G3 Headspace/Bolt Gap Question
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Thread: G3 Headspace/Bolt Gap Question

  1. #1
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    Default G3 Headspace/Bolt Gap Question

    Can you accurately/correctly measure the bolt gap with the trunnion and the bolt outside of the receiver? When I hold the trunnion and the bolt carrier with one hand and hold feeler gauges in the other hand, the bolt gap changes depending on how hard I am squeezing the bolt carrier and trunnion together. Is there a proper way to do this outside of a receiver?


    Thanks for any help!
    He who walks unarmed into paradise had better be sure that is where he is.

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    It's difficult to get a dead nuts on bolt gap reading until the trunnion is installed in the receiver, since you can flex the carrier up/down, etc. but you can get close and try to maintain the relationship between the nose of the carrier and the barrel, so that they are as close to parallel as possible(as they will be once the cocking tube is welded up). Try to end up on the high side of the bolt gap issue (.020-ish) , and then once you have pinned the barrel and welded in the trunnion, you can adjust with larger/smaller rollers as needed.

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    Quote Originally Posted by SgtHorvath View Post
    It's difficult to get a dead nuts on bolt gap reading until the trunnion is installed in the receiver, since you can flex the carrier up/down, etc. but you can get close and try to maintain the relationship between the nose of the carrier and the barrel, so that they are as close to parallel as possible(as they will be once the cocking tube is welded up). Try to end up on the high side of the bolt gap issue (.020-ish) , and then once you have pinned the barrel and welded in the trunnion, you can adjust with larger/smaller rollers as needed.
    Thank you sir. The kit I got had a cut barrel of course but before I pressed out the pin and barrel, I used a depth mic to measure where the barrel sat in the trunnion. If I duplicate that measurement with the new barrel, am I guaranteed to be in pretty good shape with regard to head spacing?

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    Possibly...probably...maybe. In a perfect world, yes. When I was building my CETME, Holescreek's tutorials listed a depth measurement of the barrel being around .015 proud of the trunnion breechface. I found that to be pretty much spot on. I'd aim for that, and then check your bolt gap. Holescreek is, IMHO, a Godsend to the aspiring CETME/G3/HK91 builder. He has many tutorials on MilitaryFirearm.com. Out of curiousity, what did the old barrel stub measure from the end of the trunnion? Then, of course, you'd also need to know the total depth of the trunnion from the rear surface to the breechface of it, and subtract the two. But .015 should be in the neighborhood, from rear of trunnion, to barrel face.

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    Check out this excellent tutorial from holescreek on MilitaryFirearm.com..."Pressing a barrel into a Cetme or HK trunnion"

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    Quote Originally Posted by SgtHorvath View Post
    Possibly...probably...maybe. In a perfect world, yes. When I was building my CETME, Holescreek's tutorials listed a depth measurement of the barrel being around .015 proud of the trunnion breechface. I found that to be pretty much spot on. I'd aim for that, and then check your bolt gap. Holescreek is, IMHO, a Godsend to the aspiring CETME/G3/HK91 builder. He has many tutorials on MilitaryFirearm.com. Out of curiousity, what did the old barrel stub measure from the end of the trunnion? Then, of course, you'd also need to know the total depth of the trunnion from the rear surface to the breechface of it, and subtract the two. But .015 should be in the neighborhood, from rear of trunnion, to barrel face.
    I'll attempt to find the tutorial. Thank you sir. I measured from the opening of the trunnion to the breach and it measured 1.216". I could check and see if your measurement on my original barrel was correct with a little math.

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    Just remember...You want your barrel to extend ABOUT .015" out from the breechface of the trunnion. If it presses in until i\the barrel is flush with the trunnion breechface, you almost CERTAINLY will have gone in too far.

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    Quote Originally Posted by SgtHorvath View Post
    Just remember...You want your barrel to extend ABOUT .015" out from the breechface of the trunnion. If it presses in until i\the barrel is flush with the trunnion breechface, you almost CERTAINLY will have gone in too far.
    I appreciate it sir. I will takes some measurements tonight to make sure your .015" and my 1.216" numbers agree with each other. Depending on how this turns out, I may be coming to you for advice on new rollers. Lol

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    This will be what you are looking for...I borrowed one of holescreeks photos from the aforementioned tutorial...
    Attached Thumbnails Attached Thumbnails G3 Headspace/Bolt Gap Question-trunnion.jpg  

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    Quote Originally Posted by SgtHorvath View Post
    This will be what you are looking for...I borrowed one of holescreeks photos from the aforementioned tutorial...
    And is he saying the magic number is the .015" you gave me?

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