I am working on a MetroArms American Classic II and have upgraded many parts and now I am looking at possibly replacing the MIM slide stop and safety.
My MetroArms (MA) slide stop measures in at .2004 that is at the high end of the .2005 -.002 spec size. This is good and I don't really want to change it.
My questions are:
What is the typical failure mode of a slide stop?
(break in half a the link or twist of the thumb stop)
The reason I am asking is I recently replaced the ignition group on the MA with hardened tool steel and noticed a significant reduction in visual polishing over MIM parts. This "wear reduction" justifies changing ignition parts from MIM to tool steel but since the slide stop and thumb safety aren't "wear" parts is it justified to spend $$$$$ on tool steel (forged) slide stop and bar stock thumb safety?
I would hazard a guess that a properly set up gun with a tool steel SS would never fail as a cross-pin. MIM, quality varies. A good steel one is not expensive.
I agree but if MIM slide stops never fails, then there is no problem, rather a percieved problem when non actually exists.
My question should be: Has anyone ever had their slide stop break in half?
Quote:
My question should be: Has anyone ever had their slide stop break in
half?
To go along with with that, is there really any empirical data that suggests MIM parts failure rates are higher than bar stock parts?
I've only seen pictures of the pin portion of a slide stop failure. That being said, I own a Kimber TLE II(7yrs old), all of the MIM parts are still in place and functioning fine. I say if you have no issues now don't worry.
We've seen pictures of broken MIM slide stops in the forum - from makers that are considered quite reputable, I might add. The breakage isn't always in the pin section, on one occasion it was actually at the inside corner between the pin and the lever - which ought to be slightly radiused, to aleviate any stress, but may have not been done correctly on that part. We've also seen slide stops that had 'chips' of metal missing from the pin section...
As I understand it the slide stop becomes a high-stress part, only when it's asked to do work that it wasn't designed to do. In an improperly setup/timed pistol, in which the slide stop and barrel link work overtime to stop the barrel moving, even a bar-stock slide stop may give up the ghost - unless the link throws in the towel first.
With that out of the way - if I remember correctly, you intend to re-finish the pistol you rebuilt, by having it Melonited. I am not sure, but I think I've read posts here suggesting that the high temperature used in the Meloniting process can mess with some small parts' hardening. I'd suggest that you discuss this with whomever you choose to refinish your pistol, in case there's something in this process that doesn't agree with MIM parts.
As far as I know, MIM steel can't be hardened as high as quality tool steels. We invented tool steel specifically because it can be hardened but steel looses its ductility as it hardens and becomes more glass like which can promote certain failures.
Hardness is also directly proportional to strength. Here is a graph showing the hardness to strength curve for steel.
My recent ignition parts purchase from Dave Berryhill were made from hardened tool steel which Dave states is around 50-53 on the Rockwell C scale. I will measure the hardness of the old MIM parts at work today and report back.
One way to get wear resistance and ductility is to case harden steel. Case hardened steels can have a HRC-50 shell over a HRC-20 core which is the best of both worlds.
Hey, I just realized that Melonite is a case hardening process so my MIM slide stop may get a hardened case that will make it stronger.
I discussed this matter with Coal Creek Armory already. They said there is no real advantage to Melonite steel that is already hard but I think they may have been referring to hardened tool steel and not MIM parts. I will ask that the slide stop and thumb safety be finished with Melonite so they match the rest of the pistol.
"Yes small parts can be treated. There is not much advantage to treating the sear and disconnector."
From my experience, it's the part that has broken the most.
But two of them were caused by an improperly fit barrel, so it doesn't matter what and how it's made.
The other two snapped at the "nub" that holds the slide open. The gun still works but it won't locked the slide back. One of them was "forged" and the other cast.
The Springfield FBI 1911, one of the most "tested" 1911's that I read about, uses a MIM slidestop.
Unless you think it's poor quality, I wouldn't replace it, but if you keep spare parts, then IMO the slide stop is one to have.