More often than you may think....they will just fall right in IF THE CYLINDER IS A LIKE-FOR-LIKE. If you use the same extractor assembly, it will work more than 50% of the time....but this is only my experience. Through no observed pattern or reason, S&W will sometimes shift the position of the ratchet-pins around and your ratchet may not fit the donor cylinder without changing pin locations. You may also find they used a "sweeping" ratchet in some models which are not pinned.
Cranes usually have numbers on them that mean something to somebody at S&W, but the differences in the crane are usually the barrel-length, or depth of the cut where the barrel attaches to the crane. If you can use your existing crane (yoke)....all you may have to do is shim, stretch or trim the yoke barrel....or maybe nothing at all.
It's a coin-flip, but if you get the exact same cylinder that accepts the exact same ratchet you stand a very good chance of success. Or....if you can get a cylinder complete with the extractor assembly that belongs with it....your chances of success are better. Yoke barrels can be adjusted on your own, but they don't usually wear-out. It's the yoke-barrel peening against the base of the cylinder that creates the end-shake. If when it was built....the BTCG was cut at the forcing cone to accept that particular yoke and cylinder assembly, there should be no reason why a clone won't work as long as it's in the same family.
Here is your problem. S&W does not like to sell cylinders. I have got them to sell me one, but usually the answer is "they have to be fitted" and we don't sell them individually. And....any used cylinder you purchase is likely for sale for a reason......if you get my point.
