Traditionally, gear rating procedures on directly consider manufacturing accuracy in the application of the dynamic factor, but only indirectly through the load distribution consider such errors in the calculation of stresses used in the durability and gear strength equations. This paper discusses how accuracy affects the calculation of stresses and then uses both statistical design of experiments and Monte Carlo simulation techniques to quantify the effects of different manufacturing and assembly errors on root and contact stresses. Manufacturing deviations to be considered include profile and lead slopes and curvatures as well as misalignment. The effects of spacing errors, runout and center distance variation will also be discussed.