A point, the kosher screws have some form of anti vibration fall out 'glue' on them. If you do use your own screws it might be sensible to apply a SMALL drop of Loctite etc. to each screw to replicate this. DO NOT excessively tighten the screws, I know that is a subjective statement but it's the best I can do.
When you remove all the prop guard screws from one motor that motor may be loose enough to carefully and gently move it out of the way enough that you can see the inner face of the motor mountings. If so put a screw through the mounting, from the outside, and measure its 'inwards' protrusion. If you do not have calipers or do not have the clearance etc. to use them try using some card etc. pressed against both the mounting face and screw and mark the end of the screw on the card. Then CAREFULLY screw a screw into the motor to the depth of the protrusion STOP IF YOU FEEL IT HIT AN OBSTRUCTION.
The screw shown in my first post was taken from the motor mount of a dummy drone (ex shop display) with coloured prop screw motors. I have just measured the length of the screw taken from a working drone that has dotted ended prop shafts and it is 6mm inc the shoulder.
The mounts of the two type of motor are shown in the attached, it looks like the screw holes in the coloured shaft motors are not as deep as those in the 'dotted' motors and there is a plastic guard inboard of the screw holes in the coloured shaft motors. I have a 'for parts' drone somewhere which I think is of the coloured shaft type, if I can find it I will check the length of its screws.
One other thing to consider is the fit of the screw heads in the recesses in the lower shell half, bearing in mind that these shells seem to be prone to cracks, it might NOT be a good idea for the screw heads to be so large that they foul the side walls of their recess and are drawn into those recesses as the screws are tightened.
All the above said if your prop guards are of the detachable type, quite a few people leave the attachment points on the drone even if they never use the guards
