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Thread: Question about replacing BCG's

  1. #1
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    Question about replacing BCG's

    I wanted to post this for a range buddy of mine. If you wanted to replace a LMT BCG with a BCM should you simply replace the carrier, or get the whole thing from BCM? The rest of the parts are LMT, and the only thing in question is a MIM gas key with some metal chipped off the face. My thoughts were that the entire piece needs to be replaced, but he feels otherwise.

    Also, when replacing a bolt after the gun has 1000+ rounds through it; does the new bolt need to be fitted, or the chamber head spaced? Or can you just drop a new milspec bolt in? The question comes from seeing those that carry a spare bolt, my guess is that it drops right in, unless the barrel is about shot out.

    Thanks in advanced for the help.
    Last edited by Apollo11; 11-14-09 at 19:31. Reason: sp

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    Unless the chamber is a match design all the parts are interchangeable without issue assuming there to spec. BCM and LMT are as far as I know but I'd invest in go and no-go gages for your chamber. Two things to check when replacing the bolt or swapping a BC, a) extend the bolt and stand the BCG on the bolt face - if it doesn't move you're gtg and b) if the BCG closes on a go gage and not on a no-go gage you're also gtg. Assuming of course the gas tube isn't mucked up, etc. I carry spare bolts in a number of rifles and they've all been cross-checked this way but then they're all NATO chambers. HTH.

  3. #3
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    If he is worried about the MIM gas key just replace the gas key with a BCM one unless he just wants justification to buying a new carrier.

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    Quote Originally Posted by Thomas M-4 View Post
    If he is worried about the MIM gas key just replace the gas key with a BCM one unless he just wants justification to buying a new carrier.
    Thanks both for your help. I don't think he wants to buy a new BCG, but he is not happy with the dings on the key. The gas key replacement is something we spoke about, but if that has to be done he would rather replace either the entire carrier, or a whole new BCG. I just wanted to see if he should buy the entire thing, or just the carrier with the key.

    I have heard rumer that the BCM carrier is made my LMT, but done with the non-MIM key and it is chrome lined...I don't know if this is true or not however.

    Also, it is good to know that you can just swap out bolts if need be, as long as everything in question is in spec.

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    If the gas key etc. is damaged or defective I'd contact LMT and get a return authorization and let them repair or replace the part way before I spent $80.00-$140.00 to replace a $25.00 part.That said its not a bad idea to have a spare.
    Just my opinion.
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    I've heard the same thing, that BCM BCGs are LMTs with some variations. They're also extensively fired, or so it's said, so it's not unusual for a 'new' bolt or BC(G) to look like crap, relatively speaking. Lots of discussion back and forth on this on TOS and other boards.

    Pick up a gas key from BCM like the other poster said and the current chipped MIM gas key becomes a non-issue as long as you have the right tools and a MOACKS, if it's really worth the trouble. I keep a couple of gas key sets as well as some stripped BCs sitting around just in case but I'm not sure what'll wear out first, a MIM key or the gas tube. Bet it's a pretty even race. Good to have spare gas tubes and roll pins sitting around too if you do much shooting.

    Oh, and don't just trust that everything's interchangable, that's a helluva way to ruin your or someone else's day. A pair of go/no-go gages costs fifty bucks, dang cheap insurance.

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    Why would you get GO-NO GO gages when as far as I know no one makes a true 5.56 mm GO- NO GO gage? In addition the tech manual only calls for the use of a Field Gage.

    In order to properly check your weapon it must be clean. Insert the Field Gage into the chamber and then insert the BCG. Applying moderate finger pressure the bolt SHOULD NOT lock up into the chamber. If it does, then you have a headspace issue.

    This topic has been addressed here MANY MANY times and I again encourage people to use the search function.

    Quote Originally Posted by justpete View Post
    Unless the chamber is a match design all the parts are interchangeable without issue assuming there to spec. BCM and LMT are as far as I know but I'd invest in go and no-go gages for your chamber. Two things to check when replacing the bolt or swapping a BC, a) extend the bolt and stand the BCG on the bolt face - if it doesn't move you're gtg and b) if the BCG closes on a go gage and not on a no-go gage you're also gtg. Assuming of course the gas tube isn't mucked up, etc. I carry spare bolts in a number of rifles and they've all been cross-checked this way but then they're all NATO chambers. HTH.



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    In order to properly check your weapon it must be clean. Insert the Field Gage into the chamber and then insert the BCG. Applying moderate finger pressure the bolt SHOULD NOT lock up into the chamber. If it does, then you have a headspace issue.

    Iraqgunz, what is a field gage? I have swapped out several BCG and never checked this. ALL where either quality LMT, COLT or Denny's, but never had a problem?

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    Quote Originally Posted by Pappabear View Post
    ...what is a field gage? I have swapped out several BCG and never checked this. ALL where either quality LMT, COLT or Denny's, but never had a problem?
    Field Gauge / Headspace Gauge

    Instructions for use

  10. #10
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    Quote Originally Posted by Apollo11 View Post
    Thanks both for your help. I don't think he wants to buy a new BCG, but he is not happy with the dings on the key. The gas key replacement is something we spoke about, but if that has to be done he would rather replace either the entire carrier, or a whole new BCG. I just wanted to see if he should buy the entire thing, or just the carrier with the key.

    I have heard rumer that the BCM carrier is made my LMT, but done with the non-MIM key and it is chrome lined...I don't know if this is true or not however.

    Also, it is good to know that you can just swap out bolts if need be, as long as everything in question is in spec.

    Negative on the rumor.

    Personally, I would just replace the gas key and be done with it.


    C4

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