Abstract: During the trial production of a four-cylinder diesel engine,cracks occurred on a cylinder block. In order to ensure the engine's safeworking, the issue must be solved. In this paper, mechanics theory is appliedto analyse the issue and it is concluded that the cracks were caused by unevenforces acting on the block. At the same time, the experimental stress anlysis iscarried out to study two types of four-cylinder engine blocks, andthe effect of cylinder washer on block's stress and cylindersheath's distortion is found. So a method is given to reduce the block'sstress and the cracks have not appeared again. Oil-leak, air-leak andcylinder scratch have not occurred either, and reliability is improved greatly.