Originally Posted by
FlyerJosh
You don't need any medical to exercise the privileges of your CFI. Unless you get paid and your instructing in an aircraft. Then you are exercising the privileges of your commercial certificate and need a 2nd class medical.
This is incorrect in every way. You don't need a medical to instruct, unless you are giving instrument instruction or acting as PIC. If you are doing one of these things, you need at least a 3rd class medical. You are being paid for teaching, not for being a pilot, and don't need a 2nd or 1st class medical.