Enter the knuckle radius r for torispherical heads, according to ASME Section VIII Div. 1. This dimension is usually referred to as IKR in many head catalogs. Even though the head catalogs list these heads as being OD heads, the knuckle radius is given on the inside diameter basis. See the illustration in the catalog and where the arrows for DR and IKR point to (the inside of the head). For more information, see Appendix 1-4 in the Code.