The key factors influencing the durability and reliability of an F6 engine during its design phase include material selection, design tolerances, thermal management, lubrication system efficiency, and component fatigue life. Engineers must choose materials that can withstand high temperatures and stresses, ensure precise manufacturing tolerances for proper fit and function, manage heat effectively to maintain optimal operating temperatures, design a lubrication system that protects moving parts, and calculate fatigue life to predict how long components will last under cyclic loading.