Installing Blended Adjustable Sights on Auto-Loading Pistols

The selection of aftermarket adjustable rear sights for large-caliber auto-loading pistols is quite extensive. Most of these sights area worthwhile investment, while a few represent a questionable compromise between price and quality. Of course, the more you know about which sights work best for the intended purpose, the better you’ll be able to help your customer with his choice. Choosing A Sight Which sight would be most suitable for a pistol with the recoil energy of a .45 auto shooting hardball ammo?

If the pistol is fed a steady diet of the same ammo, a solid, fixed rear sight will serve just fine once the sights have been targeted to the particular ammo. For experimenting with various bullet weights and different powder charges, impact will shift and the fixed sight may not be the best choice. A regular diet of 230-grain hardball ammo has been known to destroy some of the wimpy adjustable rear sights that are made to fit the factory dovetail.

Granted, recoil springs are available in a multitude of weights to produce the desired recoil absorption, but slide-return velocity can take its toll on a rear sight that’s not securely attached to the slide. Bo-Mar, STI, Wilson, and Pachmayr all market a very sturdy, accurately adjustable, rear sight that can be "melded" into the back end of the slide with a few milling cuts. When done properly, this sight will stay in place and service the guns owner for any available loading of choice.

These sights will work on any pistol slide that has at least .225 inch of material above the firing-pin tunnel and at least 1.500 inches from the face of the breech to the rear face of the slide. Getting The Measurements All of the sights previously mentioned are very similar in design with slight variations in dimensions. The example I’m using here involves a 70-Series-style, stainless-steel slide and the Pachmayr adjustable rear sight with an overall length of about 1-1/2 inches.

The distance from the front edge of this sight to the widest edge on the front of the dovetail is .433 inch as measured with a depth-recording stem on a digital caliper. The width of the dovetail, front to back, along the bottom (widest part) measures .374 inch. Adding half the dovetail width to the .433 dimension, we get a nominal dimension of .620 inch. Record this dimension for future reference.

Another dimension we need requires that the sight be taken apart by drifting the spring roll pin out of the front end of the sight. Measure from the front edge of the top half of the sight, just in front of the roll-pin hole, back to the shoulder just in front of the windage-adjustment screw. Record this dimension. In this case, the dimension is 1.036 inches. The last di- 000 inch back from the breechface.

mension we need is the depth of the dovetail’s bottom face to the inside bottom face of the sight base, which is almost always around .100 inch. Setup for Milling Once you’ve safely disassembled the slide from the frame and removed all internal parts including the original rear sight, you’re ready to set the slide up in the milling-machine vise. When doing this work, I use a Wessinger slide-holding fixture. This fixture has rails that the slide is mounted on similar to those on a 1911-style frame.

Two set screws are turned from the bottom of this fixture and tighten the slide down onto the fixture. Before I had this fixture, I made up apiece of bar stock .099-inch thick, .750 wide, and 6 inches long that slid into the slide-rail cutouts in the slide. The outer sides of the slide were then taped to prevent any surface scratching. This flat stock will prevent the slide walls from pinching together, which could ruin the slide-to-frame fit.

With either fixture, the slide is checked with a machinist’s level to verify that the top of the slide is dead level. Using the depth-recording stem on your caliper, measure from the upper, top face of the slide down to the bottom of the dovetail cut that held the factory rear sight. This dimension should be around .093 inch or so. For the first milling cut, a 1/2-inch, four-flute carbide endmill, run at around 300-350 rpm, will get the re-

quired metal removed in short order. Blacken an area with a felt-tip marker exactly 1.000 inch back from the breechface of the slide if you are dealing with a 70-Series-style slide. Using the jaws of your caliper that are used for recording outside dimensions, set the distance at 1.000 and scribe a line from the breechface back and into the area just blackened with the marker.

When dealing with the 80-Series style, this distance would need to be 1.100 inch to account for the firing-pin safety built into these slides. Begin milling off the metal from this line back toward the back face of the slide. For me, it works best when I take cuts of around .020 inch while flooding a lot of cutting oil onto the cutter and the area being cut. The oil is directed onto the cutter with a handoperated pump-oiler.

This helps to flood chips away from the cutting action and extends the life of the tool by not allowing it to get overly hot. Because the Wessinger fixture is holding the slide parallel to the longitudinal axis (left to right) of the mill, I use the cross-travel dial to get this part of the job accomplished by making the cuts across the slide. Once you reach the bottom of where the factory dovetail has been cut, quit with the 1/2inch endmill. We’re now finished with the first required cut.

The next milling cut will be made to remove material to lessen the burden for the dovetail cutter. Wipe the previously milled area clean of any oil and use the black felt-tip marker to color an area around 5/8 inch back from the shoulder we just created with the 1/2-inch endmill. This would be from where we marked our first layout line. To make this mark, use the inside recording jaws of your caliper resting one jaw against the front shoulder while scribing the line with the other jaw.

This line will be the center of the dovetail distance from the front end of the sight base. At this point, a center finder (or as some call it, a pointed "wiggler") is used to line up the quill with the scribed line. Replace the center finder with a 7/32-inch (.218) endmill. Again, I prefer a carbide four-flute cutter for this operation. This endmill is used to remove the bulk of metal, lessening the burden for the dovetail cutter.

