View Full Version : setting the bolt gap question.

04-11-2007, 08:00 PM
I am doing a build and am a little confused after reading that post about ground bolt heads. when I press in my new barrel, exactly what am i setting the gap to. I was going to shoot for .017 " . now is that from the face of the bolt? or where? and this is whith the bolt locked in to the trunion? I order a new set of standard size rollers to fit it to.

04-11-2007, 08:41 PM
read the stickies... bolt gap is measured between the bolt head and carrier.

but really... read the stickies, read all the tutorials in the gunsmithing section, read them all 3 more times so you fully understand why this stuff is so important before you throw it together.

04-11-2007, 09:14 PM
i have and I thought I did understand them. but I thought the gap was between the face of the bolt and the end of the barrel. so what gap am I setting when I press the barrel in? and how do I set it?

04-11-2007, 09:25 PM
You will need to press the barrel in. Use your eyeballs to get the barrel close to flush with the trunnion. You then need to put your completed boltcarrier in just as you normally would. Buttstock and all and let the bolt fly forward, squeeze the trigger to let the hammer fall, then measure the gap. That is of course NOT LOADED. When you get close use tiny adjustments since a few 10ths of barrel movement can equate to a thou or more of bolt gap. If you press too far as in zero gap, push it on out and start again. If you get .012 or other reasonable measurement it will not hurt to tap the barrel back, just use an old flash hider to protect the threads. Make absolutely sure to deburr the trunnion on the leading edge where you push the barrel in. Mine had some severe nicks that never went away. This caused me to gaul the first barrel bad enough to purchase another one.
By all means when you get it set use an end mill to drill the pin hole. Mine was a touch over 5mm so I had it milled with @ the standard size then reamed with a 5mm reamer extra good. A plus size standard gauge pin was used since the 5mm pin was going to be loose regardless because of the trunnion hole.
Hope that gets you going. David

04-11-2007, 09:29 PM
I will try to get you some pictures. You turn the rifle over once you get the barrel pressed close enough to start measuring. This will be with the bolt carrier and stock on it. Look down in the mag well with it rifle turned over and see where your bolthead is close to the carrier. That is the gap. There should be a really good sticky with this picture already. If not I'll try tonight to get one after midnight EST.

04-11-2007, 09:42 PM
i guess I under stand how to check the gap, one tutorial said to press the barrel into the trunion before it is installed into the reciever. the other one says to weld trunion in first and then press barrel in and set gap. I dont get how the barrel has any thing to do with the gap between the bolt and the carrier. is ther supposed to be a gap between the face of the bolt and the barrel? Also where did you get that tutorial from?

Big Steve
04-11-2007, 09:57 PM
The barrel fits tight against the bolt head. As you push the barrel into the trunion the rollers are pushed inward into the tapered part of the locking piece. That pushes the locking piece rearward. The locking piece pushes the bolt carrier rearward away from the bolt head creating a gap between them. You measure it from the bottom through the mag well opening. Measure it right in the middle at the rear of the bolt head. If you cant squeeze a feeler gauge between them then you have no gap. Push the barrel in a little bit more and it will open up.
Hope this helps.

04-11-2007, 10:01 PM
FWIW, .017 is not unobtainable. When I got mine new, it was .016 loose but barely too tight for .017, so it can be achieved. 500 rounds later and it is .015 loose but too tight for .016. Lots of life left in it. Make sure it is immaculately clean when you make the measurement for the best results.


04-11-2007, 10:26 PM
big steve... the method you described is for pushing it in from the front right? If you set the gap from the rear, the gap gets smaller the further you push it in.

04-12-2007, 06:45 AM
Roland, before you start putting it together, you need to have a thorough understanding of how it works, and the relationship between parts.

We are trying to get a lot of info back together from the old site that was lost, but it will take a little time.

To more understand how the bolt works, and how the gap is used, read thru the thread called "A petition to Perro". It talks about bolt gap, the way the bolt is in the trunnion, and has a couple illustrations.

What you want is the bolt face tight against the rear of the barrel, not the trunnion. The process of pressing the barrel is tedious, you have to move a little, reassemble the bolt and carrier and check, disassemble, press a bit, recheck, cuss because you went to far, repeat until you have a headache.
.001 of barrel movement equals .004 of bolt gap, so you can immediately see how detailed this is.

That is the only way to get it right, from the get go. Try to set it high (like .020), because they tend to close up and settle a bit once they are fired. Also use standard size rollers and good locking piece, so you have the correct wear items to replace down the road.

05-01-2007, 04:01 PM
I pressed my barrel in today, I've got it to where it is right about flush with the lip on the trunnion. The bolt gap is very large, around 1.2mm. Where do I go from here, do I press the barrel in further or did I press it in too far? Its almost perfectly flush. Thanks for your help guys.

05-02-2007, 05:00 AM
You pressed it in too far. continue pressing it all the way out, then start over.

You should stop while it is protuding a little from the trunion,and start checking there. Press in a hair at a time until you get it right.

05-02-2007, 06:59 AM
thanks rusty pirate....If I pressed it in too far, shouldn't the gap be smaller rather than larger? I mean I checked the gap several times before I reached the point I'm at and it was always huge.

05-02-2007, 08:39 AM
Was the bolt gap decreasing as you were pressing it in or staying the same?

05-02-2007, 12:59 PM
because of the way the detent is shaped in the trunnion, if you press it in too far, the rollers start hitting the front of the detent rather than the back, and the gap starts to rise again. That could be what is happening in your case. Do what rusty said, and start checking the gap sooner and more often.

05-02-2007, 03:37 PM
Also check the length of your bolt to ensure that it is not ground.

Big Steve
05-02-2007, 04:33 PM
I don't think you need to start over. I will assume you are pressing it in from the front. If so you went a little to far. Find something that wont scar the end of the barrel up and put it through the reciever and press it out a little. Something to remember is that the gap is not a 1 to 1 ratio with how much you move the barrel. For every .001 you move the barrel you will get .003 or .004 of bolt gap change.Shoot for the highest gap you can get. .020 is great!
I would stop on .022 if I hit it. You can always put in some -2 rollers for a few rnds untill your parts wear in and take a set. New parts wear in rapidly for the first 40 rnds or so and you can lose as much as .005 or so of gap right away

05-03-2007, 05:12 PM
If the trunion is in a receiver don't try to press the barrel back in even a little putting load on the back of the receiver, make sure the pressure is on the trunion. A friend helped with his press on my first build and I still need to make a fixture to straighten that receiver!

05-03-2007, 06:25 PM
hi guys, you know i am building a g-3 and when i check the bolt gap out side ,,,just putting the bolt and barrel assembly together by hand i get about .023,, now before you guys start the fire for the flaming i just would like to know when i get this thing welded,and if i still have high bolt gap can i put in some -2 or -4 rollers to bring the gap down??? seem like that would work if larger rollers make the gap bigger

i have found them and the web page says they are available

i have standard rollers in the bolt now and i have a couple sets of +2 and +4 rollers and i hope -2 and -4 rollers are available

seems like i read .003 or .004 for each size change

just thinking out loud what do you guys think


05-03-2007, 06:33 PM
no flaming necessary... sounds like plan if you have some - rollers to throw in there.