AR Sponsor
Posted: 3/1/2010 8:22:16 PM EDT
|
When you remove the handguard cap from behind the FSB, do you need to put a washer or something back in there to make sure the gas port lines up? I understand that the FSB pins will index the FSB properly, but the upper I have is an Armalite with a low profile gas block, and the gas block is attached with set screws, not pins. I'm wondering what I should do to ensure the gas port lines up with the block properly after installing the rail. |
|
I've installed a few free float tubes and had the same concern regarding relocating the gas block correctly. I've been successful by making witness marks on the gas block and barrel with a fine point pencil or a very fine scribe. Usually the gas port area inside the gas block is somewhat oversize and should be OK if the alignment is slightly off. I just leave a slight gap between the gas block and the barrel shoulder.
Another thing I do is use a couple straightedges when aligning the gas block. If you have a flat top upper and a railed gas block, it's real easy. A couple pieces of straight round stock will work fine. I use 2 steel rulers. Lay one across the upper receiver and one on top of the gas block. Sight both from either the muzzle end or the rear. Any misalignment will be obvious. If the 2 rods/rulers are not lined up, simply rotate your gas block until they line up with each other. |
| You could also spin the new gas block upside down with the set screw hole centered over the port in the barrel. Check the space with some feeler gauges, spin it back into correct position keeping the same space and tighten down the set screws. You could then use the set screw marks on the barrel to dimple it if you wish. |
|
The YHM gas block/sight installation instructions specifically say “DO NOT PUSH THE SIGHT AGAINST THE SHOULDER OF THE BARREL! You must leave a 1/32” gap between the shoulder of the barrel and the gas block/ front sight.”
Someone posted saying he had installed hundreds and they fit flush, if you had a std forearm you had to remove material from the back of the sight. The posts pertained specifically to the YHM block with a flip up front sight build in. My personal experience based on a few measured blocks/bases is YHM is right. In the ones I measured the gas port in the barrel would be on the edge of the hole in the base, if not partially obscured. Running it this way might cause issues. In the past I set by eye or feeler gage, then I had some spacer rings laser cut so now I just drop on the spacer and run the block flush against the spacer. |
|
Thanks for all the replies. Would removing one of the screws and using an automatic center punch to mark the barrel through the hole be a bad idea? I am concerned that the punch would raise too much metal up around the mark and make it difficult to remove the gas block. I don't want to dimple the holes with the gas block in place for fear of damaging the threads in the gas block. I like the laser cut spacer idea, but I don't really have enough rifles to make it worthwhile. Perhaps someone has already thought of this and sells them commercially. Perhaps I could just cut down a hand guard cap and use that instead. |
|
Quoted:
Thanks for all the replies. Would removing one of the screws and using an automatic center punch to mark the barrel through the hole be a bad idea? I am concerned that the punch would raise too much metal up around the mark and make it difficult to remove the gas block. I don't want to dimple the holes with the gas block in place for fear of damaging the threads in the gas block. I like the laser cut spacer idea, but I don't really have enough rifles to make it worthwhile. Perhaps someone has already thought of this and sells them commercially. Perhaps I could just cut down a hand guard cap and use that instead. Rather than use a center punch, use a scribe and mark the perimeter of the screw hole so you don't raise a dimple. As far as the space between the gas block and the barrel shoulder, I wouldn't sweat it- just put it where you think it matches close to the thickness of the handguard cap. You'll also have your screw hole location to go by. |
|
If you did enough rifles to make a laser cut spacer worthwhile you could eyeball it.
I just lucked out, the owner of a shop the does laser work for us likes guns and volunteered to burn some spacers out of scrap if I provided the specs. Any other time and a low production item like that would be very expensive. I would say just use a feeler gage or maybe cut the cap in half and make a gage. Use a center punch and make a very light mark in the barrel until you pull the gas block. |
|
I just free-floated my A2 with an RRA National Match free-float tube kit. Keep in mind that there needs to be a little space between the free-float tube and the gas block (in my case, 0.020" to 0.030" between the free-float tube's handguard ferrule and the A2 sight tower). You don't want the handguards to bear against the gas block or sight tower, as that would defeat the purpose of free-floating it in the first place.
In your case, I bet that, if you remove one of the set screws, you will be able to detect a mark on the barrel where the set screw was originally tightened down. Just line it back up the same way. |
AR Sponsor