M&P 5-screw issue on one chamber

[BC38]

I just replaced the cylinder on a project M&P from the 1930's.
Everything seems to function normally EXCEPT that the trigger doesn't seem to reset on one chamber about 2/3-3/4 of the time.
Cycling the action in DA it will cycle normally on 5 chambers and then the trigger won't reset on the 6th chamber.
Pushing the cylinder release will make the trigger reset, and then it will cycle normally 5 times and fail to reset again on the same chamber again. After resetting it with the cylinder release, sometimes it will fail to reset on that same chamber, but other times it will cycle normally for 11 trigger pulls in a row and then the trigger will fail to reset again ON THE SAME CHAMBER.
It never seems to have the problem on any other chamber, just on the one chamber - and even then it doesn't hang up every time even on that one chamber. So it is inconsistent and somewhat random, but when it doesn't reset, it is always on that same chamber.
Any thoughts?

 

[pantannojack]

Avoid tension of the springs with the side plate off.

 

[armorer951]

I'm assuming you mean the trigger doesn't reset on the same chamber. If this is the case, the ratchet for that particular chamber may be damaged or may be "long". (oversized)

Long ratchets often interfere with proper trigger movement to the rear during cocking, and trigger reset.

Less likely causes would be damage to the hand or extractor, improper headspace, or a yoke that is out of alignment.


Carter

 

[BC38]

Avoid tension of the springs with the side plate off.

Thanks for the cautionary advice. I've been around long enough and read enough here to know not to cycle the action with the sideplate removed, for fear of breaking off the hammer stud.
But thanks for reinforcing that warning.

 

[Venom6]

Might check to make sure the rebound slide spring has not been shortened.

 

[BC38]

I'm assuming you mean the trigger doesn't reset on the same chamber. If this is the case, the ratchet for that particular chamber may be damaged or may be "long". (oversized)

Long ratchets often interfere with proper trigger movement to the rear during cocking, and trigger reset.


Carter

You are correct - the problem is with only one chamber. I edited my original post to clarify the issue.

So from what you are saying, it sounds like there is one ratchet tooth that needs to be filed down to correct this trigger reset issue?

Any suggestions or links to tutorials or references to the procedure that is required to correct the problem?

It seems like it should only require a tiny bit of filing to reshape that one ratchet tooth to correct this issue, but I really want to make sure I completely understand what needs to be done before I attempt any modifications.

Any additional educational material that anyone can point me to will be much appreciated.

 

[BC38]

Might check to make sure the rebound slide spring has not been shortened.

Thanks for the input.
The rebound spring doesn't seem to have been modified - based on its length, and how hard it is to compress and pop into place
It also seems to me that if a modified rebound spring were the issue it wouldn't just be an issue with the same one chamber consistently. It seems more likely that if a shortened rebound spring were the cause that it would show up as consistent problems with all the chambers - or possibly random problems with different chambers - not just a fairly consistent problem with only ONE chamber.
At least that seems logical to me, but I'm certainly no expert revolversmith. That's why I'm looking for advice from the folks who are.

 

[jmiles1960]

I agree with Armorer951. Sounds like a "long" ratchet on the sticking chamber. A Barrett file is used to file the ratchets. Understanding the ratchet / cylinder geometry is key to locating and tweaking the correct ratchet. It is not a difficult fix, but must be done with caution, patience and again, complete understanding. I remove the side plate and watch the hand as it engages the ratchet. Good luck and continue to ask questions.

 

[BC38]

I agree with Armorer951. Sounds like a "long" ratchet on the sticking chamber. A Barrett file is used to file the ratchets. Understanding the ratchet / cylinder geometry is key to locating and tweaking the correct ratchet. It is not a difficult fix, but must be done with caution, patience and again, complete understanding. I remove the side plate and watch the hand as it engages the ratchet. Good luck and continue to ask questions.

This makes sense to me, based on my understanding of how it operates and my basic mechanical instincts.
Do you know of any reference materials I can read or watch (u-tube?) that will detail the exact procedure I need to follow to make the necessary modification?

 

[armorer951]

You are correct concerning the importance of confirming that an offending ratchet is the source of the problem. Mark the problem chamber, and then take a close look at the ratchet for that chamber. The contact surface for the hand on that particular ratchet will likely have damage, or show signs of peening on the top. (peening ratchets is very common attempt used to cure DCU, or "carry up" problems)

You can determine where the contact between the hand and ratchet is by adding some Dykem or "sharpie" ink to the side of the offending ratchet and then carefully cycle the action with the sideplate on. You'll need a magnifier to check for the resulting rub marks. on the hand and the side of the ratchet.

Removing material from an extractor ratchet is a "last resort" to restore proper function. Be sure to rule out all other causes before proceeding.

Also, be sure to check the suggestion in post #5 about verifying that the rebound spring is a full length OEM spring.


Carter


Determine ratchet for a particular chamber.......

 

[Protocall_Design]

With the sideplate off, hammer and mainspring out, you can look in the hand window from the back and watch how the hand interacts with each ratchet. Then you are not blindly guessing, you can see what's happening in real time. See if there is any difference between the normal 5 chambers and the 1 chamber. See if the hand is pinched between the ratchet on the left side and the frame on the right side on that 1 chamber. If not pinched, it's something else. Don't file anything until you determine exactly what is causing the problem. We think it's the hand/ratchet, but could be something else.

 

[BC38]

With the sideplate off, hammer and mainspring out, you can look in the hand window from the back and watch how the hand interacts with each ratchet. Then you are not blindly guessing, you can see what's happening in real time. See if there is any difference between the normal 5 chambers and the 1 chamber. See if the hand is pinched between the ratchet on the left side and the frame on the right side on that 1 chamber. If not pinched, it's something else. Don't file anything until you determine exactly what is causing the problem. We think it's the hand/ratchet, but could be something else.

Sounds like a good diagnostic process to follow. Basically you're saying the hand is rubbing on the right side of its slot - due to a ratchet tooth issue?

Hammer and mainspring? You lost me.
Per the schematic below, the main spring is in the grip and attached on one end to the hammer stirrup, and on the other end to the cutout in the front bottom of the grip frame, so that makes sense.
But this schematic doesn't show a "hammer spring". So I'm not sure which other spring you are referring to - unless you are suggesting I remove the actual hammer along with the mainspring. That seems like it would be removing too many of the moving parts of the action for an accurate diagnosis...

 

[Protocall_Design]

If you remove the hammer and mainspring, you eliminate the chance of breaking off the hammer pivot pin by cycling the action with the sideplate off. We are trying to determine if there is a hand and ratchet interface problem, so it won't matter if the hammer is out of the gun for that. I didn't say hammer spring anywhere that I can find.

 

[BC38]

If you remove the hammer and mainspring, you eliminate the chance of breaking off the hammer pivot pin by cycling the action with the sideplate off. We are trying to determine if there is a hand and ratchet interface problem, so it won't matter if the hammer is out of the gun for that.

OK, I'm following you now. I can try that. I'll et you know. Thanks

Anyone else have any other thoughts or suggestions?