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The gradual loss of preload of steel fasteners set into zinc alloy pressure die castings and held at constant temperatures of 50°C or 80°C was continuously monitored over a period of 150 hours using a calibrated load cell. For the commercial alloys Alloy 3, Alloy 5 and ZA-8, the degree of load loss was found to obey a power-law kinetic relationship with time. Choice of alloy had a powerful effect on preload relaxation: Alloy 5 was markedly superior to Alloy 3 with ZA-8 best of all by a small margin. The effects of changing the geometry of the screws and the methods of producing the threads in the alloy castings were determined for the three alloys in a series of detailed experiments on M6x1 screws torqued to give an initial preload of 6 kN (1349 lbf.). These tests showed that the worst resistance to load relaxation was obtained by using a self-tapping screw in a plain hole in the casting.