Sideplates, yokes, cylinders, barrels and ratchets are not drop in parts. You can get lucky with any one of them but that is unlikely.
Unless your blessed with luck, "fitting" a cylinder can be a can of worms. If your new cylinder slides a few thousandths forward on the yoke it can be shimmed. If the yoke barrels is a few thousands too long for your new cylinder it can be shortened and your old cylinder shimmed. After both cylinders are correctly held back the B/C gap is adjusted. If it's good with one cylinder but too small or non existent with the other the longer cylinder can be shortened in a lathe. If the B/C gap is too big with either cylinder the barrel has to be set back before the longer cylinder is shortened. Setting a barrel back is a lathe job that runs $100 to $150.
Generally it's advised that you reuse the old ratchet but that obviously can't be done with a different number of chambers. If my fuzzy memory of what's been posted is good, 6 and 7 shot 686s use the same hand. You can look in parts lists on S&W's web site to verify the part number is the same but that doesn't guarantee your new cylinder will carry up correctly. For the two cylinder convertible I created the first ratchet I tried was unusable because the subtle shape of ratchet teeth had changed over the years. I found one that worked great but I couldn't predict that before purchasing it. Similarly if you buy your second cylinder with a yoke the new yoke may not even swing into the frame. That's one reason yokes, frames and side plates get an assembly number to keep them together after they are fitted.
So, you want to buy a random cylinder and plop it right in. Are you feeling lucky?
