HighFlyer,

Don't know that I agree with you. First of all, not all CFIs require a medical (glider CFI or Balloon CFI, FAR 61.3(c)). Also, if the examiner agrees to ACT as PIC on any practical, a medical is not necessary (see FAR 61.47(b))

What regulation requires the CFI to exercise the privileges of a commercial? If you must ACT as PIC (private student pilot, for example), then you only need a 3rd class medical. If you are a required crewmember by regulation -- a "safety pilot," for example, then you would only need a 3rd class medical. The only time you would need to have a second class medical is when you are exercising the privileges of your commercial pilot's certificate. I'm not sure I know of an example when this would happen or be required. Instructors don't exercise the privileges of their commercial certificate when they instruct. See FAR 61.23(a)(3)(iv) which indicates that only a third class medical is necessary. Go on to read 61.23(b)(5) as well.