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How to Convert a Scope Base to 8-40 Screws

How to Convert a Scope Base to 8-40 Screws

Often, gun owners will not bring a firearm to a gunsmith or machinist until it has already been damaged by someone without the skill or proper tools who attempted to modify it and failed. Such was the case with the Ruger 10/22 rifle that appeared in my shop a week ago with butchered scope mounting holes in the receiver. Threads had been stripped, and an attempt at re-tapping them had been made, but the culprit backed off and sought help before any permanent damage occurred.

At this point, I had two options: weld up the holes and re-drill them or drill and tap to a larger screw. Welding would have meant lugging the receiver to my welder and probably refinishing it, so I opted to convert the threads from the original 6-48 size to 8-40. The conversion is simple if you have the proper tooling and we're dealing with an aluminum receiver in this case so tapping it is a breeze.

A milling machine or a drill press with a cross slide table is recommended to keep the holes in a straight line. You wouldn't believe some of the receivers I've seen with botched scope base holes where someone used a hand held drill motor.

A dedicated drilling jig such as the Forster Universal Sight Mounting Fixture works well on traditional bolt actions in a drill press. I use a Grizzly G1007 mill drill in my little shop with excellent results. It's not as nice as a real knee mill but it's also not as expensive and with a digital readout, life is good.

Our scope base was designed for the original 6-48 screws so the screw clearance holes need to be opened up for the larger 8-40 screws and the counterbores for the screw heads will need to be enlarged. Clamping the scope base in the mill vise, I located the first hole with a .146-inch gauge pin and zeroed the readout. The gauge pin was removed from the chuck and replaced with a #19 drill and the screw clearance hole was drilled.

The drill was replaced with an 8-40 Weaver counterbore and the screw head seat was bored so the 8-40 Weaver Torx head screw will sit flush or slightly below flush.

This process was repeated for the next three holes and the distance between them was recorded from the digital readout so the exact hole pattern can be transferred to the receiver. If you are using a Weaver base the hole patterns are available at Brownells.

With the base finished, I clamped the receiver in the vise, located the first hole with a gauge pin and zeroed the readout. The hole was drilled through with a #28 drill and then the drill was replaced in the chuck with a tap guide and 8-40 tap and the hole was tapped for the new thread turning the tap guide by hand.

The process was repeated for the next three holes, moving the table in the pattern we already recorded from the readout. The scope base was then test fitted and the screws had to be shortened so they didn't protrude into the interior of the receiver. Everything fit up nicely in the final installation and probably no one will ever notice the screw heads are slightly bigger than normal.

The chuck was centered on the scope base holes with a .146 gauge pin.
The scope base holes were drilled with a No. 19 drill to fit the 8-40 screws and the pilot on the Weaver counterbore.
The counterbores in the scope base must be opened up to fit the new 8-40 screw heads.
Counterbores for Weaver or fillister head screws are available from Brownells.
Screw heads should sit slightly below flush with the base. You don't want them protruding and potentially interfering with the ring when installing the scope.
The original screw holes in the receiver were drilled with a No. 28 drill.
Then Norcross used a tap guide to keep the 8-40 tap straight.
The completed base hardly looks different.
A bench-top mill like this Grizzly G1007 is handy for scope base and sight work, and its price is hardly out of reach for the average home gunsmithing hobbyist.

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