Skip to main content

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.

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended Articles

See More Recommendations

Popular Videos

Leupold Freedom RDS

Leupold Freedom RDS

Patrick Sweeney reviews the Leupold Freedom RDS optic with bullet drop compensator!

Shooting the 20mm Lahti

Shooting the 20mm Lahti

David Fortier had the chance to fire the Lahti L-39 while filming the 2018 season of Guns&AmmoTV while on location at DriveTanks.com.

3 Ring Silencers

3 Ring Silencers

Patrick Sweeney heads to the range to look at suppressors from 3 Ring Silencer Corporation.

Firearms News Reviews the S&W M&P 2.0 9mm Compact Pistol

Firearms News Reviews the S&W M&P 2.0 9mm Compact Pistol

Firearms News editor Vince DeNiro runs the Smith & Wesson M&P 2.0 9mm compact pistol through a self-defense drill.

See More Popular Videos

Trending Articles

This article will walk you through the process of building a Glock pistol from an 80 percent frame.How To Build a Glock From an 80 Percent Frame Gunsmithing

How To Build a Glock From an 80 Percent Frame

Gus Norcross - August 13, 2020

This article will walk you through the process of building a Glock pistol from an 80 percent...

Developed to replace the venerable AK-74, the Blow-Back Shifted Pulse AN-94 Nikonov is a fascinating hyper-burst design which faded away after the death of its designer.Russian 5.45x39mm AN-94 Nikonov Review Rifles

Russian 5.45x39mm AN-94 Nikonov Review

David M. Fortier - August 14, 2020

Developed to replace the venerable AK-74, the Blow-Back Shifted Pulse AN-94 Nikonov is a...

The Masada is IWI's first striker-fired pistol and is an accurate, good looking polymer handgun that has a great trigger.IWI Masada Pistol Review Handguns

IWI Masada Pistol Review

Bob Taubert - August 19, 2020

The Masada is IWI's first striker-fired pistol and is an accurate, good looking polymer...

Virginia Republican Rep. Glenn Davis introduced two pro-2A measures in a special session that are designed to partially roll back some restrictive laws.Fighting Back in Virginia 2nd Amendment

Fighting Back in Virginia

Mark Chesnut - August 24, 2020

Virginia Republican Rep. Glenn Davis introduced two pro-2A measures in a special session that...

See More Trending Articles

More Gunsmithing

On the bench in front of me was a customer's M&P Shield he'd dropped off complaining about theApex M&P Trigger Kits Gunsmithing

Apex M&P Trigger Kits

Gus Norcross - July 11, 2014

On the bench in front of me was a customer's M&P Shield he'd dropped off complaining about...

AGI's Certified AR-15/M4 Riflesmith Course includes DVDs with Complete Disassembly and Reassembly,AGI AR15/M4 Riflesmith Course Gunsmithing

AGI AR15/M4 Riflesmith Course

SGN Online Editors - July 06, 2014

AGI's Certified AR-15/M4 Riflesmith Course includes DVDs with Complete Disassembly and...

Lone Wolf Distributors are now offering the Advantage Arms .22 Conversion Kit for Glock pistols.Lone Wolf Advantage Arms .22 Conversion Kit Gunsmithing

Lone Wolf Advantage Arms .22 Conversion Kit

SGN Online Editors - April 16, 2014

Lone Wolf Distributors are now offering the Advantage Arms .22 Conversion Kit for Glock...

 Hobby gunsmithing will help you develop a variety of mechanical skills in shaping parts, attachingBasic Gunsmithing Skills Gunsmithing

Basic Gunsmithing Skills

Steven Matthews - April 07, 2016

Hobby gunsmithing will help you develop a variety of mechanical skills in shaping parts,...

See More Gunsmithing

Magazine Cover

GET THE MAGAZINE Subscribe & Save

Digital Now Included!

SUBSCRIBE NOW

Give a Gift   |   Subscriber Services

PREVIEW OUR CURRENT ISSUE Arrow

Buy Digital Single Issues

Don't miss an issue.
Buy single digital issue for your phone or tablet.

Buy Single Digital Issue on the Firearms News App

Other Magazines

Special Interest Magazines

See All Special Interest Magazines

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Phone Icon

Get Digital Access.

All Firearms News subscribers now have digital access to their magazine content. This means you have the option to read your magazine on most popular phones and tablets.

To get started, click the link below to visit mymagnow.com and learn how to access your digital magazine.

Get Digital Access

Not a Subscriber?
Subscribe Now