A CTi failure loss analysis is made on the motorcycle engine cylinder head bolt fracture. The material, hardness, tensile strength, inclusions in bolt split are qualified; the tempered sorbite of matrix microstructure is normal; surface decarburization depth (8.3 um) also meets the requirements. But there is a crack folded at the bottom of the screw tooth of screw tooth defect, which is not in conformity with the GB5779.3 requirements. Presuming folded crack as the origin, fracture occurs resulting from crack propagation when it is being used. Through the orthogonal experiment, the six parameters of thread rolling technology are improved and the thread folding is eliminated.