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Autumn has come, but it looks like my summer carry will be with me for a few more weeks. Pictured below is my P229 which has been milled for a Trijicon RMR. I removed the RMR to replace the battery, and when I did so, both the mounting screws broke in half. You can see the bottom halves still embedded in the slide. I removed them with a torque driver, so I know exactly how much force I used and it wasn't excessive: 25 in-lbs. Seems the screws just failed under the impact of the 2k or so rounds I've put through the gun since the optic was mounted.

I can't even use the gun with the backup sights now, because of that spring you see sticking out. That's the safety spring, and it needs something to press against — either the base of the optic or the top of an unmilled slide — for the internal safety to operate properly.

It's going back to the gunsmith to mill out and replace the broken screws. Sigh.
 

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Dang! What a pain.

Was a strong thread lock used on those tiny screws?

It may have been just the mass of the optic applying shearing forces to the screws. Then the torque of turning them finished them off.
 

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Yes, there was some kind of Loc-Tite on there. But like I said, I didn't use a whole lot of force. The screws have to have already been compromised somehow.
 

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OP,
It may be the light,but what was that slide machined with a chain saw perhaps?

Allen head screws are Grade 8 fastener.

Torx,depending on where they were sourced(China) at best are a Grade 5 fastener.

Have the screws removed by milling w/ an end mill or you'll have a mess to repair.
 

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Can you send it back to the place that milled it?

I would be concerned about a local smith making it worse. But you may have some good smiths in your area. Where I live, I wouldn't trust any of them.

I have an FNH FNX-45 that has the safety stop spring held in by the rear sight. Had I not searched to see which way to remove the sights and stumble across that "feature", I might have lost the spring. Man, trying to hold that sucker in place, while cranking on the new sight with the sight pusher, and not bending/crushing the spring with the sight, was a serious pain in the butt!! It is a small diameter spring.
 

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Discussion Starter #6
My local smith is the guy who milled it. He's an 07 and has a pretty well-equipped shop.
 

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I’m not very familiar with mounting an RMR, but, many optics rely on pins, a recoil lug, or a snug milled body fit to the optic or mounting plate to help take the recoil stress off of the screws alone.
 

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Wow, that sucks. I’ve never seen that before. I’m new to pistol optics and just sent my 229 slide out to be milled for an rmr.
 

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OP,

Allen head screws are Grade 8 fastener.
Not all Allen head screws are Grade 8 fastener.

I had been using grade 12.9 metric Allen screws (M3, M4, M5 and M6) for my RC truggies and buggies for RC racing. Since I have a lot of these different screws, I also used them for my optics especially the slide mounted ones for my X5 and shadow 2.

I bought all of my 12.9 metric Allen screws from boltdepot.com. I bought boxes (100 screws) in different sizes many years ago and still have a lot of them since I don’t race RC anymore.
 

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Discussion Starter #12 (Edited)
Do you mind if I ask who did your milling?
I'm going to hold off on answering that. I'm currently trying to determine whether it might have been done wrong (e.g. pocket not deep enough), and if that turns out to be the case then I want to try to get it resolved amicably first before I start dumping all over him on the internet.

EDIT: Now that I've exonerated them: it was Saxonville Armory in Framingham, MA.
 

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Metric & SAE hardware do NOT have the same properties-
meaning a 12.9 grade metric fastener is NOT an SAE Grade 12 or even a Lab 9 grade fastener.

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I understand that. In my materials science and metallurgy classes back in the day, we used to argue about the different grading for fasteners.

In the end, it’s was proven that grade 8 is similar to 10.9. Along those lines, we also proved in our class that grade 12.9 metric screws are stronger and more durable than grade 8 fasteners.

OP, what screw were used in the installation of your optic? I would also recommend installing bosses to mitigate the shear forces acting on the screws when the slide moves fore and aft.

Good luck.
 

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Discussion Starter #14
I understand that. In my materials science and metallurgy classes back in the day, we used to argue about the different grading for fasteners.

In the end, it’s was proven that grade 8 is similar to 10.9. Along those lines, we also proved in our class that grade 12.9 metric screws are stronger and more durable than grade 8 fasteners.

OP, what screw were used in the installation of your optic? I would also recommend installing bosses to mitigate the shear forces acting on the screws when the slide moves fore and aft.

Good luck.
It's a black Torx-head screw. I forget whether it came with the RMR or if the guy who did the milling provided it.

I can't figure out how the bosses would work, geometrically. The RMR has a couple indentations in the front corners but they're spaced wider than the screw holes, seemingly too wide for the slide.
 

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It's a black Torx-head screw. I forget whether it came with the RMR or if the guy who did the milling provided it.

I can't figure out how the bosses would work, geometrically. The RMR has a couple indentations in the front corners but they're spaced wider than the screw holes, seemingly too wide for the slide.
The "bosses" or "locating pins" take the fore and aft shear stress off of the fasteners, whose only purpose then is to hold the optic "down" on the slide. In this case for an RMR the threaded holes would be at position "D" and locating "pins"/"bosses" would be in position "Y".

It looks as if it was attempted (silver "circles") with only 1 to any depth.EDIT: It appears the circled area on the right is your Extractor pivot pin, rather than for a locating pin. From the chart I included, you should be able to tell where the locating pins belong. It could be that your slide cannot accept them. In that case you will likely be plagued with this re-occurring.
 

