Bent Piston? Alignment issue? New ACR ownder
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Thread: Bent Piston? Alignment issue? New ACR ownder

  1. #1
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    Bent Piston? Alignment issue? New ACR ownder

    Hello,

    New ACR owner and first post here. Former Military, SAR, and lots of wilderness experience. Comfortable and familiar with a fair amount of weapon platforms but no gunsmithing experience beyond basic maintenance. I hope to both learn and contribute where I can.

    I recently purchased a used ACR and when doing a detailed field strip and cleaning (before shooting) I noticed something that seemed odd. (Not sure if I have all the terminology right so please feel free to correct me). After removing the gas regulator (plunger) from the gas block, the piston was very tight and difficult to remove. I removed it and cleaned all the carbon out. When reinstalling I noticed the fat part of the piston rod slides freely in the gas block and the thin part of the piston rod slides freely through the tube/guide attached further down the barrel when tested separately...but, when I fully install the piston through the gas block and into the other tube/guide it really tightens up.

    It seems that either the piston is bent and/or the tube/guide is slightly out of alignment with the gas block. It's hard to tell, but it's the only thing I can think of that would make it tighten up like that when you fully install it and feed the piston through both. Is this normal? Is there a way to get the gas block and tube/guide aligned properly?

    Any feedback would be appreciated! Really excited to have found this community and to dive into the ACR platform. Finding this forum was a big part of deciding to take the plunge!

    Ronin

  2. #2
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    One easy way to tell if the piston is bent is to remove it and roll it on a smooth, flat surface. I have heard of bent pistons but generally it's after modifying the gas system for caliber conversions etc. If the piston is straight you almost certainly have a misalignment issue. Maybe someone more well versed on the platform can advise you better, but I think you hit the nail on the head when you tried it and felt resistance. Maybe loosen the assembly and straighten it the best you can, tighten everything backbup and see if it slides.

    For what it's worth, I ran the calculations and the piston guide seems absolutely necessary to prevent buckling of the piston when under compression.

  3. #3
    Forum Lurker inferno999's Avatar
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    I've never tried but it should be fairly easy to test if the piston guide is offset. Just loosen it while it's mounted in the gun. It's just a couple Torx, maybe t30 if memory serves... If the piston moves freely when it's loose, tighten it up in the corrected position. Done.
    Last edited by inferno999; 03-27-2017 at 02:02 PM.

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  5. #4
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    If you remove the piston and regulator, remove the bolt and look down the gas block, hold it up to the light, you will easily see if the support is aligned or not, and then are able to adjust it perfectly. Hard to describe in words, but it should look like concentric circles all the way down, gas block, support, trunnion.

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    Thanks for all the replies. The support wasn't lined up. Once I broke it lose and re-aligned it was nice and smooth. Piston still seems straight from the roll test.

    Got a chance to take it out to the mountains and snow today for some shooting and movement drills. I'm really liking the ACR platform so far! It ran everything I put through it: Steel cased Tula, Brass PMC, Fiochi and some Winchester White Box and, with every magazine combo I had on hand (PMAGs, steel surplus, and Lancers). Not a single hiccup. I heard read some rifles, especially the earlier ones didn't like the cheaper ammo (mine is Windham BL016xx so I think that means it's early production). Ergonomics are great though the safety can really dig into your finger! Do you all just grip low/readjust as needed?
    mxracer72 and sharpcoolman like this.

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    I installed the 45 degree safety, but it still barely hits my hand. I might buy the right side safety delete or cut mine down more.

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    I accidentally broke mine off. It was my fault completely, I was prying on it to get it out because I forgot the correct location for removal.

    On a lighter note, Nexus Tactical and Dana (Bigdog) sell right side safety "deletes" that insert a flat where the safety used to be.

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    For the record, I like it better off altogether. Lol

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    I think that'll have to be my next purchase. Are the 45 degrees from Big Dog as well? I sometimes use my index finger (or closer to the knuckle) kind of like on an AK. Does that still work with the 45?

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