I've searched this forum and was only able to find the spark plug gap for the 4g63T and 420a. Since the shop I work at only does work on Prevost coaches (Harper Detroit Diesel motors), I dont' have access to the automobile catalouges. The guy at the auto parts store had no clue what I'm talking about. Anyhow, if anyone knows the spark plug gap for a 4g63NA it would be greatly appreciated.