Smoothing the action in a gun with MIM parts

[Hearsedriver]

Seems like I read here or somewhere else that guns with MIM parts do not respond as well to action work as those with forged parts. Ive had really good luck slicking up DA pull on older S&W's with forged parts . Yesterday, I bought a nice used 60-9 that has MIM parts. The action is pretty nice but, there seems toi be some drag on the trigger rebound. I popped the sideplate and did a quick inspection and found everything toi be clean and lubed. I removed the rebound slide and checked the spring. I compared it to a new 14 lb Wolff spring and found it to be slightly shorter so, I popped in the Wolff spring. Everything feels about the same . My quesion is this. Would it be worth my time to do a complete teardown with emphasis on smoothing the contact points, excluding the sear and trigger contacts ? When I had the RB slide removed, it felt pretty smooth and had factory radii on the edges. If anyone has successfully slicked up thier modern J frame action, I would appreciate any tips. Thanks!

 

[chief38]

I do not own a MIM firearm and so I don't know the quality of the MIM parts in question here. I would think that MIM parts are not as "dense" as forged parts are, and since they are basically made of powdered metal, would not be as easy to smooth out or take a super smooth face. Before stoning anything I always like to shoot the **** out of a gun with at least 600 - 700 rounds. I'll bet by that round count your gun will have been smoothed out fairly well and you will not have to be concerned any further.

The other win - win situation here is that you will get plenty of shooting practice and have some fun while smoothing out your gun.

Chief38

 

[Tyrod]

I think you'll find that stoning the DA sears on both the trigger and hammer will work to some degree. The 3 j frames where I stoned the DA trigger sear, DA hammer sear and dehormed the outside of the rebond slide had these result:

Reduction in DA trigger pull
442 (1994) forged parts
.65lb reduction

642 (2011) MIM parts
1.08lb reduction

317 (2011) MIM parts
.50lb reduction

There are other parts where I could have smoothed the action out even more, but for the time being, this is all I did. Be prepared though the trigger & hammer on MIM guns are constructed differently.

 

[tomcatt51]

The MIM parts "smooth" just like the forged parts. Use a stone and just take down the high spots. They even weld. I got curious and welded (TIG) a couple cut off hammer spurs together to find out.

If you have an MIM parts J frame you might try: https://apextactical.com/store/product-info.php?pid49.html

 

[Hearsedriver]

The MIM parts "smooth" just like the forged parts. Use a stone and just take down the high spots. They even weld. I got curious and welded (TIG) a couple cut off hammer spurs together to find out.

If you have an MIM parts J frame you might try: https://apextactical.com/store/product-info.php?pid49.html

Thanks for the Apex link. I ordered the kit and will give it a try. A 3 lb. reduction of the DA pull would be nice.

 

[Nightowl]

I've only worked on a few MIM part guns, but have found them pretty smooth before I did anything to the parts. They are not through hardened, so it would be easy to cut throught the hardened surface and reduce the life of the part.

 

[dfariswheel]

The reason for smoothing gun parts is to remove or at least reduce sharp marks left by machining that can cause catching or rough movement.

Since MIM parts are cast, there ARE no machine marks.
It's been my experience with Colt's sintered steel and S&W's MIM that the actual operating surfaces are already about as smooth as will do any good.

If you insist, remember that many of these parts have a thin surface case hardened coating. That coating can be easily broken through, exposing the soft inner steel.

Per the S&W factory and master gunsmith Jerry Kuhnhausen, ANY signs the hammer notch or the sear area of the trigger has been stoned or altered is cause for automatic rejection and replacement of the parts due to the risk of break-through of the hardened surface.

 

[scooter123]

Jerry Kuhnhausen wrote his shop manual years before the implementation of MIM. What he is referring to are the case hardened Forged parts. Depending on the process and alloy used, Case Hardening can produce a hardened layer that runs from a couple of thousands of an inch in depth up to a full 1/16 inch in depth.

One of our members has posted that the MIM parts are through hardened because of the difficulty he has experienced in drilling the hammers in order to lighten them and reduce lock time. From what I know of MIM, I'm inclined to believe this, with most MIM processes through hardening is a normal result of the process. I've also read that most MIM processes result in a high content of imbedded Carbides in the microstructure that results in a material that is extremely wear resistant due to those carbides.

Personally, I've found that the MIM lockworks are very easy to tune up because the parts rarely require any additional fitting and they incorporate features that used to have to be added to the older forged lockwork components.

 

[BruceM]

"One of our members has posted that the MIM parts are through hardened because of the difficulty he has experienced in drilling the hammers in order to lighten them and reduce lock time."

Roger that!!

Forged steel parts are case hardened and once you cut through the hardened surface, they wear very quickly. Hardened MIM parts are hard thru & thru. Further more, high quality MIM parts should need less smoothing because they are more dimensionally stable than their forged counterparts. I do believe that JM's guns are built with MIM parts and that should tell you something. In my view, junk parts are junk parts whether they be forged or MIM. My beef is using MIM parts in the applications for which they are not suited just for the sake of economy. They also cannot be color case hardened to my knowledge so they look like hell IMO when used on top of the line guns where CCH hammers & triggers were standard. Aesthetics do matter, believe it or not, Smith & Wesson.



Bruce

 

[Hearsedriver]

Yes, MIM parts can be color case hardened. The ones on my M60 are.

 

[BruceM]

"Yes, MIM parts can be color case hardened. The ones on my M60 are."

They may be colored in some manner but CCH; I'm pretty sure not. You can spot a MIM hammer & trigger as opposed to CCH forged a mile away, even in the generally poor light of a gun show.

Bruce

 

[cjtraining]

FWIW I hold a sheet of 2000 grit 3M wet/dry paper flat on a piece of granite and polish the parts that move against each other. I don't polish until totally smooth, only the high machine marks.

 

[WR Moore]

I installed trigger stops in several DAO 3rd generation with MIM triggers. I found the surface of the triggers too hard to drill, I had to use a milling cutter to create the hole for the stop. YMMV.

 

[yamadeal]

Mim parts respond well to a little stone work, if in doubt check it out with a before and after trigger pull test. Worth the time.

 

[Ed45]

MIM or whatever, the new guns coming out will be very difficult to do tuneups on. As I learned with my BG38, there are NO MORE SCHEMATIC PARTS DIAGRAMS because the legal guys at S&W removed them. New guns like the BG38 are a mystery that you use with faith and no knowledge of what makes it tick. Amazed we have not seen more on this (I posted about the BG38) and am frankly outraged. Ed

 

[woodyplacks]

trigger job spring kit

Greetings, I recently bought a as new in box 629-6, and installed a jm trigger spring kit, as well as polish the rebound slide, per you tube instructions. The result is a very smooth and tunable da-sa action- actually, it was very smooth before, just heavier than what I wanted. $18.00 plus shipping from Brownells, and 15min. time, really improved my 629, to where it is a joy to shoot. Mim parts not looking good, hmmm; I called S&W, and asked if the mim parts could be case colored, and they said yes, but they would not do that. Farm out the job? You tube has a video of someone polishing the mim parts, so that they match the rest of the gun. I traded a Ruger sp101, that had mim parts, and some cash to get the 629, and while I had the 101, I never had an issue with mim parts. `just my thoughts,...........Art.