Conventionally, the engines are calibrated under the assumption that engines will be made exactly to the prints, and all the engines from the same batch will be identical. However, engine-to-engine variations do exist which will affect the engine performances, and part-to-part variations, i.e., the tolerance, is an important factor leading to engine-to-engine variations. There are researches conducted on the influence of dimensional tolerances on engine performance, however, the impact of straightness, which is an important geometric tolerance, on lubrication is an unsolved issue. This study presents a systematic method to model the straightness and to analyze its effects on the friction loss. The bearing model is built based on elastohydrodynamic (EHD) theory. Meanwhile a novel modeling method to represent any form of straightness in three-dimensional space is proposed. Then the meta-model with straightness as the input and friction loss as the output is built based on Kriging interpolation theory. Genetic algorithm (GA) is utilized to search for the straightness forms that lead to the best and worst lubricating conditions, respectively. Results show that straightness of the U-shape could lead to an increase or decrease of friction loss and this is mainly determined by the straightness phase. It is also shown that the degree of friction variation at a certain straightness phase is determined by the shape and amplitude of straightness.