Old M&P Problem

[Gazz]

I have an old M&P, sn248XXX that does not index correctly. It seems to be okay in DA mode but in SA mode, the cylinder is just a degree or two short of lining up with the barrel. I tried a new extractor star but that did not help. I have heard that a oversized hand is the way to correct this which would require opening the hand window in the frame. This should be easy enough to do if it is the solution. I have also noticed that the bushing in the frame where the ejector rod pin (which also shows some slight wear) locates is sort of egg shaped and wonder what effect this may have on operation and in fact if it is the real cause of the problem. The revolver is nickel plated that is a bit spotty and has a later barrel on it so it would be just a shooter. I need to fix it though to make it just a shooter though. I posted this originally in the smithing section but never received a response so thought I might try it here. Thanks for any comments or advice.

 

[tbury]

An oversize hand is only oversize so it can be fitted to the gun not the gun to the hand. The timing issue you describe is due to the hand being worn so it no longer fits the ratchet part of the cylinder. If you search YouTube for S&W Timing you will find a MidwayUSA video describing this process; but I would still get the part and find a "good" gunsmith to do the fitting.

 

[mikepriwer]

It could be other problems, as well. If the cylinder is sitting slightly
too far forward, you get timing errors. Shims inside the cylinder
will push it back to where it is suppossed to be.

Also, if the yoke is slightly bent, the cylinder does not line up right,
and again you get timing errors.

later, Mike Priwer

 

[Gazz]

The reason I thought that the hand window in the frame had to be opened up for the oversize hand is because a service guy at S&W told me that is what they did. There is some barely barely perceptable end shake in the cylinder so I do not think that is it. The crane does not quite mate up perfectly to the frame when the cylinder is closed so that is something I am going to have to look at.
I will be doing the work myself. I have many years experince as a jeweler and servicing high tech equipment (SEM's and FIB's) as a mechanical engineer. So far, I have been able to fix all of my guns that require repair just that this problem is a bit perplexing to me. Actually now that I thnk about it, I had a very similar problem with a 1895 series Colt DA revolver that I did give up on. I did not want to shoot it it any case but did want it to work properly. This S&W should be a nice shooter.

 

[mikepriwer]

There are a couple of tests for a bent crane. One is to open the
cylinder, rotate to the next charge hole, and then close it. Observe
the seating of the extractor lug retaining pin. Nominally, it should
seat just a tad further out with the cylinder closed, than with it
open. Repeat this for all 6 charge holes, observing how the pin
seats iftself. If they all do not seat the same way, then something
is bent.

Next, open the cylinder, and gently rotate it. Try to sense if there is
any drag, or resistance to rotation, at any particular rotation point.
If there is, then something is bent.

Finally, letting the extractor rod rest on your right hand, single-action
the gun through all 6 charge holes. For a gun in proper time, you will
hear the click as the cylinder stop snaps into place; if you try to
further rotate the cylinder just a bit, the stop would prevent it. If you
have a click when starting to rotate the cylinder just a bit further, then
that charge hole is out of time. (The idea of resting the extractor rod
on you other hand is to put a very slight drag on the cylinder rotation,
to keep it own intertia from carrying the cylinder to lockup.)

If all 6 charge holes do not lock up properly, then either the hand is
worn, or something is holding it back. If only one (or two) do not lock
up, then its probably a worn extractor star cut for that charge hole.

Later, Mike Priwer

 

[twaits]

You might try Robin Dietrich up here in Maine:

Gunsmith in Maine. From repairs to full customization

He is working on one of my Colts right now. There was another guy I sent his way that had a Python that was out of time and he repaired it for around $50 or $60. I've heard good things about him.

 

[Gazz]

Mike, the problem you describe is exactly what is happening. When I thumb cock the revolver, the cylinder rotates but not all the way. Just a slight turn of the cylinder and I can hear and feel the bolt lock into position on the cylinder. If I run the revolver DA, the cylinder is in the correct position when the hammer (prehaps immediately after - hard to tell) drops. Twaits, while that sounds quite inexpensive for a repair like this, I will have learned nothing by taking it to somebody else other than I can get it done by somebody else. There is a small amount of personal pride here. I want to fix this myself. I just need a bit of help from my fellow enthusiasts.

 

[mikepriwer]

Gazz

It sounds like the gun is going into single-action position before the
cylinder stop has set itself. You might try the following.

Remove the side plate, and remove the hammer mainspring. The idea
here is to try to understand where the cylinder really is, when the
hammer flies forward. First, applying gentle forward pressure to the
hammer, double-action the trigger. At the exact point where the
hammer is being released by the trigger, and wants to move forward,
let go of the trigger, and see if the cylinder stop has really locked in.
If its not, that would indicate that the gun is really out of time in
double action.

Now, do the opposite. Slowly pull back the hammer until it goes into
single-action lockup. Now, with gentle forward pressure on the hammer,
pull the trigger until the hammer wants to release and move forward.
Check the cylinder-stop lockup. Presumably, the cylinder will not be
locked up yet, as you already have observed. If the double-action is
working right, but the single action is not, then something is wrong
with either the single-action slot on the hammer toe, or the knife-
edge at the rear of the trigger. Either someone has altered them,
or what you have is not a matched hammer and trigger; ie, one or the
other has been swapped out.

In double action, the hammer is raised by the contact of the double
action sear (on the hammer) being lifted by the rear part of the
trigger. Eventually the sear slides off the end of the trigger, releasing
the hammer. In single action, the knife edge at the rear of the
trigger slides out of the hammer-toe slot, allowing the hammer to
fall forward.

The cause of your single-action timing error seems to be that the
trigger knife-edge is engaging the hammer-toe slot too early. Either its
a mismatched hammer and trigger, or the wrong hammer and trigger,
or the hand is too short - perhaps from wear. If the hand is too short,
I would have expected that double-action would also show a timing
error.

By studying the interaction of the hammer and the trigger carefully,
and without the mainspring, you may be able to ascertain what is
happening.

These guns had several engineering changes, and some of those
changes affected the configuratioin of the critical parts of the
hammer and trigger.

Later, Mike Priwer