Feed the 7/32 cutter down and across, removing .020 of material at each pass while pumping cutting oil on the cutting action to help remove chips. We need to feed this cutter down until we reach the measured dimension we recorded earlier of the bottom of the sight dovetail to the inside bottom face of the sight base, about .093 to .100 inch. When you reach this depth, measure with your caliper to verify the dimension. It’s better to be .001 or .002 inch short than it is to be too deep.

Once the cutter reaches our target depth of cut, the 7/32-inch endmill is replaced with the dovetail cutter to prepare a resting place for the new sight. It’s obviously very important that you not move the left/right dial of your milling table. For the next part of the job, I again prefer the carbide, four-flute cutter with a 60-degree-included cutting angle and a diameter of .359 inch. Don’t take the .359 inch diameter on the bottom of the dovetail cutter as literal; measure it yourself with a micrometer.

The first time I did this job, I used a high-speed steel cutter of the same dimensions, mainly because it was cheaper in price. Throughout the cutting process, I worried that the cutter would shatter and ruin an expensive Colt slide. The cutter made it through the job just fine, but I now use the carbide cutter because it cuts more freely into these hard slides.

I recommend that this cutter be run at not more than 600 rpm, but 300-350 rpm works fine for me with a slow feed, and again, flooding with lots of cutting oil. With the dovetail cutter tightly colleted in the mill and with the motor off, ease the cutter down until one of the cutter’s bottom faces touches the slide where the cut from the 7/32 inch endmill ended. Lock the quill in this position and move the milling table toward you, which will move the cutter back and away from the

Right top: Cutting scallops along the sides of the sight with a 1/4-inch ball endmill adds a nice cosmetic touch. Right bottom: Here, the job is finished with the sight in place, ready for business. slide. Turn the motor on and feed the carbide cutter into the 7/32 milled cut slowly while pumping oil onto the cutter and just ahead of the cutting action until the dovetail cutter comes through the right side of the slide.

The first pass with the dovetail cutter will probably not provide the needed fit for the sight base. Remember our earlier measurements? The bottom of the sight dovetail measured .374 inch and the bottom of the dovetail cutter measures about .359 inch or whatever dimension you measured it to be. The one I have measures .356 inch, so I need to subtract the actual cutter dimension from the .374 sightbase dimension to see how much material remains to be removed.

As an example, lets say we have around .018 inch left to remove. There are two options we can look at: 1) We can use the cutter with a dial indicator setup to mill the excess material away one or two thousandths of an inch at a time until we reach the fit we desire; or 2) we can mill material away until we have a few thousandths of an inch left and then fit the sight into the dovetail by means of a dovetail file.

Whichever method suits you, keep trying the sight base after each cut until you reach a fit whereby the sight base will almost begin to enter the dovetail. Try tapping the base in with a plastic mallet gently to make any high spots on the sight base more visible. Swipe these areas lightly with a triangular dovetail file until the sight base goes in with help from the plastic-faced mallet. The last milling cut we need to make is for the top half of the sight that holds the rear sight blade and windage adjustment.

For this part of the job, a 3/16-inch ball endmill will work just fine, but I prefer to remove most of the metal with the 7/32-inch carbide endmill first, and then blend in the radius with the 3/16 ball endmill. This time, a two-flute endmill works best, as it isn’t as likely to load up with chips as a four-flute would. The bottom front of the rear sight blade housing has a 45-degree chamfer at the bottom of the front face.

This can certainly be left as is, but it looks better to use a narrow pillar file to blend the corners of the 45-degree angle into a radius that fits the 3/16inch endmill cut. The depth of the area to be cut will need to be the same as the measurement taken from the bottom of the sight-blade housing just below the windage-adjustment screw to the inside bottom face of the top half of the sight.

Reassemble the top to the bottom half of the sight, and tap it into place with a plasticor rawhide-face mallet until you have it "eyeball" centered. Using a depth micrometer or the depth-recording stem on your caliper, measure from the side of the slide to the side of the sight base on each side of the slide. Tap the sight base over with the plastic hammer until you get an equal reading on each side. Once this has been achieved, lock the sight into place with the set screw(s).

Depending on which sight you are installing, find a drill bit that will slip fit into the tapped hole in the sight base for the elevation screw. Twirl (continued on page 18)

this drill with your fingers to establish a witness mark for spot-drilling. Remove the sight and drill the spot with a #31 drill, being careful to stop just short of breaking into the firingpin tunnel. The Pachmayr sight has the sight base tapped at #6-48 for the elevation screw, so if you are using this sight, the hole just spotted does not need to be tapped but should be drilled with a #29 hole to allow the elevation screw some clearance here.

The firing-pin stop plate will need to be reduced in height after this cut is made. The easiest way to measure how much it needs to be reduced is to reassemble the stop plate in place along with the firing pin and firingpin spring, and then scribe a line along the back face of the plate. Remove it again and then either file, grind, or mill away the top so that it won’t interfere with the downward adjustment of the sight.

There are two additional milling cuts that can be made with the 1/4inch ball endmill, but these are purely cosmetic additions and don’t contribute to the functionality of the sight at all. Both cuts are made along each side of the sight base to create a transitional radius that blends the look along the side of the sight into the slide. On a stainless-steel slide, all that’s necessary now is to bead blast the milling cuts with #270 glass beads for a satin finish.

If the slide is blued steel, it will now need to be blued. Conclusions If you do much work on the 1911style auto and have access to a milling machine, installing these sturdy sights will be welcome news for some of your customers needing this type of service. The things to remember are, measure often and use lots of cutting oil while feeding the cutters slowly through the work. I