[Reaper6]

I discovered I have an unusual gas piston, looks modified or maybe wear?

Took a trip to the range yesterday and was zeroing a new to me AR TEL scope. I experienced a few failures to extract and a couple that did extract/eject, but failed to load the next round, these were intermittent failures. Cases that didn't extract required a stout pull of the op rod to remove. Rifle has an SAK NM barrel, I'm assuming a fairly tight chamber. Ammo was hand loaded, Hornady 168gr HPBT with 41gr of IMR 4895 and CCI #34 primers. These loads worked well in my other M1As.

Once home I cleaned the rifle and investigated for the possible culprit. Gas system was tight, but when I disassembled it I noticed the gas piston had a bevel in it I hadn't noticed before (on the left in the pics). To my knowledge this is original to the rifle, which is a 1973 vintage Devine. Could this gas piston be the cause of my failures or was it my hand loads? I also shot a M1A five liner which ran flawlessly with these loads, this one has a regular gas piston (middle on in the pic) and SAI barrel.

I'll swap out the gas piston before my next range trip, but that might be awhile before I can test it.

Gas piston in question is on the left, middle is from my five liner, right is from my parts bid (all pictured for comparison)

Close up

Rifle 1973 Devine with AR TEL Scope

 

[Reaper6]

I am waiting to hear from the more experienced M1A/M14 gurus, but your gas cylinder port appears to be out-of-alignment.

That "gas stain" above the port on the left piston makes me think "misalignment" of the gas cylinder.

Gas cylinder seems to be in alignment. When mounted and without the piston, I can insert a thin Allen wrench straight through the gas cylinder vent hole and into the bore.

Did you measure the diameter?

.498 dia. Same as my other piston.

Have you tried that piston in a different rifle? That way you can tease apart is it the piston vs cylinder vs something else.

No yet. I ran out of time, but will try that for my next range trip, might be a couple weeks before I get out again.

 

[Reaper6]

Thanks for the inputs, gave me a few things to check out and rule out. Here's what I've found so far:

Your gas piston doesn't align with the gas port hole based on the white arrow on the bottom pointing to the location where I think the combustion gases are landing (dark circular area). or perhaps the gas cylinder is not aligned properly?
View attachment 468418
Again, a careful inspection is required to determine the root cause as to why the combustion gas is being dumped about 1/8" too far forward relative to the gas piston's inlet hole.

I don't think the dark carbon spot is from a direct blast from the gas port, but maybe the bevel is redirecting some gas there. I've gone through the gas system and all looks properly aligned.

Gas cylinder appears aligned with gas port, here a small Allen wrench goes cleanly through the vent hole and through the gas port.

With everything assembled, I put a small dot of white paint on the on the piston through the vent hole.

Once the paint dried, I removed the piston and compared the location of the paint mark to another piston.

As far as I can make certain, the gas system seems to be in proper alignment.

The gas piston is sitting way too far rearward relative to the gas cylinder port. One possibility is a 're-welded' op rod that is a little too short?

Op rod is SAK, I don't think its a re-weld. I compared it to a TRW op rod I had laying around and dimensionally, they matched.

Op rod alignment to gas system

Does the gas piston rattle with bolt closed on empty chamber?

Probably.

No, nothing rattles.

Try a new piston...

^^^This is what I'll do. Seems the most logical step at this point.

I was kind of hoping someone here would have recognized the mod and could tell us what it was for. It will remain a mystery for now.

 

[Reaper6]

Try a new piston...

Problem solved. I installed a new piston and rifle functions fine now.

No idea how long the old piston was in there, since I bought the rifle over ten years ago I guess. I haven't fired this rifle for a couple years and I don't remember it having issues when I did shoot it or maybe I did have issues with it and that's why I put it away? Either way, the rifle has very little wear and probably hasn't seen a lot of use. Good chance the piston was original and installed by Elmer in 72 or 73. No idea what the purpose of the mod was for, but didn't seem to do much good.