I recently bought a P365 SIG and it was designed from the beginning with a safety in mind. Usually they only ship to Massachusetts IIRC but as the parts became available more have been installing them.
It's not rocket surgery, the initial decision is do you keep it ambidextrous, or just left side? Which involves Mr. Dremel, and we all appreciate just how boisterous he can get. Looking at my array of tools vs what kind of bits and stones I had, the grip got filed with literally a utility lawnmower sharpening file. I was using a 3D printed jig to confine my cut, it pins in and precisely controls what you do. I chose to do only left side, and cut off the right side lever where it would stick out of the FCU. Assembly was no problem - at least at first. Safety went up and down, detent snapped into the notches, and I worked it with some pressure until I felt it smoothing out.
From there you simply insert the FCU, pin up, and then install the slide, where things got interesting.
Let me say that if you push the slide on and it gets stalled at the slide stop, take it apart, check to see if the slide stop has downward spring pressure. If it fell out while you were installing the safety, nope, the spring leg gets under the stop, and jams the slide. That leg goes on top of the mag follower hook sticking out into the well, giving it downward pressure. Otherwise, no go.
Second issue was the slide stopping at the sear - goes to the next item. Does the slide lever act with spring pressure pushing it away from the 7 oclock position? It too should be spring loaded to go back to the default position. If not, check the sear leg under the FCU to see if the lever strut is engaged - which allows the slide lever to pull down the sear. That strut can pop out and get on the wrong side, making the sear jam in the up position. There is a 90 leg that engages the sear leg to connect the motion, and the other end a circular knob to connect the lever. Properly oriented, pushing the lever down should cause the sear to retract and the slide to move fully to the rear.
So, there is a reason why SIG says "Don't remove the FCU to clean it." Three hours into researching it online and fixing each step as I discovered answers, I was finally able to function check the safety. Now I need to find time to go shoot it. Right, I haven't even done that yet. Oh well.
TL;DR check to see if the lever and catch are spring loaded and functioning correctly before reassembly.
It's not rocket surgery, the initial decision is do you keep it ambidextrous, or just left side? Which involves Mr. Dremel, and we all appreciate just how boisterous he can get. Looking at my array of tools vs what kind of bits and stones I had, the grip got filed with literally a utility lawnmower sharpening file. I was using a 3D printed jig to confine my cut, it pins in and precisely controls what you do. I chose to do only left side, and cut off the right side lever where it would stick out of the FCU. Assembly was no problem - at least at first. Safety went up and down, detent snapped into the notches, and I worked it with some pressure until I felt it smoothing out.
From there you simply insert the FCU, pin up, and then install the slide, where things got interesting.
Let me say that if you push the slide on and it gets stalled at the slide stop, take it apart, check to see if the slide stop has downward spring pressure. If it fell out while you were installing the safety, nope, the spring leg gets under the stop, and jams the slide. That leg goes on top of the mag follower hook sticking out into the well, giving it downward pressure. Otherwise, no go.
Second issue was the slide stopping at the sear - goes to the next item. Does the slide lever act with spring pressure pushing it away from the 7 oclock position? It too should be spring loaded to go back to the default position. If not, check the sear leg under the FCU to see if the lever strut is engaged - which allows the slide lever to pull down the sear. That strut can pop out and get on the wrong side, making the sear jam in the up position. There is a 90 leg that engages the sear leg to connect the motion, and the other end a circular knob to connect the lever. Properly oriented, pushing the lever down should cause the sear to retract and the slide to move fully to the rear.
So, there is a reason why SIG says "Don't remove the FCU to clean it." Three hours into researching it online and fixing each step as I discovered answers, I was finally able to function check the safety. Now I need to find time to go shoot it. Right, I haven't even done that yet. Oh well.
TL;DR check to see if the lever and catch are spring loaded and functioning correctly before reassembly.