Originally Posted by
Velo Mule
What?? Wait!! What is that? I'm agreeing with [MENTION=348133]panzerwagen[/MENTION] that this split is from the stem expander, however, it looks like there are no threads in just that area but are around the rest of the steerer tube. That area also looks to be painted rather than bare metal.
Was this section of the steerer tube indented, which would be why it didn't get threaded and then got painted? Then being indented, cracked when the stem expander was tightened?
Can we get a few more pictures from different angles. This sure is odd.
It is fairly common for French bikes to have a flat milled on the steer tube with a matching washer, to serve the same function as a slot on most other forks, fitted with a keyed washer. The steer tube is threaded, and the flat milled through the threads prior to painting.
The damage was caused by a combination of overtightening the stem bolt and setting the stem too high in the steer tube. The expander wedge or cone on the stem should sit below the threaded section of the steer tube.