Hi everyone
I bought a bike from ebay and thought the headset was loose so i took the topcap off and i think the compression plug was too low in the carbon steerer according to the manufacturer's instruction (cannondale) the plug should support the lower stem bolt, the top cap the upper bolt, i rectified that. However after inspecting the fork steerer I noticed a line where the stem clamped the fork, see attached pic. Is that something i should be concerned about? can the fork break on this line? the line is not deep and it seems purely cosmetic but i know nothing about carbon ...

I my experience (Let me start by saying I'm nowhere near a carbon expert) if you do the tap test and make sure there's no separation, bubbling, etc, it should be ok. So, tap with your finger or a penny up and down that steerer and if there's any 'different' sound specifically on or around that line, there could be an issue. Inspect for any separation or bubbling as well. I had a nasty fall once that knocked my rear wheel out of alignment and I rode for a little bit with it rubbing on my frame.. wore a nice little spot out, too.. But according to a carbon expert, there was nothing to worry about, just superficial. What you don't want is separation or delamination or a crack/hole.

There's a fella or two on these forums that work with carbon that can probably give better information, but hopefully this helps anyways.

if you are referring to the line near the middle of the steerer tube, you have nothing to worry about. It's from the manufacturing process, and would have been sanded smooth if it was in a visible portion of the fork.