Freddy, thanks again for posting this. I'm having a bit of trouble with an 8mm BREVET crown and referred back to this for help.
On my 8mm crown the central stem tube is square and does not protrude from the crown post very far at all; its face is nearly flush with the crown post face, perhaps proud of that face by 1/10mm or less.
When I unscrew the crown from its tube there is good spring action to extend the crown from the tube. But often when I attempt to set the time or wind the watch, the crown just spins as if it's disconnected from the stem. Investigation reveals that the stem tube is down inside the crown post, so the square portion of its shape does not engage the crown post. If I pull out slightly on the crown and turn it, I feel it spin and then click out into place so the stem tube is again protruding from the crown post. Then I am able to wind and se the watch.
This makes it seem like the inner post spring is too short. But like I said there is good spring tension to pop the crown out from the tube when I unscrew it from the closed/sealed position.
Any thoughts? I suspect (sadly) that I have got a very sophisticated and nearly perfect fake crown here. All other details are exactly right.