MHA Custom Universal Stoning Fixture

Cleaning up afirearm’s sear engagement surfaces requires careful attention in order to avoid damaging or changing critical angles. While a jeweler’s visor, good lighting and high-quality stones are vital tools for the job, relying on steady hands alone for proper alignment can lead to excessive metal removal and/ or surfaces that have been stoned unevenly or at incorrect angles. The result may be a trigger is no better than before you began or, even worse, inadequate sear engagement and an unsafe firearm.

Having a fixture that presents the engagement surfaces in parallel alignment to their pin bores for stoning and polishing is a helpful way to prevent these problems. The result is a savings of time, effort and the potential cost of replacement parts. I recently came across anew universal stoning fixture from a very talented gunsmith in North Carolina that makes hammer and sear work much easier and more uniform.

Mike Allen of MHA Custom, LLC (4t5acp.com) has been building custom handguns for sixteen years, specializing in the 1911 family of semi-automatics and tactical handguns in general. He’s a graduate of the very well-regarded Gunsmithing program at Montgomery Community College in Troy, NC (montgomery. edu/acgunsmith.htm) and has heavy custom and general gunsmithing experience under his belt. Mr.

Allen’s Universal Stoning Fixture was originally developed to deal with the very heavy trigger system found in the FN FAL rifle but it was soon apparent that it was ideal for correcting the troublesome hooks of the 1911A1’s hammer, where it is not uncommon to find one hook doing all the work against the sear due to uneven contact. However, as the "Universal" in the name implies, the fixture’s application far exceeds one model or class of firearms.

My interest in the MHA fixture was for work on the trigger components of various non-AR tactical rifles Ifi nd myself accurizing from time to time. Unlike the many drop-in, aftermarket triggers available for AR rifles and carbines, most semi-auto sporters and surplus rifles require careful work to smooth out their heavy, creepy, "Mil- Spec" triggers. I used the MHA Custom fixture on both handguns and rifles and found it to be a worthy tool for trigger work.

I’ll admit up front that I approached it with a minimal level of experience in this arena. Aside from working a handful of trigger groups, the bulk of my custom work involves building and accurizing ARs. Due to the surface-only hardening of sear engagement surfaces on most stock AR triggers, stoning them is not normally advisable.

Though minor polishing and match springs will usually clean one up and lighten overall pull, the myriad of drop-in replacement triggers largely eliminates most AR trigger work of this type. So while I carried little baggage into this evaluation, I also had no other fixture work to compare it to.

This hammer from a pile of old M1A parts shows uneven wear of the sear notch prior to stoning. This MHA Custom Universal Stoning Fixture is designed to hold a hammer or other trigger group component in place via a gauge pin placed through the component’s bore hole and locked in place within V-notches cut into the fixture’s slide under a pair of retaining rods.

MHA Custom includes a complete set of precision gauge pins with each fixture as part of the purchase price, which I found greatly eased the burden of figuring out what would work best for my applications. The included instructions easily coached me through proper setup and use, so I’ll give a thumbs-up for clarity, completeness and the ability to teach even a rookie to succeed.

Multiple adjustments—gross and fine— allow movement of the firearm’s component up or down in order to align the work surface with the level top of the fixture. I found it easiest to place the fixture on my tallest workbench where I could easily view it from the side to ensure alignment. Though the metal base assembly wears a protective back finish, the top surfaces will lose their coating as you work because they’re designed to support the actual stone on both sides of the work piece.

The two sides of this top surface pro- Two brass thumb screws apply spring tension to the gauge pin rods to retain the component while the fine adjustment knob provides incremental up/down adjustments to bring the work surface level with the top of the fixture.

While the provided screw was too long for most parts I stoned for this article, I plan to add a couple of shorter screws to my kit for pieces needing an extra degree of stability. vide a perfectly level plane to guide the removal of metal without changing the engagement surface angle of the component being stoned. The use of this top surface is what removes the error created when stoning by hand and eye alone. When setting up to true a component, once I saw the

The small size and limited engagement surface of this component make honing by hand a tough process, serving as an example of the MHA fixture’s usefulness. work surface was close to even with the top surface of the fixture, I placed the stone on top of the fixture and above the hammer notch to fi nalize alignment by adjusting the fine adjustment knob in small increments until the work surface was perfectly level with the top of the fixture. This process takes only a few seconds.