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It's a black Torx-head screw. I forget whether it came with the RMR or if the guy who did the milling provided it.

I can't figure out how the bosses would work, geometrically. The RMR has a couple indentations in the front corners but they're spaced wider than the screw holes, seemingly too wide for the slide.
Did you use a mounting plate, with the built in pins that match the holes in the bottom of the sight? Just from what I can see in your picture, it looks like the broken screws barely clear the milled surface.
 

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The "bosses" or "locating pins" take the fore and aft shear stress off of the fasteners, whose only purpose then is to hold the optic "down" on the slide. In this case for an RMR the threaded holes would be at position "D" and locating "pins"/"bosses" would be in position "Y".

It looks as if it was attempted (silver "circles") with only 1 to any depth.EDIT: It appears the circled area on the right is your Extractor pivot pin, rather than for a locating pin. From the chart I included, you should be able to tell where the locating pins belong. It could be that your slide cannot accept them. In that case you will likely be plagued with this re-occurring.
That’s the diagram which comes with CZ Customs Multioptic mount. It uses both screws and bosses which come in different sizes. The forward holes are for the bosses which lines up with the holes on the forward (Trijicon RMR) or center/rear (Romeo 1/Romeo 1 Pro/ DPP).

I have two of those Multioptic mounted on my TSO and Czechmate. The TSO has a trijicon RMR in 6.5 Moa while the Czechmate has a Romeo 1 Pro mounted on it.

Here are pictures showing the guns, different size bosses and screws.





Screws and bosses:



Yours didn’t have the two bosses at the front of the RMR to keep it in place. The screws are for mounting of the red dot.

Edit - I removed the trijicon on my TSO so I can take a picture of the bosses’ location. This gives a clear view. I didn’t use the screw that came with the trijicon and Romeo 1 pro and replaced them with 12.9 grade M3 and M4 respectively.
 

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That’s the diagram which comes with CZ Customs Multioptic mount. It uses both screws and bosses which come in different sizes. The forward holes are for the bosses which lines up with the holes on the forward (Trijicon RMR) or center/rear (Romeo 1/Romeo 1 Pro/ DPP).

I have two of those Multioptic mounted on my TSO and Czechmate. The TSO has a trijicon RMR in 6.5 Moa while the Czechmate has a Romeo 1 Pro mounted on it.

Yours didn’t have the two bosses at the front of the RMR to keep it in place. The screws are for mounting of the red dot.

Edit - I removed the trijicon on my TSO so I can take a picture of the bosses’ location. This gives a clear view. I didn’t use the screw that came with the trijicon and Romeo 1 pro and replaced them with 12.9 grade M3 and M4 respectively.
Yes, your photo shows the "locating pins" or "bosses" that locate the position for the RDS.

Again, many 'smiths won't mount certain RDS on certain slides, likely because these locating pins cannot be fit into some slides.

In the OPs case, even his hold down screws will only be allowed a shallow depth on the "right" side, due to the external Extractor location. I know in some cases some of Sigs slides have lightening cuts from below that created problems.

dfranke, I wish you luck getting this ironed out!
 

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Discussion Starter #19
I've more recently had another Sig, a P226, milled for an RMR by Burke. I don't have that gun in front of me because it's currently with my local smith for Cerakoting, but I'll be picking it up this Friday at the same time I drop off the P229. I'll take a look at how the two jobs compare. I don't recall Burke installing bosses either, and I think I'd have noticed if he had, but I'll double-check.

Somebody on another forum noticed that the screw holes on the P229 aren't flush with the milled surface: the lines between the bare steel and the nitron finish cut half-way through the screw holes. This seems wrong to me and could be remedied by milling a little deeper. Again, I'll check how Burke's work compares.

I'm hoping that deeper milling for a flush surface + stronger screws should be enough to prevent this problem from recurring.
 

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I removed them with a torque driver, so I know exactly how much force I used and it wasn't excessive: 25 in-lbs. Seems the screws just failed under the impact of the 2k or so rounds I've put through the gun since the optic was mounted.
A couple points come to mind that don't appear to have been mentioned in the responses so far.

-While it's a sampling of just my own optics milled or optics ready pistols I have or have had, every one I've worked with has recommended either 10 or 15 in-lbs of torque along with either Loctite Blue or Vibratite VC3. Mostly 10 in-lbs. Those flavors include RMR, Holosun 507's, and RMSc on P229, P225, P365XL, Glock MOS plate (Gen5 MOS), CZ OR factory plate, C&H plate for CZ OR, and LTT 92 RDO. Could be wrong but 25 in-lbs to remove seems high. Perhaps over-torqued or a more permanent thread locker was used.

-Regarding bosses or lack of bosses, L&M Precision did the work on my P229 and P225 and they didn't include bosses as part of the milling work. What they do however is measure the exact size of the optic you send along and cut the channel to the exact size needed. Removing or reinstalling the optic, it's a perfect snug fit where even without the bosses the only thing the screws are doing is holding it down. Just because a cut doesn't have bosses doesn't mean the mounting screws are taking on the full load. All depends on who did the work and how well it was done.

Side note - Mark / L&M Precision are outstanding across the board and I can't recommend them enough.
 
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