49 CFR § 230.24
Maximum allowable stress
November 18, 2020
(a) Maximum allowable stress value. The maximum allowable stress value on any component of a steam locomotive boiler shall not exceed 1⁄4 of the ultimate tensile strength of its material.
(b) Safety factor. When it is necessary to use the code of original construction in boiler calculations, the safety factor value shall not be less than 4.