A B777 EGT slightly overheated to 680 degrees on an ETOPS takeoff. The Captain was concerned that continuing ETOPS with that event was not possible but was told that because the MEL did not prohibit operation and the actual limit was 685 so the flight continued.

Narrative

Takeoff OAT 93F; max power; packs off; max allowable takeoff weight. As the gear was being retracted the left engine EGT gage displayed a full red indication. The EGT limits came back within the normal range when the throttles came to climb power after the flaps moved to position 1 climb power. The crew felt the overheat lasted approximately 5 seconds and reached approximately 680; 5 degrees above the limit of 675. The onboard maintenance computer registered 676; 1 degree above limit. Crew felt comfortable with the engine's performance during the rest of the climb. Captain asked for a conference call to be set up between Maintenance; Dispatch; the 777 Chief Pilot. I wanted an answer to the question 'Is it legal to go ETOPS with an engine that had exceeded a published limit.' Maintenance said that there was a 20 degree buffer built for exceedances; and that 685 was the real limit. The 777 GURU joined the conference call and said he thought it was legal to continue if the MEL did not prohibit it. Crew decided to continue.