As far as I know it's a 1/16th - but BEWARE, not all 1/16th allen keys are the right size, so you need a decent quality Allen Key (lots of cheapies are just the right size to strip the screw - it needs to fit tightly) and remember LocTite is your friend.
Or you could have gone with a quality pistol like a sig or glock and wouldn't have had this problem!!!

enefgee, thanks.
It took literally hours of Google searching to find a few more posts that also say the rear sight set screw is a 1/16 Allen. Wow. That was harder to find than it should have been. The Smith & Wesson manual doesn't even say. I may be wrong about the 9 Pro, but many M&P's use this size screw if anyone ever loses it: # 5-40 or a metric pitched 0.6. The S&W part #108010000 is listed at 27 cents cost.
Q. Where is a good place to find Loctite 242 (Blue)? CanTire and HomeDepot do not list it online.
...I used to build RC cars and the only 1/16th allen keys I could find that wouldn't strip the head were sold in the RC stores.
Or you could have gone with a quality pistol like a sig or glock and wouldn't have had this problem!!!
Thanks wherermykeys, I know where you're coming from. This is what my toolbox looks like. Even still, I had no 1/16 SAE hex in there. Shocking when you think of all those pinion gear set screws that must have been about that size.
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Thanks wherermykeys, I know where you're coming from. This is what my toolbox looks like. Even still, I had no 1/16 SAE hex in there. Shocking when you think of all those pinion gear set screws that must have been about that size.
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Does anyone know what size allen key fits the set screw on the M&P 9 rear sight? I have tried ALL of mine (both metric and empirical).
Or you could have gone with a quality pistol like a sig or glock and wouldn't have had this problem!!!



























