prob a dumb question, but is the purpose of having a locking bolt pin to prevent the ejector rod from backing out if it were to come loose?? which would cause the centre pin to 'unlock' the cylinder

Not a dumb question. Based on the wording in your question, I'm assuming you are referring to the front locking bolt.

The front locking bolt in the Smith and Wesson revolvers, which is basically a spring loaded detent, was designed to help stabilize and support the extractor rod, and add support to the front portion of the yoke during the stresses of firing. Due to it's design, the front locking bolt is only marginally effective in assisting in stabilizing this area at the forward end of the extractor rod.

Technically, in terms of nomenclature, the locking bolt "pin", as you described it above, serves only to hold the spring loaded front locking bolt in place inside it's cavity in the barrel.

The center pin itself is spring loaded inside the extr. rod, but is not attached to the rod, and moves forward and back inside the cavity in the extractor and the attached extractor rod completely independent of these other parts that make up the assembly. Spring tension on the center pin is provided by it's own coil spring.

If the extractor rod unscrews and "backs out", this has no effect on the center pin's position in the breechface. If the extractor rod continues to unscrew, this eventually prohibits the free rotation of the cylinder, locks the end of the extractor rod over the tapered end of the front locking bolt, and prevents the user from opening the cylinder using the thumbpiece. (which, when pushed forward, pushes the center pin forward to release the locking bolt)

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Ret. LE, FA Instr, S&W Armorer

Cheers, I have a custom smith 686 (customed barrel to comply with uk communist laws )
The smith didn’t put a front locking bolt or a ball detent . He said it doesn’t need it ???

This is an opinion and is worth exactly what you make of it.

The very first S & W Hand Ejectors (.22 HE or Ladysmith, model 1896 in .32 Long, and the model 1899 in .38 Long Colt/Special) did not have the forward locking point for the extractor rod.

Adding this feature onto later guns may have been a marketing point against Colt, but every S & W revolver since then has had either the front and rear locking points (the vast majority of HEs), the rear with a ball detent yoke lock (the newer model 19s, 66s, and the 460 and 500), or three locks (no fair guessing which model ).

So, you may wish to ask your gunsmith if he thinks a second lock of some form is not necessary because he thinks the gun will never shoot loose, even with heavy .357 use, or because he does not install them regularly.

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Alan
SWCA LM 2023, SWHF 220

For a revolver to ‘shoot loose’ only happens due to the wear in the
Cylinder stop stud and or on the ‘grooves’ in the cylinder where they cylinder stop stud locks into I believe?

Will not shoot loose so I was told

I can also develop end shake where the cylinder moves forward and aft. and the parts you are talking about are know as the cylinder stop and stop notches. If you will observe the hand or small pawl which comes through the recoil shield portion of the frame and engages the teeth on the ratchet rotates the cylinder on a S&W counter clockwise and against the yoke that holds the front of the cylinder in position to the barrel. This can cause to yoke and of course the cylinder to move very slightly away from the frame and alignment. The lug under the barrel was designed to minimize this. Totally necessary? No. A good idea? Yes. Although slightly difficult to install on a custom barrel, a ball detent setup in the yoke is really easy. I think it is more a case of don't want to bother with it that anything else. The number of revolver smiths in the UK has got to be pretty small these days.

Colt has never used them. True. But, then Colt has its hand on the other side of cylinder and rotates its cylinder so that it presses crane into the frame.

unfortunately, thanks to the treasonous and unlawful firearms act 1997 which banned short barrel handguns ie any barrel under 12inches there are literally only a handful of revolver / pistol smiths . funny enough, there are many many AR-15 builders now. ARs are getting popular here now, that US companies like spikes , LMT , lantech are making UK legal versions

Does anyone have a cross section view showing how this locking bolt pin works??

It's actually called the "locking bolt". It resides in the barrel, is held in place by a cross pin, and powered by a small coil spring. Fitting them is a chore. They are rarely fit correctly.

Most of the time they are fit to extend too much, which causes the locking bolt to bear too heavily against the cavity in the extractor rod. Occasionally they are too short, and do not accomplish the task they were designed for, which is to support the end of the rod, without impending on the cylinder's rotation.

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Ret. LE, FA Instr, S&W Armorer

do you think my custom 686 without having any support for extractor rod will be ok without it ?

Without the locking bolt option, I've always installed a ball detent in the yoke, which locks into a small cavity placed above it in the frame. This supports the yoke, and holds the cylinder firmly in alignment with the chambers of the cylinder, assuming that the timing is correct and parts have been correctly fit.

I never did any testing to confirm that accuracy and consistency were improved by this add on, but I personally believe that to be the case. Purely an opinion though.

Here's a photo of a ball detent I installed in the yoke of one of the many PPC revolvers I built back in the '70's and 80's....... when dinosaurs roamed the earth.

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Ret. LE, FA Instr, S&W Armorer

Ok cool, is installing a ball detent hard ? Is this something a general gun smith could do ? the original pistol smith who built it doesn’t want to know (there are loads of general gun smiths here though )
Will the 686 be ok without a ball detent ?

Installation involves some precision drilling and fitting. Any trained, viable gunsmith with the proper equipment and tools should be able to do the work.

The decision about the support of the yoke will have to be made by you. Do some research...."Google it".

I would not regularly fire a revolver in .357 caliber without such support. That is only my opinion based on my training and experience, and the revolver's design and limitations.

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Ret. LE, FA Instr, S&W Armorer

You can get a kit for the ball and it is inexpensive. Basically some small ball bearings, springs and a special cupped bunch. Drill the correct sized hole to the depth in instructions, insert spring with ball on top, press punch on top of ball till punch touches yoke and give punch a firm tap with hammer. You must use a drill vise and drill press to drill the hole. It must be perfectly vertical and centered so it doesn't go outside of yoke or hit hole for ejector rod. Once yoke has ball, install it in fram without cylinder, put some layout dye on top of yoke cut and open and close yoke a few times. This places a line in layout dye where ball rides. Udsing a small ball carbide cutter in a Dremel type tool make a small round pocket at far back on the very end of that line, checking to see when ball enters and holds the yoke in place. Takes maybe a hour total with no rush.

Here is a link to kits. The fixture has a setup for positive location of hole.
POWER CUSTOM CRANE BALL DRILLING FIXTURE for S&W | Brownells

Should be able to ship anywhere as it is just springs and balls and a punch

cheers for the info. I've got a gun smith who will do the job using the brownells ball kit. Now will have piece of mind of having a second lock up .

I just acquired a model 66 no dash 2.5" and the tail of my locking bolt sticks out just a little too much. It functions fine...just looks a little strange. IF I can learn how to upload a pic I will. Anyone have any input on this?

The rods and center pins are fit pieces. You could remove a bit off end of rod and center pin the bolt would go farther forward and leave less tail. Better would be to simply remove it, take a bit off the end of the end ejector rod locking bolt that is sticking out and re round it. Grind of the right amount, Chuck tooth portion in small drill and spin it against abrasive to re profile. Being a stainless gun no refinish needed It only need a little bit to protrude when cylinder is open. How much sticks out varies from gun to gun. I never worry about that ens as long as the other end is doing its job.