It needs some careful measurement as you dont want the crown to bind with the case. When the stem is shortened the 1st position becomes the winding function. So if the crown binds you’ll be grinding the crown against the case or worse still the crown won’t wind the movement if it’s cut too short. Most decent watchmakers will know what to do. When it’s done right the crown sits nice and snug and looks great. As long as there is also an original gasket in the crown it should also be good enough to keep general moisture and dirt out as well. Obviously not as good as having a screw down crown but it does the job.