Not timing in the usual sense, but here's the story:

Recently I bought an 18-3 from the internet in 99% condition. If it's been fired at all, it was very little.

When I removed the sideplate to look inside, it was perfectly clean and bone dry- not a bit of oil inside or outside. Fortunately, no rust. My guess is that it got sold to a gun store and had 50 years of dried oil and dust inside, so they removed the grips and put it in the ultrasonic cleaner for a while, wiped it off and put it up for sale. I oiled everything inside and outside.

The problem is that the cylinder stop drops down when you start to pull the trigger (or thumb-cock the hammer), like it should. But as you pull the trigger just a little more, the stop jumps back up again way too soon and sometimes re-engages cylinder notch before the cylinder even starts to rotate. That locks up the gun.

With the sideplate removed, I can see the cylinder stop slip off the nose of the trigger before it should, but can't figure out why. Nothing looks modified and the gun still has the heavy factory trigger pull. I didn't disassemble any further, but tried some blasts of Gun Scrubber into that area, including the area of the cylinder stop spring, then re-oiled. Still does it.

Can anyone tell me what's causing the problem, and how to fix it?
Thanks!

(Pictures coming soon)

If that is truly what's happening it could be a problem with the hand.

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Normally, this condition is caused by the bevel/point on the cylinder stop being too short. The "point" of the bevel needs to extend out further, so that the hook on the trigger will keep the stop down just a bit longer, as the hook travels down, and pulls down the cylinder stop at the start of the trigger pull.

Properly fitting a new cylinder stop should solve the issue.

__________________
Ret. LE, FA Instr, S&W Armorer

^^^
I just did some cycling of the gun with the cylinder open, so it's easier to see what the stop is doing:

As the trigger starts rearward, the stop drops fully down (flush with the frame) like it should. Then as the trigger is pulled just a tiny bit more, it jumps back up about 1/3 to 1/2 way to the fully up (locked) position. Then it jumps up all the way at the normal point about 1/3 of the way to the next notch.

There is enough variation in that partial jump up that usually the cylinder can rotate, but not always.

Does this support the idea of the bevel/point on the stop somehow being too short?

I believe so. RG makes a good point about verifying that the hand is also working properly, and the headspace is correct. If the hand is late, then the cylinder misses the opportunity given when the stop does drop down.

The most common situation here though, I believe, is that the point of the stop is not quite long enough. The other possibility is that someone has modified the hook on the trigger by stoning....or perhaps it is damaged. This could also cause the stop to be released too soon, because the hook is not shaped correctly, or has been shortened.

__________________
Ret. LE, FA Instr, S&W Armorer

If you cycled the trigger with the cylinder open that means you pulled the cylinder latch to back to be able to work the trigger. If so the hand has at this point nothing to do with the cylinder bolt/stop cause the cylinder is not interfering with the hand and the trigger is the only thing that works the cylinder bolt/stop if you have the cylinder open then the only added strain on the trigger and hammer mechanism is the hammer cycling as you pull the trigger. So, I would say remove the cylinder bolt/stop and clean it and the spring and make sure the spring is not damaged cause that will cause the cylinder stop to hang or not work as it should

The only thing that moves the cylinder stop is the trigger nothing else. So, if you remove the hammer spring and hammer and cycle the trigger and it still acts up it’s a problem with the cylinder stop. When the hammer is pulled back it moves the trigger to the back slightly before it locks, that in turn moves the cylinder stop and allows the cylinder to be spun by the hand. But, ultimately the trigger is the only thing that moves the cylinder stop. So, check the cylinder stop and spring. After, of course removing the hammer spring and hammer and cycling the trigger, the hand should still spin the cylinder and cylinder stop should still engage the cylinder and stop it

Please don’t cycle the trigger with the side plate off cause the hammer will try to move on the hammer pin and the hammer will strike the edge of the side plate edge. If you remove the side plate just go ahead and remove the hammer and hammer spring, then you can cycle the action without any real problems, the hammer spring adds pressure on the hammer pin when it’s unsupported

I've received my homework assignment for tomorrow, I'll check all those items.
Thanks for all the quick responses here.

EDIT:
I didn't fully cycle the action with the side plate removed, just enough trigger movement a couple of times to watch the cylinder stop action.
Cycling with the sideplate removed is also said to stress the pins, since they are supported on only one side. I don't plan to experiment much.

Yesterday I took it all apart, the insides looked pristine. Except for the cylinder stop, which looked like it had been "fitted" by a blacksmith. Lots of nasty file marks and unusual angles. It still jumps up and locks the cylinder fairly often.

So I called S&W today and explained the situation to the nice lady, she's sending me a prepaid FedEx shipping label. That's a nice money saving touch. But when I asked how long it would take until the gun was repaired and returned to me, she said six MONTHS!
Do you think that's an accurate time?

It's tempting to have the gun repaired locally, but good revolver smiths are a rare animal. Most "gunsmiths" today are just Glock/AR15 parts changers. Do you have any recommendations in the NW Chicago 'burbs?

EDIT-
I'd also like to replace the skinny grooved trigger with the slightly wider smooth trigger on most modern S&W's. Is that an easy replacement?

If you want to try to change the cylinder stop yourself, you can PM your address to me and I will send you one and if you can fix it. Pay me $10.

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Thanks, that's a very kind offer. If I decide to go that route, I'll PM you.