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Gunsmithing

The 1911: Bo-Mar Rear Sight Installation

by Gus Norcross   |  August 5th, 2013 1

After a couple decades of 1911 work, I can say say that the most popular adjustable rear sight for 1911s is the Bo-Mar. I use the name “Bo-Mar” to identify the particular style rather than a brand, since the Bo-Mar company has been gone for years.

Bo-Mar sights are square with a prominent rear blade popular with bullseye shooters and an elevation screw that extends through the sight into the slide.

With the demise of the original company and demand still high, several manufacturers released copies of this classic sight. Some are good, some not so good.

Here I’ll demonstrate installation of Ed Brown’s version of the Bo-Mar, blending it into the slide in the popular “lo-mount” position. The Brown sight (Brownell’s 087-000-050) is American-made and may be better than the original, with a nicely staked hinge pin and very positive click adjustments for the windage and elevation screws.

This operation requires a milling machine and basic knowledge of machine tools, but you don’t have to be an old school tool and die maker to get it right.

The installation can be broken down into four machine operations: milling the slide to accept the sight body, cutting a pocket for the rear blade, the dovetail cut and drilling and tapping for the elevation screw. The original rear sight, firing pin stop, firing pin and spring should be removed from the slide.

The first task is to level the slide in the mill vise and touch off on the breech face with an edge finder and zero the readout or dial. Start the cut 1 inch to the rear of the breech face and mill off all the material to the depth of the original rear sight dovetail, which in this case was about .110″. I used a 1/2″ carbide cutter. Carbide tools are recommended when milling  modern slides, which can run over 40 RC in hardness.

The second operation is the clearance pocket for bottom of the sight leaf at the rear end of the slide. This cut will be about .140″ deep and a 3/16″ radius at the front of the pocket will match the contour of the sight for a custom look.

I use a 1/2″ carbide end mill that was custom ground by my local tool grinder KV Tooling Systems (kvtooling.com) with a 3/16″ radius. Mill forward from the rear of the slide to a position 2 inches behind the breech face.

The third operation is the dovetail cut. We will use a carbide .359″x 60 degree dovetail cutter (Brownell’s 080-621-060), set to the depth of the sight dovetail (.100″).

Measure the sight carefully and determine the location of the center of the dovetail from the forward edge of the sight (.467″ in this case) and add 1 inch. With the cutter 1.467″ back from the breech face and lowered .100″ below the cut from the first milling operation, go straight across through the slide, keeping the cutter flooded with oil.

Next, hand-fit the sight to the slide. The dovetail cutter is a few thousandths smaller than the sight, allowing a custom fit to the slide. The sight should be tight enough that a punch and small hammer are required to position it, basically a press fit. To fit the dovetail, a 60 degree sight base file will be needed ( Brownell’s 080-648-160). Once the sight is fitted, center it in the slide.

The last machine operation is drilling and tapping the hole in the slide for the elevation screw. Center a #21 drill in the elevation screw hole and spot the slide through it. Then switch to a #31 drill and drill all the way down into the firing pin tunnel so you can see the drill break through when looking through the tunnel from the rear. Tap the hole full depth with a sharp 6-48 tap and install the elevation screw and springs.

One tip here: elevate the forward end of the slide slightly when drilling and tapping the elevation screw hole so the screw head is level with the surface of the sight leaf. Otherwise, the elevation screw may bind a bit.

Aside from refinishing the slide and checking zero, we’re done.

  • todd c

    Very well done! On a separate note, when will the final article (Part 5) about fitting the 1911 bushing be available? Thanks!

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