as I pressed). So in other words you go from unable to lock the bolt to bolt locked with no gap at all in less than .010 of barrel pressing movement. I hope that makes sense.

I noticed with my cetme and my cetme only do I say this applies for, is that if the barrel was pressed past flush with the end of the trunion I would achieve the same gap every time. This was due to the fact that the bolt head contacts the trunion if the barrel is pressed past flush. I did this at least three times while pressing, then having to press the barrel all the way out and start over.

My advice is to press the barrel to within and eighth inch of being flush with the trunion, then check gap. Most likely at an eighth inch the bolt will not lock. Then press a hair more and check gap. By a hair I mean one pop of the barrel. I noticed when I was pressing the load would increase until the barrel sort-of popped. Moving most times around .020. Which could take you right through the gap "window". So I does take a little touch and a lot of patience.

Hope this sheds some light.

[mprtech]

Forgot to mention, your bolt gap will drop .004 to .008 after 20 to 50 rounds. You need to build that into the pressing. So If you gap after pressing before shooting is .023, you should be perfect.