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Thread: Short stroking Bolt Group?

  1. #1
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    Short stroking Bolt Group?

    I was running my Sabre 11.5" barreled SBR on Friday. (I haven't really tried to run it very hard yet) I started getting some short stroking.. Bolt overs, failure to lock back, etc. So Saturday I take out another lower to make sure that it's nothing to do with the buffer/spring combo in the host gun. Same problems! Great! I've narrowed it down to the upper.

    But for the hell of it I pull the bolt group out of the SDI upper and drop it into a known functioning carbean. SHORT STROKE city.

    So I have a Colt M16 bolt group that is giving me short strokes in two different guns? Bolts are staked on it. The one thing that I did notice is that there's a slight gap between the carrier and the key in front of the bolts.. in the area just behind the cam pin slot. I'm wondering if gas could be leaking there. ALL other parts on the group check out.

    Rob? Grant?
    "You people have too much time on your hands." - scottryan

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    Quote Originally Posted by demigod View Post
    I was running my Sabre 11.5" barreled SBR on Friday. (I haven't really tried to run it very hard yet) I started getting some short stroking.. Bolt overs, failure to lock back, etc. So Saturday I take out another lower to make sure that it's nothing to do with the buffer/spring combo in the host gun. Same problems! Great! I've narrowed it down to the upper.

    But for the hell of it I pull the bolt group out of the SDI upper and drop it into a known functioning carbean. SHORT STROKE city.

    So I have a Colt M16 bolt group that is giving me short strokes in two different guns? Bolts are staked on it. The one thing that I did notice is that there's a slight gap between the carrier and the key in front of the bolts.. in the area just behind the cam pin slot. I'm wondering if gas could be leaking there. ALL other parts on the group check out.

    Robb? Grant?
    I'd pull the carrier key. If it's leaking as I believe it is, then you'll see carbon tracking from the hole in the carrier usually going from the hole to the front edge of the carrier key. I'd replace the key with a BCM unit. If you don't have a BCM to replace it with you can reuse the key but hone/lap the finish off the bottom of the carrier key on an Arkansas stone knife sharpening stone (this insures the key is totally flat, not warped), degrease the key, screws and carrier. Reinstall using red loc-tite on the bottom of the key and torque to 55-60in lbs and stake it well. Let it dry for as least 12hrs, lube it up at the range and test fire.

    Robb
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  3. #3
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    Thanks, Robb.

    I'm pretty sure it's leaking there too! (BTW, BCM keys are out of stock... I just checked)

    I'm thinking... possibly the front bolt snapped and the key is lifted in the front?

    The key and bolts feel tight.. no wiggle anywhere.

    I can't figure out what else would cause the key to lift up like that.
    "You people have too much time on your hands." - scottryan

  4. #4
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    Demi,

    FYI- my TM indicates 35-40 in./lbs for the bolt carrier keys.

    Quote Originally Posted by gotm4 View Post
    I'd pull the carrier key. If it's leaking as I believe it is, then you'll see carbon tracking from the hole in the carrier usually going from the hole to the front edge of the carrier key. I'd replace the key with a BCM unit. If you don't have a BCM to replace it with you can reuse the key but hone/lap the finish off the bottom of the carrier key on an Arkansas stone knife sharpening stone (this insures the key is totally flat, not warped), degrease the key, screws and carrier. Reinstall using red loc-tite on the bottom of the key and torque to 55-60in lbs and stake it well. Let it dry for as least 12hrs, lube it up at the range and test fire.

    Robb

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    Quote Originally Posted by Iraqgunz View Post
    Demi,

    FYI- my TM indicates 35-40 in./lbs for the bolt carrier keys.
    Thanks. I'm just going to order a BCM bolt group for the immediate fix.
    "You people have too much time on your hands." - scottryan

  6. #6
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    Quote Originally Posted by Iraqgunz View Post
    Demi,

    FYI- my TM indicates 35-40 in./lbs for the bolt carrier keys.
    Noted.
    After having 35-40in lbs come loose even when well staked, I've since used 55-60in lbs and red loc-tite. I believe Armalite also put out a memo to the same finding for torque spec. You won't have broken screws at 55-60in lbs if using high quality screws.....
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  7. #7
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    Thanks for that update. I'll file it in the mental repository for the future.

    Quote Originally Posted by gotm4 View Post
    Noted.
    After having 35-40in lbs come loose even when well staked, I've since used 55-60in lbs and red loc-tite. I believe Armalite also put out a memo to the same finding for torque spec. You won't have broken screws at 55-60in lbs if using high quality screws.....

  8. #8
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    It turns out my guess was right. I pulled off the gas key today and the front bolt was snapped. The key felt tight.. I guess from the back bolt hanging on pretty good. But the small gap was enough to shut the bolt group down.

    I took it to S.A.W. and pointed out the gap before I performed surgury myself. They looked over the bolt group and said nothing was wrong with it.

    Lesson learned??? Never trust the experts!
    "You people have too much time on your hands." - scottryan

  9. #9
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    Quote Originally Posted by demigod View Post
    It turns out my guess was right. I pulled off the gas key today and the front bolt was snapped. The key felt tight.. I guess from the back bolt hanging on pretty good. But the small gap was enough to shut the bolt group down.

    I took it to S.A.W. and pointed out the gap before I performed surgury myself. They looked over the bolt group and said nothing was wrong with it.

    Lesson learned??? Never trust the experts!
    Good to hear, glad I recommended pulling it!
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  10. #10
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    I'm guessing that the bolt snapping would just be a fluke?
    "You people have too much time on your hands." - scottryan

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