In motorcycle engines with aluminum crankcases, fatigue fractures at the roots of the internal threads of the fastening bolts used for the cylinder head and crankshaft main bearing often occurs during the durability tests at the prototype stage. A technology that evaluates the fatigue strength of the entire crankcase including the roots of internal threads using a large-scale and nonlinear finite element method (FEM) analysis is established by this research. Parallel process computation by a cluster server enables the evaluation of the fatigue strength of the crankcase in a short time suitable for the development process even when using a model that faithfully reproduces the shape, the contact property, and the elasto-plastic material characteristic of the threads. This technology enables the efficient design of crankcases that are light and durable. Moreover, the mechanism of fatigue fracture at the root of the internal thread is clarified because a quantitative evaluation of thread root stress is possible.