I just bought a fine used S&W 13-2 : seemed to need nothing but a good cleaning, so I got to it : I removed the front sideplate screw to slip out yoke & cylinder... cleaned & cleaned & oiled... etc. & finally put everything back together :
Now the cylinder won't open all the way, although it freely swung open all the way before.. It swings to about 45°, but I can't get it horizontal (90°) without pushing -HARD- unless I UNscrew the yoke screw 2-3 turns (at which point it is quite loose).
I disassembled again, & saw nothing wrong with the screw : BUT : it had gouged out a portion of the inner face of the screw slot on the yoke button. WTH ??? From that ONE in-and-out swing ?
Now I know about mixing-up of the front & middle sideplate screws and I am absolutely certain I didn't : all screws went back where they came from originally.
What have I done wrong ? Is my yoke damaged beyond usefulness ?
I am not inclined to shoot that gun with a loosened yoke screw !
What should I do ? Help !......anyone....?

More than likely the screw was looser than it is after being re-installed and therefore in a slightly different position - a bit farther in. This is not a big deal and not uncommon. Remove the screw and where it is "marked", remove a VERY SMALL amount with a stone to "fit" the screw. It does NOT have to be tight up against the yoke to retain it - just well inside the groove.

Smith has since changed the design of this screw to a spring loaded pointed version which is sort of self adjusting. Just remove a slight amount at a time and try it after each few strokes of the stone.

Chief38

You could also try swapping the two round head screws with each other and see if that works.

I agree with bgrafsr,
Try the screw swap before filing away at any screw. There is the possibility that the previous owner had swapped the screws and just backed out the wrong one to make the cylinder swing out properly.