Hi Danny - yeah something seems to be wrong with the bottom surface of the sector part - that surface is not getting shaded properly and it almost seems to be getting shaded with a little bit of stuff going towards the outside of the surface. That could mean that some process constructed it incorrectly with the trims going in the opposite direction from what they are supposed to (normally outer trims are supposed to go in a counter-clockwise sequence in UV space).

I've attached a repaired version - I extracted that bad face, copied its edge curves and untrimmed the surface, then pasted those edge curves back in and retrimmed to them, and then joined the newly trimmed surface back into place.

This version should work as expected now - you can now select the sector and the cylinder and use the boolean Merge command to generate a cut sector and also keep the plug as well.

So it looks like some kind of geometry bug happened at some point in construction of the sector, unfortunately it's difficult to figure out where the bug was at just by examining the end model...

- Michael