S&W 638 Cylinder Swap

[Olon]

I recently bought a 340pd cylinder from midway with hopes of swapping it into my 638. I assume the ratchet they send with it has excess material and requires fitting, but I'm wondering if there's anything wrong with using the old extractor star/ratchet that came from the factory, and just putting the new cylinder on that.

I checked the gap from the cylinder to the forcing cone and it's 0.006" for the titanium cylinder and 0.007" for the stock one. When installed, everything seems to line up as it should (visually, not sure how to check this exactly). There are no drag marks from the cylinder stop. Maybe I'm taking too much of a leap here but I'd think that the relationship between the cylinder notches and bores is a tightly controlled one in terms of manufacturing tolerances.

A friend of mine bought a 340pd parts kit and swapped the crane, cylinder, ratchet et. al into his 642. Much to my surprise it doesn't seem to have any issues with timing, at least not that I can tell by shooting it. I'm not so bold but his experience makes me wonder if I'm overthinking this.

So in short: can I just drop a new cylinder into a j frame with the old ratchet? Is there something I should be checking for other than drag marks from the cylinder stop or cylinder gap? Thanks in advance

 

[BBBBill]

WEll, sometimes you get lucky and sometimes you don't. The chances of getting lucky seem to wax and wane with the furtunes of the company. The modern machinery hold tighter tolerances, but the specs have changed rapidly over the last few years. And the workforce has turned over repeatedly. So I'm not inclined to trust anything to be drop in even if it is advertised to be so.
The proper way to fit the cylinder is will involve checking cylinder to barrel clearance, endshake, headspace, yoke alignment, rotational alignment of the individual chambers using a close fitting range rod, bolt drop, etc. The best source of info on all that is the Kuhnhausen S&W revolver manual. Well worth the price for the depth of info presented. Even if you never work on your own gun the info will educate to be able to filter out the wannabees who call themselves gunsmiths and select someone knowledgeable.

 

[Olon]

WEll, sometimes you get lucky and sometimes you don't. The chances of getting lucky seem to wax and wane with the furtunes of the company. The modern machinery hold tighter tolerances, but the specs have changed rapidly over the last few years. And the workforce has turned over repeatedly. So I'm not inclined to trust anything to be drop in even if it is advertised to be so.
The proper way to fit the cylinder is will involve checking cylinder to barrel clearance, endshake, headspace, yoke alignment, rotational alignment of the individual chambers using a close fitting range rod, bolt drop, etc. The best source of info on all that is the Kuhnhausen S&W revolver manual. Well worth the price for the depth of info presented. Even if you never work on your own gun the info will educate to be able to filter out the wannabees who call themselves gunsmiths and select someone knowledgeable.

Thanks for the input. I went ahead and ordered that book. Even if this cylinder drops in, it can't hurt to know that stuff I suppose

 

[Jim Watson]

What does transplant of a 340PD cylinder gain you? Are you buying titanium?

 

[Olon]

What does transplant of a 340PD cylinder gain you? Are you buying titanium?

Yes, I bought the titanium cylinder to shave some weight

 

[BBBBill]

What does transplant of a 340PD cylinder gain you?

A hard kicking gun. Those little beasties are a handfull for me with my old injuries and arthritis. I need a bit of heft in my guns.

 

[westernrover]

I've swapped cylinders while keeping a ratchet on a couple of S&W L-frames. I didn't do it permanently, but to test whether it would resolve an issue with one of them. Before I shot them, I checked headspace with a piece of brass in each chamber. At one point I bought a go and no-go gauge, but found them unnecessary. I also checked the timing (made sure the bolt locked the cylinder before hammer fall) and the chamber alignment using a Brownell's range rod for my specific caliber (357). There may have been some difference in barrel-to-cylinder gap and endshake (easy to measure with a feeler gauge), but nothing that would make it unsafe. I shot them that way for a while and had no issues. For the sake of my experiment, the problem one gun had did not migrate to the other gun when I swapped the cylinders, probably ruling the cylinder out as the cause.

More recently, I had a cylinder swapped by a good gunsmith that specializes in revolvers. Their company is well-known. They could have swapped the old ratchet to the new cylinder, but didn't even consider it. They fit a new ratchet. They also shimmed the cylinder for endshake. I know because they asked me if I wanted the endshake set for the old cylinder or the new (the new, of course). The old cylinder and ratchet are still usable, but they'd have more endshake. The other reason I don't use them is because of machining defects in the chambers (courtesy of S&W's Performance Center who charged me for that defective cylinder and refused to fix it). My point here is that a good revolversmith will do the swap the right way with a new ratchet and endshake shims to give the best result. Swapping it without fitting it carefully will probably work and even be safe if checked carefully, but the result could be premature wear of the hand, the timing going out sooner (even if it's ok at first), and even more wear or stretching of the frame if endshake is greater. The more you shoot it, the bigger the difference it makes.

 

[Olon]

@westernrover this is exactly the advice I was looking for thank you