View Full Version : Cetme Bolt Gap!!!!????
05-01-2007, 06:04 PM
I pressed my barrel in today, I've got it to where it is just about flush. The bolt gap is huge, about 1.2mm. Should I continue to press the barrel further in, most likely past the "shelf" or should I tap it back in a bit. I know that the closer the barrel is to the back of the rifle, the larger the gap should be. I just felt like my gap should be smaller when my barrel is this far in. Any input would be great, I'd like to wrap this project up.
05-01-2007, 06:24 PM
What do the headspace gauges indicate?
05-01-2007, 06:50 PM
yea.....I don't have .308 headspace guages. everywhere I've looked on building Cetme/hk91 style rifles has said to rely on bolt gap and not use headspace guages.
05-01-2007, 08:00 PM
I need help here badly guys, PLEASE!!
05-01-2007, 08:13 PM
You don't use headspace gauges, the bolt rests against the bbl when fully forward. Headspace is set by the depth of the chamber in the bbl and is not adjustable without major surgery. What bolt gap measures is the degree of roller engagement with the trunnion and is done on an empty chamber. Gap closes up with wear, so for a new build you want to set it to the max spec or close to it.
The gap between the rear of the bolt and the front of the trunnion should measure a max of .020 inches or .5mm. Moving the rear of the bbl closer to the bolt will increase gap, moving it farther away will decrease gap. Once you snap the bolt/carrier assembly into the trunnion to check gap, smack the back of the carrier where the hammer would fall with a small hammer, this will seat the rollers fully. You are measuring on the bottom of the assembly?
05-01-2007, 08:28 PM
yes, I am measuring from the bottom through the magwell. I guess I need to push the barrel in further but I am afraid that its going in too far.
05-01-2007, 08:40 PM
Assuming the bbl is made properly and you are using a CETME trunnion and bolt, press it in until bolt gap is at max or just under and you will be fine. What receiver are you using? Original trunnion installed from the kit, or new US made one in the receiver? US made ones AFAIK are G3 spec and won't work with a CETME bolt.
05-01-2007, 09:00 PM
I am using a century stamped receiver, original trunnion, new spanish barrel. I assume that the barrel should not be below the "shelf or whatever" its called. It should be somewhere around flush.
05-02-2007, 05:22 PM
Got a picture of the bbl/trunnion from the rear?
05-03-2007, 09:43 AM
I don't, I think it'd be hard to take a picture since its in the receiver. I'm slightly stumped. I continuously took the assembly out of the press and checked bolt gap, now I'm at the flush mark and its still HUGE.
05-03-2007, 12:40 PM
The trunnion is already welded into the receiver? How did you align it?
05-03-2007, 07:24 PM
I pressed it to meet up with the rails? I was pretty sure the trunnion was supposed to be welded in before the barrel was pressed in?
05-04-2007, 01:11 AM
Many ways of doing things, the factory built rifles welded the trunnion into the receiver then pressed the bbl in. They also have the jigs necessary to ensure proper alignment. Homebuilders generally do things a bit differently out of necessity.
This is the generally accepted method:
First step in fitting the trunnion to the receiver is to set the depth in the receiver with a magazine. There is a lip on the bottom rear of the trunnion that fits into a recess on the front of the mag. With the mag release not installed, the trunnion is butted up against the rails. A magazine is rocked into place. If the trunnion was in too deep, it will be levered forward to the proper position. If it is not in deep enough, the mag will not be retained by the lip. Pull straight down on the mag, if the lip is engaged in the recess, the mag will not come out. If it does, the rails will need to be trimmed back until it doesn't. We now have the depth properly set.
The bbl is then pressed into the trunnion to the proper depth, using bolt gap as a measurement of that depth. Once this is set, the bbl pin hole is drilled and the pin installed. Now the bbl and trunnion is placed in the receiver as an assembly, along with the cocking tube. The front sight triple frame is then installed,. This lets you use a laser boresiter to align everything. The sights are centered then the position of the trunnion in the receiver is tweaked until the laser dot lines up with the sight picture. Locking the bolt and carrier into place will also help line everything up. The trunnion is then welded in place, alternating from side to side to prevent drawing of the metal in any one direction. Dot alignment is rechecked after each weld and the trunnion position is tweaked as required.
The cocking tube is then welded in place, some people put a brass rod in it to support it and back it up if the weld melts a hole in it.
From that point, it's just basic hand fitting to get the other subassemblies mounted onto the receiver.
I've built two G3s this way and not had any problems. Used the original bbl and trunnion assembly so I didn't have to press the bbl or do the triple frame install but everything else is the same. IIRC, the Century receiver I did required the rails be trimmed back about a 1/16" to get the proper mag fit.
To get back to your original question, the bolt is resting against the back of the bbl when you measure bolt gap. Regardless of how deep it ends up, it has to let the bolt go deep enough into the trunnion to get the proper bolt gap. If pressing it in deeper closes up the bolt gap, this is what will be required. The only problem might be if the bolt is not resting against the back of the bbl, but something else is stopping it from fully seating. In that case pressing the bbl in deeper would not cause a change in bolt gap and you would know to check for binding elsewhere.
05-04-2007, 08:35 AM
what could the bolt be getting stopped by, I mean the bolt head seems to be free from obstructions, when I look up through the magwell, everything looks good. I guess I need to do some more detective work. Thanks for the input though.
05-04-2007, 10:29 PM
There could be a brass shaving or something inside the bolt roller mechanism. Snap the bolt to it's rearward position outside the rifle, there should be no gap between the bolt and carrier. If there is, something is binding up in there. There could also be a buildup of carbon in the trunnion recesses for the rollers or a burr in there could be stopping the bolt.
In any event, if pressing the bbl deeper closes up the gap the bolt was stopping against the rear of the bbl as it should be and you just haven't pressed it in deep enough. If the gap doesn't change, then I would try to get a better look inside the trunnion as the bolt is obviously not stopping against the bbl.
05-04-2007, 11:24 PM
Thank you very much for putting all this down in writing. I'm going to be doing this next week and by going through this info I've got a great understanding of what I'm going to be doing.
I'm using a cast stainless CAI receiver so I'll let you know how it goes.