The relationship of J factor and internal casting quality in high pressure die casting was investigated by leakage testing conducted after the machining process. The trial die, used for motorcycle engine applications, was a single closed-deck-type liner-less cylinder block with an expendable salt core. The J factor was calculated by using the VanRens equation with varied gate thickness, gate speed, plunger diameter, and casting pressure. To confirm casting quality by leakage testing, we conducted X-ray computerized tomography (XRC), micro-focus XRC, and thermal decomposition gas chromatographic determination analysis. In addition, XRC was used to calculate the total shrinkage volume. A strong correlation was found between the J factor and tightness against pressure leakage.