I started with a scrap 1911A1 hammer just to get my feet wet. I had several more old hammers to continue familiarizing on, including one each from a Browning Hi power and an M1A rifle. While the engagement surfaces looked good when viewed under magnifi cation before and after stoning, visual inspections alone didn’t confirm how they would feel in afirearm, so more testing using trigger components from working firearms was next on the agenda.

I had previously done some spring work on a S&W Model 638 revolver but the double action pull was still fairly gritty so that seemed like a reasonable place to start. Prior to working over the hammer in the MHA fixture the single action pull was 33 ounces. After truing and polishing the hammer notch the weight was reduced to a smooth 24 ounces. I felt that was pretty good for my first real try.

Since my primary work centers on the aforementioned tactical semi-autos, I next worked over a pair of rifles from the HK-91 family. I’ve found that HK91 variants and clones to have some of the most lawyer-proof triggers available. Since replacing a trigger group with a match MSG-90 or PSG-1 trigger pack costs $800+ for parts alone, stoning and spring work are the only affordable solutions I know of. The first trigger group was from an HK91.

I was able to reduce much of the gritty second stage in this trigger but I wouldn’t consider it to be perfectly crisp. After stoning the hammer notch and sear engagement surface the second stage weight was reduced from seven pounds, fi ve ounces down to fi ve pounds, thirteen ounces. This is still heavy in my book but without additional spring work this was the best I could achieve and still far better than stock. The second trigger group came from a PTR-91, which is a very wellbuilt, US-made, HK91/G3 clone.

Starting weight for this trigger’s second stage was much better at fi ve pounds, nine ounces. After stoning the hammer and sear, second stage weight was reduced to four pounds, two ounces. Additional spring work on this trigger group reduced the weight down to three pounds, six ounces but that was done independent of the fixture. Again, the gritty feeling and stacking of the second stage was also nearly eliminated.

Since one of the recommended uses for this fixture is to remedy the difficulty in properly stoning 1911A1 hammer hooks, I couldn’t finish the evaluation of this handy tool without tearing into one of my own 1911s. I have a very well-used Kimber Ultra CDP that has suffered from a lessthan-stellar trigger since I bought it. Once my daily concealed carry handgun, it’s now relegated to duty as a demonstration piece for shooting drills when I conduct concealed carry certifi cation courses.

Cleaning up the trigger has been high on my "need to do" list for quite awhile. When selecting the proper gauge pin for the fixture, it’s important to use the one that fits tightest without having to force it into the bore hole. I found the 0.157 diameter gauge pin fit the Kimber’s hammer pin hole best. Again the 1911A1 hammer’s legs were easy to hone evenly. A few passes of afine India stone followed by light polishing cleaned the once (Continued on page 22)

creepy trigger up very well. After reworking both hammer hooks and the sear’s engagement surface, also made easy by the MHA fixture, the trigger was reduced from a creaky fi ve pounds, two ounces to a much smoother four pounds, four ounces. The actual stoning process with these parts occupied only fifteen minutes of my time! No doubt the ability to use the fixture’s top surface as a guide allowed me to work much more effi ciently than if I had tried this freehand.

Though not mentioned in the manual, the MHA fixture could be used to change the original hammer or sear angle to give either an overcocking condition or, conversely, a rolling let off. This could be accomplished by simply raising or lowering the slide from the position identifi ed as mirroring the original angle. Care must be taken with either of these procedures to ensure the correct angle is maintained and that an unsafe condition is not created when the firearm is reassembled.

Do not attempt this type of hammer or sear adjustment unless you can do so safely. When I started this project I planned to simply evaluate the MHA fixture and return it to Mike Allen with some novice-level feedback, for whatever that might be worth. Once I saw the utility and multiple applications for this tool, the decision to purchase it was easy to make. Having ruined apart or two over the past couple years through inexperience, shaky hands, or just plain being a dummy, I need every bit of help I can find.

I believe the very reasonable purchase price of $350 will more than pay for itself in the years to come, especially as I discover new uses for the MHA fixture. AG