My Open Gun Keeps Breaking C-More Mount Screws

[DyNo!]

I'm using this mount and the right most screw (and only the right most screw) has broken off for the second time.

This is the screw:

http://www.mcmaster.com/#92210a131/=8qaopj

The first time, I installed the screw and loctited it. I did not over-torque it. It lasted for about 500 rounds and one day, the head snapped off during firing in a practice session and I caught it.

During my conversion from 38SC to 9mm major, I had the smith remove all the screws including the broken one and remount the C-more.

I fired about 200 rounds in a match yesterday and I was wondering why my accuracy got pretty bad on my last stage. I didn't look at the gun until I got home and I was not surprised to see that the screw was broken again.

Am I using the wrong hardness of screw?

Could I be breaking the screw by riding the thumb rest too hard? (I don't think I can break a stainless steel screw with my grip )

The Georgia State match is on Friday for me and I really don't want to shoot production.

Also, what is the best way to remove the broken screw without damaging the threads in the frame? It is loctited so I'd be able to put a lighter to it but after that, I want to be really careful and I'd like to know the best method of getting it out. (Power drill it with a small bit from the other side?)

Edited September 6, 2010 by DyNo!

[Aircooled6racer]

Hello: I can give you 2 option that will work. First use steel screws instead of stainless ones. Next drill and tap 2 more holes on either side of the center screw. You can get the screws from Shooters Connection I had my C-More mount screws come lose and it does suck when you can't hit what you aim at Thanks, Eric

[DyNo!]

I can get steel screws tomorrow - I don't know about drilling and tapping the other holes though.

Could I expect it to stay on with steel screws?

How should I go about removing the broken screw?

Thanks!

[G-ManBart]

I was going to make a cheap shot about the penalties of switching to 9 Major, but I'll save that for later

I would suggest you get a set of mount screws from Brazos. They use the torx bit, which is far less likely to strip, and you know you're getting a screw that's intended for the purpose. I put them on pretty tight...snug them up and then one good twist, with a little blue locktite and they have never loosened up on me (tried several mounts and changed a couple of times).

Just a thought, but if you weren't really getting the other one tight enough, it could have let the mount wiggle a tiny bit, and that could have been causing the failures...or the other screws could be too hard/brittle. It also makes a strong argument for mounts with 5 screws. You won't even notice if one of them loosens up or breaks.

I'd try an EZ out on the broken screw before trying to drill it out. R,

[Aircooled6racer]

Hello: I would take the whole mount off and see if the broken screw is sticking out a little from the frame. I would try a pair of pliers if it is sticking out a little. You may have to heat the screw up a little to loosen the thread locker. If it is not sticking out any I would take it to a machine shop and have them remove it for you. If you have the skill and the machines you could drill the screw out yourself. Make sure that you have both. I have made some money drilling out broken screw extractors and broken drills in engine parts and machine parts. Repairing threads is no fun for you and me as the song goes Hope this helps. Thanks, Eric

[Pat Miles]

Heed Eric's advice. The odds are that the screw broke off flush with the frame. Being able to put the frame in a vise on a milling machine or good drill press will up the odds of removing the remnants without scarring the frame. I've made a few bucks removing broken and/or wallowed out hex head screws from C-More mounts also.

Pat

[DyNo!]

One of the other two screws was loose and the other came out with ease. He had loc-tited them and had probably tightened them.

The third screw is flush with the frame. I'm going to give up trying to shoot it this weekend because even if I did get it mounted and sighted in on time, chances are it would probably make it 3/4 through the match.

On top of steel screws, looks like a I need to go back to a two sided mount or make this one a 5 hole-er.

Poll added - I think it would be easier for me to stick this on the classifieds and get a new mount for this gun.

Edited September 6, 2010 by DyNo!

[yoshidaex]

Once you get that broken screw out, you should be good using steel screws. The steel screws have double the tensile strength as the stainless screws you were using.

Use some loctite, re-sight the gun and you should be good for the entire match.

[Aloha Robert]

All posters have had some good advice.

I think we should hear from the mount manufacturer of that mount for guidance. Perhaps this is a problem others have had with this set up. I dont know the specifics in this case but if its the most rear (aft) screw that is breaking, the cause might be the mounting itself or installation problem. That particular design put a LOT of stress on the aft screw. I assume the manufacture has taken that into account but perhaps the wrong screw is not strong enough.

