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Discussion Starter · #1 ·
I'd like to get some opinions regarding the importance of the operating rod cylinder tube (when fully forward) being concentric with the gas cylinder tube. I notice when I install different operating rods on my SAI M1A rifle that the cylinders line up slightly differently. Some are very concentric with each other while others are slightly off (i.e., rod cylinder's center axis is not perfectly concentric with gas cylinder's center axis). All combinations seem to function OK.

In one case I attempted (via bending) to perfectly align a "Fulton Armory" operating rod cylinder with my rifle's gas cylinder. While I was successful with the alighment, the bending caused excessive drag by the rod (i.e., it would no longer pass the 60 degree tilt tests). After the realignment and even with the drag, the rifle shot and cycled reliably. That makes me wonder what's MORE important... concentricity of the operating rod and gas cylinders OR the rod passing the tilt tests.

Hope the above makes sense. I'd appreciate your thoughts on this.
 

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You bent the rod? Wow. You know you can shift the op rod guide a little left or right to get the op rod to align with the gas cylinder better, doesn't require you to destroy any parts either. Usually helps with the tilt test too when done correctly.
 

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It is good to have the op-rod shank and the gas cylinder aligned. Misalignment will cause uneven wear on the piston tail and you will be able to see that only part of the piston tail is abosorbing the impact. This means your op-rod is striking the piston at an angle and inducing a non-concentric impact on the piston. For the NM rifle, this could be bad. It may or may not affect accuracy but I don't like the wear that will be induced.

I adjust the position of the op-rod guide to align the cylinder and the op-rod. After that I square up the mating surfaces of the piston tail and the op-rod shank.

I had to bend my operating rod to pass the tilt test but it was only in one axis; vertical. Luckily, my op-rod was already aligned with the piston. On my SAI, I adjusted the guide and it passed the tilt tests. I also squared the mating surfaces on that rifle.

Again, it's a good idea to have everything perfectly aligned but it may or may not affect accuracy and the degree of the affect will differ from rifle to rifle.

Tony.
 

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Discussion Starter · #4 ·
Good information. Thanks guys.

To clarify... my rifle's current set-up (as received from SAI) is dead-on for concentricity and passes all tilt tests. It's only when I temporarily install other operating rods (e.g., the Fulton Armory rod) that I notice the lack of concentricity. I haven't made any adjustments to the operating rod guide because all's good with the current set-up, and I don't want to upset that. If I ever decide to permanently swap out the original SAI operating rod with a replacement, I'll likely just adjust the operating rod guide as you guys have suggested to make the parts line up correctly.

I like to occasionally "test" my spare parts to boost my confidence that they'll work when I need them.
 

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Concintricity is good, but more important is consistancy. A little misalignment isn't going to cause major malfunctions. A lack of cosistancy will cause inaccuracy. The overall fit of the op rod in the receiver op rod way, no rotational movement of the op rod guide, and no rotation of the gas cylinder result in consistant positioning of the op rod in it's relation to the piston.
 

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opinions regarding the importance of the operating rod cylinder tube (when fully forward) being concentric with the gas cylinder tube.
No importance, the only important thing is that is aligns to center with the piston tail itself, in the center of the op rod. Too many variables on gas cylinder splineways, gas cylinder splines, rear D hole machining in the gas cylinders, op rod boss pin grooves cut off axis, too shallow, too tight, too large or small diameter, barrel makers dimensional differences, and op rod guide and op rod variations in dimension. So line up the rod to the tail of the piston and let er rip, as long as the op rod tab does not bind in the receiver rail, and op rod doesn't bind at the guide, and it passes tilt test.
 
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