Cracks on bolt face

[Artiz]

Hi guys, I was just inspecting one of my AR's that has a BCM BCG and lo and behold, I think the bolt has cracks running from both corners of the ejector hole where the flat face meets the sides. The gun has been sitting for a good long while since it's last been shot, so I think what we're seeing here is carbon being pushed out of the crack by oil through capillary action, making it apprear pretty obviously. I took the best picture I could, then I cleaned the face residue with Qtips and I can still see further parts of the crack ever so faintly. I would need a microscope to see it better.

BCG has less than 2000 rounds through it of various factory ammo. It's been paired since new with a new LMT 10.5" barrel.

Here are two pictures to illustrate what I'm saying. First picture is the bolt as I pulled it out of the gun, second picture is after I cleaned the residue off.







Has anybody here seen this before?

[alx01]

I've never seen this before, but this definitely looks like micro fracture in the bolt. Is this a 158 or 9130 bolt? Even with a shorter 10.5" a quality bolt should last at least 5-7k rounds. I would replace it if in doubt.

[markm]

What is going on with the ejector? Is it removed?

As far as the crack, it looks to disappear after cleaning. I might run a few hundred more rounds and re-check the bolt.

[Toonces]

I've been a quality engineer in tier 1 or OEM diesel engine manufacturers machine shops for for 20 years. I've looked at a lot of defective parts under a stereoscope. The crack is plainly visible after cleaning, so I don't doubt that it's actually there. Email the pics to BCM. I would expect them to send you a new one, and want the defective bolt back, both ASAP.

[Wake27]

I've never seen this before, but this definitely looks like micro fracture in the bolt. Is this a 158 or 9130 bolt? Even with a shorter 10.5" a quality bolt should last at least 5-7k rounds. I would replace it if in doubt.

Pretty sure BCM only makes bolts in 158.


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[markm]

The crack is plainly visible after cleaning, so I don't doubt that it's actually there.

I must be nuts because it looks like most of it swabbed off except the little portion nearest the ejector.

[Artiz]

What is going on with the ejector? Is it removed?

As far as the crack, it looks to disappear after cleaning. I might run a few hundred more rounds and re-check the bolt.

The ejector is just out of focus.

The crack doesn't dissapear, it's the carbon that was pushed out of the crack by the oil and accumulated on top of it that wiped off. The crack is still there. I'd bet good money it will reappear by itself because of the residual oil and carbon in it.

I must be nuts because it looks like most of it swabbed off except the little portion nearest the ejector.

It's the carbon that was taken out of the crack by the oil that swabbed off, not the crack itself.




I've contacted BCM through their online form and included a link to this thread so they can see the pictures.

[markm]

Ok. Interesting. May just be a bad bolt. I've become really bad about cleaning/inspecting my bolt groups.

[MegademiC]

Im interested in what they say. Thats a wierd place to have cracks, assuming thats what we are looking at.

It would be cool if you knew someone that could mpi the thing.

[Toonces]

I must be nuts because it looks like most of it swabbed off except the little portion nearest the ejector.

No, I don't think you're nuts. But I've looked at a lot of parts as they were returned, then cleaned, then at the dye penetrant/mag particle results, all under very nice stereoscopes. The pics from Artiz are way better than what most diesel engine OEMs send when complaining of a defect. I would guess there's a 99% chance a dye pen check would show that those cracks are real in short order. I would also guess those cracks are a metal defect, not a use or stress defect. I've looked at a lot of defective bar stock through the years on metal that was very high grade, meant for diesel engine injector bodies. 100,000 pounds of metal from one heat squeezed into 1" diameter bar stock equals a lot of linear feet and a lot of possibilities for a piece of slag to produce a defect.