In CNG steel cylinders produced by the hot spinning method from seamless tubes, homogeneity of the material in the closed-end region is a significant factor. Therefore, in the production process, the produced cups are immediately under a hot heavy press, and in addition to forming the bottom shape, the heterogeneous area is driven out, finally, this region is eliminated via machining. Ideally, the depth of this milled area should be to the extent that the heterogeneous region is completely removed, also according to the designing standards, the minimum bottom thickness shall be more than that of wall thickness. However, the problem is that controlling this parameter is impossible in the machining process, and in some cases, it may become more than the specified limit. In this research, the minimum allowed remained thickness after bottom milling has been obtained by analysis and also compared with real results acquired from static and fatigue tests. Deducing the results of the FEM analysis, it can be seen that the remained bottom thickness up to 6mm has no limitation for the endurance hydrostatic pressure, and that was supported by experimental burst tests. Considering this amount for the bottom thickness, the percentage of rejected products declined considerably from 4% to 2% in the factory. Manuscript profile