Replacing a Front Sight Base...

[Quarterbore]

Issues with replacement (Overview):

Replacing the AR-15 gas block (or front sight base) comes with issues as the gas block is mounted to the barrel and drilled during assembly. This means that the gas block is placed onto the barrel and drilled through the gas block and barrel. The holes are then reamed to size with a tapered reamer and special tapered pins are installed.

Removal of the AR-15 Gas Block:
To remove the gas block it will be first necessary to remove the two tapered pins from the front sight base. This is done by driving the taper pins out with a pin punch. The tapered pins are removed by driving from left to right; the small end is on the left side of the barrel, the large end is on the right side! It will often require considerable force to get these pins out but with olid blows in the correct direction they will come free! After the pins are removed the gas block will slide off the barrel.

Replacing the gas block:

Because the gas blocks are drilled while mounted to the barrel the holes on two different gas blocks are not likely to have their holes drilled in exactly the same location or angles. The place to start is by installing the new gas block to the barrel and look at the alignment of the holes through the front sight base. With luck, you will be able to see the holes in the barrel through the holes in the new gas block.

From there, I like to tap the hole on the bottom of the gas block for a small set screw and take the weapon to the range to confirm that the cant of the front sight is correct. I am sure this could also be done with loc-tite but with the set screw it is easier for me to make adjustments and keep the gas block from moving when I have it where I want it. I will also scribe a small mark on the barrel.

To install the new gas block, drill out the taper pin bores. I have not done a gas block replacement in years but I believe I used a 3mm drill bit but what is important is to try to use a bit that is about the same size as the original "SMALL" hole in the original gas block and barrel. After the hole is drilled, ream the hole with a taper pin reamer until the newly reamed hole is deep enough to allow the taper pins be fully engaged and drive the taper pins back in.

Where to buy the tools:

Brownells:

Front Sight Base Taper Reamer: PN: 080-726-004
Front Sight Base Tapered Pins: PN: 080-725-001

http://www.brownells.com/aspx/NS/sto...il.aspx?p=5613

[Quarterbore]

Note, it should be pointed out that in the directions above I had used a used FSB that was already drilled... I need to mount a bayonet lug FSB on my AR-10 soon so I may try incorporating some of the advise from the following link when I do it next time!

Originally Posted By Extracheeze:
http://ar15.com/forums/topic.html?b=...207&page=1

Very Carefully. I have been a toolroom machinist since I can remember, so I have an AWESOME set of tools. If you do not know how to chase a thread to a shoulder or bench out a taper hole Do Not Attempt This Yourself!

First, I memorized this webpage: Replacing AR15 / M16 sights

Since I dont have a Bridgeport with digital readouts in my shed, I did it by hand.

"Slip the replacement base on and check for the alignment of the holes." My new FSB had no holes in it from the start, (Special Thanks to Yankee Hill Machine) so this step was tricky. What I had to do was make sure the gas hole lined up properly. With the original FSB still on, I scribed a tiny circle on the barrel in the hole under the sling swivel pin. Then all I had to do was visually line up the hole on the bottom of the new FSB when loc-titeing it on. Double check the sight base alignment from varying angles and line up the FSB if it leans to one side or the other just a bit. I screwed a tap into the sight post hole so it was sticking up to help make sure it is as straight as humanly possible. No second chance. Do it kinda snappy too cause the loc-tite gets sticky pretty quick. Allow the loc-tite to set over night

"Once the loc-tite cures, re-drill out the taper pin bores and re-ream, manually" I measured and scribed little x's on the new FSB to match the pinhole locations of the original FSB. I did one hole at a time, not drilling the front hole out till I had the pin seated in the first hole. Drill through from each side starting with a .050" drill bit. Use EXTREME caution so as not to break the bit when it breaks through the inside of the hole. Then on the BIG side of the pinhole you drill it out to .075", so you can see the .050" hole to determine how off center it is on the small end of the taper. Hopefully it is pretty close, since you took an hour measuring the locations you laid out and reassuring yourself it was going to be OK. Whichever way the hole is off center, widen it in the opposite direction using the side of the spinning .050" drill bit like a power file. You can now look through the little hole and do the same to the big hole with the .075" drill bit. Once you get your holes roughed in to center you can start stepping up the drill size drilling both holes from the same side to ensure proper alignment, till you get to around .110"-.115" MAXIMUM on the small side of the hole, which will alow you to start using your size #2/0 taper reamer for final hand fitting of the pin.

For a couple days after I did this, while waiting for a lathe, this rifle looked confused. Bayo Lug, No Threads. I finally threaded it using a lathe in a good friend's machine shop. He wanted to talk me into chucking it up in the CNC and running a thread program, I did it the old fashioned way on a South Bend lathe. I used .496" as maximum thread diameter, hitting on the high end of the thread specification which calls for .492"-.498" diameter, starting off with .620" Overall Thread Length so it was on the low side of blueprint specification of .630" +- .010". I faced off the shoulder just a bit to index the flashider and ended up at .630".

Later, at the range, I set it to mechanical zero and it hit paper right where it should have, which for me is a little low and a little to the left. I made the appropriate clicks for windage and elevation to dial in my personal zero and Bingo! Special Thanks Also to BrightFlashlights for the non-eye-poker-out Phantom. Looks Good on there, Don't it.