Methods of assembly -- question about gas port alignment

[SHIVAN]

What method ensures proper gas port to gas block alignment? Assuming no pins are present, how do YOU ensure that you have aligned the gas block to the gas port?

I usually trust that the gas tube and gas tube hole should get me "close enough" if lined up properly, but I suppose you could be a millimeter out and never know it...

So how do the "pros" do it? Is there a tool? A feeler gauge? Something more precise than the method I use??

[K.L. Davis]

The best bet is to have a barrel that is "indexed" with a small plunge cut 180 degrees opposite of the gas port... you use the hole in the bottom of the gas block to line up with that hole -- of course, some blocks use a set screw in that hole, which is all the better for alignment and retention.

You can make a jig to hold the FSB or gas block on place on the barrel for drilling the pin holes -- it is a rather simple arrangment really.

If those are not an option... I usually do a quick measurement of the center of the hole in the gas block to one edge, then slap some layout dye on the barrel, make a reference mark for depth and TDC, then line the block up with that.

eta: crap... I just noticed that you asked how the pros do it... disregard my previous.

[Griz]

On every gas block I've seen, the hole in the FSB that mates with the gas port is much bigger than the gas port in the barrel, so there is plenty of room for error without affecting function at all.

I'm no pro, but I always align the FSB by boresighting the iron sights. I mainly do this so that the irons are on target when centered, but I guess a fringe benefit is that it seems to help line up the FSB with the gas port. I check that zero at the range and if no adjustment is needed, I dimple the barrel for the set-screws and apply Loctite. (I guess I'll be SOL if I ever install a low profile sight )

[Robb Jensen]

What method ensures proper gas port to gas block alignment? Assuming no pins are present, how do YOU ensure that you have aligned the gas block to the gas port?

I usually trust that the gas tube and gas tube hole should get me "close enough" if lined up properly, but I suppose you could be a millimeter out and never know it...

So how do the "pros" do it? Is there a tool? A feeler gauge? Something more precise than the method I use??

I'm far from a pro.
But what I do is mic the gas port to the shoulder, then mic the gas block to make sure it'll be over the port. I then mark the barrel and the block and then counter sink where the set screws will go so the gas block can't walk forward. Then upon final assembly I use Rocksett to keep them from coming loose during operation. This method seems to work well so far. The gas port in the gas block is usually quite a bit bigger than the port in the barrel so even if it's a degree or 2 off it'll still work.

[Nitrox]

My hack-Fu is strong, I use a grease pencil and a feeler gage.

[K.L. Davis]

My hack-Fu is strong, I use a grease pencil and a feeler gage.

If those are not an option... I usually do a quick measurement of the center of the hole in the gas block to one edge, then slap some layout dye* on the barrel, make a reference mark for depth and TDC, then line the block up with that.

Okay... so I dont sound all like Mr. FancyPants

*Sharpie

[Todd.K]

I drill the gas port, then index the barrel 180deg and use a ball endmill for the dimple.

There are no feeler gauges that I am aware of. The only good thing I can tell you is that if it's a carbine gas system the port is small and you have a good bit of wiggle room. You should be able to get it plenty close with your eye, of course I have a machinist eye....

[Todd.K]

I thought of one more idea.
Get a spare gas block and drill all the way through from the bottom.
Put it on the barrel and line up the gas port in your new hole on the top of the gas block.
Use the forward set screw to hold it in place and use the rear hole as a jig to drill the detent for the rear set screw.

[Robb Jensen]

I thought of one more idea.
Get a spare gas block and drill all the way through from the bottom.
Put it on the barrel and line up the gas port in your new hole on the top of the gas block.
Use the forward set screw to hold it in place and use the rear hole as a jig to drill the detent for the rear set screw.

That would be the very best way.

[Nitrox]

There are no feeler gauges that I am aware of.

Just make one...cut an HG cap in half. This is where Hack-Fu and Dremel-Fu converge.