After changing gas blocks, other than bottoming the new one to the barrel flange, how do you know the holes are lined up?
There is no guide for getting it vertically aligned and I have heard that you should back the block off of that flange about a razor blade's thickness also.
Is there one correct way to do this to ensure reliable cycling with no short stroking?

this is from a post of mine back when i was tinker with my M&P
I belive most of us were not experiencing "short stroking because a single round loaded into the mag and fired would give you lock back, indicating the BCG was traveling all the way back. what a lot of us discovered is that the M&P10 being overgassed,and the BCG and or buffer system being too light, that the BCG was traveling too fast... this was proved out by using two different methods.
1. was to increase the weight of the BCG, I used the Tubb's carrier weight system to increase the weight of the BCG

2. was another user disassembled the stock buffer tube and added weights to it

both methods work for fast stroking...
I wish you luck

when changing mine i set the gas tube in the block then slide it on the barrel.

I was going free float so got ride of the front rail keeper and took a size of it spaced the block and no issues at all. the tube will insure the block is lined perfectly straight then spacing got me dead on.

hope that helps some.

Thanks. I have not shot mine since replacing hand guard and gas block. It all went together ok so shooting it will tell.
I was curious about the "razor blade" spacing trick. Is that to line the holes up exactly, or slightly offset them to reduce gas pressure, thus slowing the BCG down a little?
If its the latter, is there a chance of gas cutting erosion opening it back up in time?

when I removed the set screws< i noticed that the gas port in the block and the one set screw hole lined up.
when I rotated the block 180 so I could looks at the relation of the barrels gas port and the setscrew hole, I found that a razor blade was the spacing required to center them up. this made sense, because of the thickness of the original handgard end cap...
clear as mud?

Ok, that makes sense. Thanks. I need to recheck mine, although based on another reply here, the gas rod will bottom out where it needs to be putting the block wherever that results? Apparently the rod can be backed out that razor blade width with no problem, but the holes lining up is mandatory regardless.
Don't mean to beat a dead dog here, just want to get it right.
Thanks again everyone.

I do a couple of things that have helped me with AR builds and have always seemed to work.

I draw a light pencil line on top of the barrel that intersects the center of the gas port. When I install the gas block I just align the top/center of the gas block with the pencil line.

I also measure the thickness of the original handguard cap and back my new gas block away from the barrel flange that amount.

__________________
Cliff
CCSO, FPD ret, USN ret.

That makes sense too. Nice to have some reference points.

Are any of you guys dimpling the barrel for the set screws to counter sink them?

as per post #2 yes I did...just made since to do so
the unit can't rotate, run in or out, if I ever need to disassemble, it will go right back to where it was with out any effort

I did this to my gas block. Make sure you have perfect alignment with one screw set and make the dimple with drill bit. I then removed the gas block and cleaned up any shavings. Then I put the block back this time with the dimpled screw set and again checked for alignment and drilled the next dimple. Once satisfied with the dimples I cleaned and degreased around the area and blued the shiny parts. Don't forget to blue the metal or it will likely rust very quickly.

Have fun. It was the first time I've done anything like this to a barrel and I really did enjoy doing the work myself.

Ah, missed it in the quote block....

Shooting the gun to verify propper alignment would be the way to go before dimpling. Then just take one screw off, dimple barrel and replace screw. Then move on to the other. Good advice removing and cleaining out threads and sealing finish. I use a buddy that does Cerekote to hit my barrel before final reassembly. Good to have friends with cool equipment!!

I just installed the Troy .75 and bottomed it out on the barrel flange and it worked fine. The hole in the gas block is larger than the hole in the barrel so no problem getting them to line up.

Yes, the port in the gas block is generally a bit larger than the port in the barrel, so you can be off a bit and the rifle will still cycle, however, you would do well to offset the gas block a bit so that it is centered over the gas port in the barrel.

The easiest way to do this is to remove the set screw from the gas block that corresponds with the port (that's how they drill the port). Slide the block onto the barrel upside down and look through the screw hole to align the hole with the gas port in the barrel, then, with a pencil, mark the barrel where the front edge of the gas block is. I also take a straight edge against the barrel shoulder and make an intersecting line showing the center of the gas port in the barrel. Then, rotate the gas block into proper position and tighten the screws. Ensuring proper gas flow is the way to ensure that your rifle stays semi-automatic.

I did this the first time with mine, and my semi auto became a bolt gun. I then indexed accordingly taking the original handguard endcap into place and its back to cycling properly.

I can't believe that troy doesnt take and factor in that the low profiles are used without a cap into their length. Will some one mic the lengths?

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It would be a nightmare if they did because it would create a whole other spec. This gas block alignment thing is very simple as my post above explains. I don't mean this to be rude, but if you have that much difficulty with it, maybe you're better off sending it to someone who know's what they're doing.

I didn't have trouble with mine I was simply asking a question about the length. You are mistaken.

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several months ago, some one did measure the cap thickness with a pair of calipers, and applied that thickness to the new block setting