Abstract: After comparing the requirements for the specimen processing tolerance as specified under Specification API RP 5C5-Edition 2017 with that as specified under the 2003 edition of the same specification,it is concluded as follows. The critical parameters to be controlled for the specimen processing as specified under both the said latest edition and the old edition of API RP 5C5 mainly include the seal diameter,the thread meddle diameter and the thread taper,etc.,among which the control of the thread taper as described under both editions of the API specification are the same,whereas the seal diameter and the thread middle diameter are different;in the old edition the thread diameter and seal diameter deviations are emphasized,while in the new edition,the magnitude of interference is emphasized. And the old edition specifies two limit tolerances i. e.,the interference magnitude and the low interference magnitude,while the new edition specifies four limit tolerances,i.e.,the super high interference magnitude,the extreme low interference magnitude,the interference magnitude,and the low interference magnitude;and in general,the tolerances as specified under the new edition are stricter and more rational,while the old edition-specified tolerances are just special cases of the new edition.