AR Gas Key

4

4T4MAG

New member
I just received a new bolt carrier group for a new ar build. It does not have a name attached to it like DD or LMT or RCA. The description states HPT, MPI, Shot peened, Carpenter 158, 8620 steel, chrome lined carrier key and bolt, grade 8 screws and so on. I put it all together all torqued to proper spec, finished the trigger work and thoroughly cleaned the BCG as I normally would and headed for the hills. I only fired 40 rounds as I had received a few short strokes, which usually means gas leak. I checked the gas block alignment, and gas tube. Then I noticed some fouling around the front base and sides of the gas key. I thought I had the proper amount of lube inside the bolt, it was not dripping wet. I took a small air bulb that I use to clean lens, closed the bolt and blew air into the gas key, it started bubbling around the base and the front base of the gas key. All the screws are still staked and tight.

I admit, I've never been this ocd about a gas gun build. I also know that gas leaks cause premature failure. Many websites state that there are only a few companies that actually make bcg's and this bcg states that all testing and QC has been done.

After all that, are all bcg/gas keys going to leak a little? Short of buying the raw parts and hand lapping the key and the carrier and staking can I expect that a BCG from Daniel Defense or LMT to not have this happen to? Or am I too OCD?
 
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About all you can do, is check the bottom of the bolt key for flatness, and its seat on the bolt. I would imagine the seat to be the flattest of both.

If the bolt key is not flat, you might take a file to it, and flatten the base. The best way, would be to put it in a mill, and do a facing cut across the bottom. You won't be able to take much, maybe -0.002", as you could throw off the hole for the tube.

Another thing to check, is the roughness of the machining on the bolt and the key. If it is too rough, you may not be able to get a seal. It should be about 63 micro-inch or less.
 
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Thanks for the help. I'm going to start with raw parts ( bolt carrier and key that have not been fit or staked) and assemble it myself. I'm going to mill it and lap by hand.
 
I'd send the bolt carrier back to whence it came exactly as it is w/o tampering. Sometimes assemblers will take shortcuts like using thread locker to cover up a bad fit hoping the BCG won't get hot enough to melt the sealant. If the parts came from someplace like Brownells, Midway, or even an ebay seller, there will most likely be a replacement policy.
 
When we assemble the carrier we place sealant around the gas hole, then install the gas key, torque it down, stake it, and then check the staked screws by trying to back them out of the key. Sounds like they missed the sealant around the gas hole. Things will get missed when you are assembling 1,000 units at a time.
 
As Don P said, the proper way is to apply sealant between the bolt carrier proper and the gas key. That's the original specification. I think that some/many/most manufacturers skip that step, figuring that any carbon blow by will seal up any leaks within a short period of time. As the mating surfaces of both pieces are machined flat, any leakage is quite likely to be very small compared to the actual gas ports.

In short, it's not right, but it won't really affect anything either.
 
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