The star fangled nut which comes with the M40 headset is NOT suitable for carbon steerers. If you have a carbon steerer you will need to purchase a compression plug designed for carbon steerers.
The M40 uses Sealed ANGULAR CONTACT BEARINGS. Angular Contact bearings are better for headsets because they distribute both radial and axial loads evenly prolonging bearing life.
Stack Height: 9.6mm
See our HEADSET CHART if you are not sure about the spec you need.