Edited September 6, 2010 by Aloha Robert

[Aircooled6racer]

Hello: What I have seen on the stainless screws is the threads are cut deep and very close to the head of the screw. This may be causing a stress area that is causing the screw to break. Another possible problem may be the holes in the mount are too large in diameter putting more stress on the screw head. Having only 3 holes doesn't help either. Order the steel screws and clean the screws and the holes with brake cleaner spray and blow dry with compressed air. Do this twice then use red Loc-tite on the threads and the head of the screws. I think you will be fine until you can drill and tap the other 2 holes. Thanks, Eric

[Pat Miles]

One of the other two screws was loose and the other came out with ease. He had loc-tited them and had probably tightened them.

The third screw is flush with the frame. I'm going to give up trying to shoot it this weekend because even if I did get it mounted and sighted in on time, chances are it would probably make it 3/4 through the match.

On top of steel screws, looks like a I need to go back to a two sided mount or make this one a 5 hole-er.

Poll added - I think it would be easier for me to stick this on the classifieds and get a new mount for this gun.

If one of the other two screws was loose and the third came out easily they were probably not Loctited. That in itself may have contributed to the breaking of the rear screw.

Before you run out and buy a new mount you may want to verify that the three holes you have in your frame now will line up with the holes in the new mount. If they don't you will have a helluva mess on your hands.

Given that we know at least one of the three screws was loose (maybe the broken one was also but we'll never know now) that might have had some bearing on the rear one breaking. I would buy some carbon steel screws and use Loctite 271 after cleaning the holes well. As Eric said put a drop underneath the head itself as well as on the threads.

Pat

[Aircooled6racer]

hello: I voted for 3 steel screws to get you threw the match then drill and tap the other 2 holes after or even before the match. You have a good looking C-More mount that you like I am guessing so go with it. Or you could just get a Quinn II mount if the hole spacing is the same as you have. I don't like the looks of the 2 sided mounts anyways Thanks, Eric

[Flyin40]

I can get steel screws tomorrow - I don't know about drilling and tapping the other holes though.

Could I expect it to stay on with steel screws?

How should I go about removing the broken screw?

Thanks!

I have an Alchin mount and have only used 3 screws for the last couple of yrs. No problems. I did strip a head a couple of yrs back. I just drilled a small hole in the broken screw just far enough to let a screw extractor get a bite. It came right out. It was loctited in also. You just have to make sure the length is right on the screws. Make sure they don't stick out inside the dustcover.

Flyin

[Ming the Merciless]

If the countersunk screw is protruding too far through the mount, the screw is actually tightening when the head of the screw hits against the frame instead of as designed. This leaves the mount loose to move and you shear off the head of the screw. Depending on how much interference you can slightly countersink the threaded holes in the frame to clear the screw head.

If you have a drill press and a drill vise (even a cheapie Harbor Freight will work), remove the mount and grind the broken screw end flat. Use a propane torch to heat the screw until the Loctite just starts to smoke (approx 250 - 300 degrees.) Re-install the mount, select a drill bit that just fits the screw hole in the mount and use the mount to center the drill bit on the flattened end of the broken screw. Just drill a small divot and stop. Remove the mount and use a smaller bit like a 5/64 to drill through the broken screw. If the hole looks well centered you can use a larger bit like a 3/32 to enlarge the hole before using an EZ-Out.

[DyNo!]

Braxton1 invited me over for Labor Day to get the broken screw out.

He proposed that the other two screws backed out enough to place all the stress on one screw and I agree.

I stopped by McMaster-Carr today and got some steel screws and green loctite 290.

I've cut the screws to the right length, cleaned them, doused everything in green loctite, and tightened the heck out of them.

We'll see what happens this weekend at the Georgia State match and more importantly, tomorrow when I take it out to zero.

Edited September 8, 2010 by DyNo!

[B_Seehawer]

Thinking that maybe your holes aren't lining up perfecly or your countersinks in the mount are not perfectly round or cockeyed. This will put tons of stress on those little screws too.

Brad

[Ed Robinson]

Anyone who has ever dealt with stainless hardware (screws, nuts/bolts, etc) especially marine grade knows how easy it is to twist the head off of something. Stainless being so "hard" has very little give or twistability. You will probably find the carbon steel screws will never give you a problem as long as they stay tight. With the green loctite you should not have that problem either. I think you have solved your problem.