I have a 20" barrel with iron sights on it and I want to replace them with a railed gas block. I assume that to remove the original block/sights I need to remove all the pins and slide it off. But when installing the new block, what is the best way to assure proper alignment of the gas port and gas block? Thx in advance for any suggestions.

You need to squirt some oil on the two taper pins and let it soak through. Then you need to tap out the pins from the LEFT side as you are lookind down the barrel from the rear. An easy way I do it is lay the barrel across a couple old leather boots to cushion the barrel and tap them out with a hammer and punch.

After that lube up the whole area under the block. You then can try and twist it off towards the muzzle, you might need to hit it with a rubber mallet.

To install the block you need to draw a line from the gas hole on the barrel towards the receiver so you can see where the left/right orientation is. And you need to measure the distance to your new block's hole from the edge of the new block and then measure the same distance on your barrel and mark it. Then simply slide on the new block and line it up with your pencil marks and you should be set. To test it, squirt some Break Free down the gas tube and hold the muzzle down. If it is lined up then the Break Free will run through the tube down the gas hole, and out the muzzle GOOD LUCK!

usually the hole in the gas block is bigger than the barrels gas port so you have room for error.  If you intend to install a front sight or anything like that then the critical thing will be lining that up not your gas port.

Agree with curt, the step in the barrel that front handguard cap rests on should keep you from putting the gas block on too far for it to obstruct the gas port, especially if you keep the handguard cap in place.  Granted, this only applies if the gas port is the same distance from the rear of the new block as it is from the rear of the current front sight base.

If the gas block will have a front sight attached to it and is held on with set screws, before drilling the dimples for the screws, you may want to take the rifle out and sight it in first.  That way you can trial and error the final location of the block for windage and then drill the dimples to make its position more permanent.

Thanks for the input guys. I think I may need to read up further before I get into this. I dont think the screws on the bottom of my block require any kind of dimples, as they appear to be completely clear of the barrel.

If the gas block clamps on instead of being held on with set screws then ignore the bit about dimples.