I was just doing a bit further tuning on my 620 today and found that the previous contains some misinformation. The crane screw is a 4mm standard pitch, the remaining sideplate screws Appear to be a #5 thread. The diameter checked at 0.123 inch with digital calipers, which corresponds with an oddball size, the #5. As for the pitch, if it's a standard #5 screw it should have a 40 pitch thread. However, my thread pitch gages are at work so that's an assumption that may not be reliable.
I also took the time to remove a sideplate screw from my 1988 vintage model 67. Diameter on that screw checked an 0.120 inch, close enough to the 620 that one might think they are the same. However, the screw from the later gun is too large to easily thread into the frame of the older revolver and I didn't force it. As for pitch, both screws have the same pitch and the older screw fits the later gun easily. My conclusion is that the reason for the difference in diameter is most likely a result it different "classes" of the #5 thread being employed. Within the specification for screw thread sizes that is an allowed variance to accomidate coating thicknesses or size changes due to heat treating. What's odd is that "class" is normally determined for the tapped hole and the male thread is held constant. Obviously, S&W has been doing a bit of tinkering within the thread size specifications and used to use what would might be considered slightly undersize on the older guns.
I also dug into my spares and checked the size of a sideplate screw recently purchased from Brownells. That screw checked at 0.123 inch in diameter, same as on my 620.
Now, if you are trying to repair a tapped hole in the frame, the starting point should be a #5 tap. Note, this assumes that the thread size between 1981 and 1988 hasn't changed, I don't have a S&W of this vintage at cannot say for sure. Then you should be able to purchase a newer sideplate screw without having the "lap" the screw to fit, which I can assure you is a real PITA. As for finding a #5 screw locally at a hardware shop, it won't happen, the #5 thread is distinctly an oddball. Taps also won't be easy to find, however I suspect that Brownell's will stock this size.
Now, on the chance that you've already purchased a new screw and don't want to purchase a tap, I'll describe how to "lap" two mating threads together. First, you need some lapping compound which can be purchased at machine shop suppliers such as Production Tool. Warning, it isn't cheap because it uses a diamond abrasive. In a pinch rubbing compound for automotive paint can be used but it will be much slower. As for procedure, you coat the threads with the lapping compound and screw it in until it just starts to bind. Then you go in 1/2 turn further and work the screw back and forth until that binding is greatly reduced, then you back the screw out, recoat it, and go in just slightly deeper. You keep repeating this cycle until the screw will go in full depth, then clean everything as well as possible and your parts will fit nearly perfectly. BTW, once lapped together, screw and tapped hole must be considered as being married for life. I'll also warn you that when lapping two threads together you can't try and rush it by forcing it, if you overtighten a screw coated with lapping compound it will most likely sieze beyond any hope of removing it, you are basically putting wet sand into those threads.
PS; if I can remember it I'll take a spare into work Monday and confirm the thread pitch is a 40 pitch.
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