Mandatory Appendix XXVI of the ASME Boiler and Pressure Vessel Code (BPVC), Section III, Division 1 currently permits the use of high-density polyethylene (HDPE) in buried safety Class 3 piping systems. There have been concerns about how the slow crack growth (SCG) of HDPE emanating from surface scratches that may occur during fabrication or installation. The current allowable scratch depth in Appendix XXVI is 5% of the pipe wall thickness for pipe 4 inch and less in nominal diameter pipe and 0.040 inch (1mm) for pipe greater than 4 inch nominal diameter. This report presents the results of further investigation into the SCG rates by testing of notched PE 4710 HDPE pipes made from PE 4710 cell classification 445574C bimodal resins that meet the requirements of Appendix XXVI. These tests were conducted by first making razor-cut surface scratches in 4 inch, 8 inch, and 16 inch nominal diameter piping made by three different piping manufacturers using three different resins. The pipes were end-capped and pressure-tested at elevated temperature until failure, or until a prescribed number of test hours were reached. The razor cuts were at different depths and lengths so as to result in a variety of stress intensities, net section stresses, and nominal stresses. Following failure, or after the prescribed number of test hours was reached, the pipe specimens were inspected to determine the amount of SCG. This paper presents the results of the testing and recommended ASME BPVC Appendix XXVI changes for allowable crack depth based on the testing are provided.