Abstract: A Belleville spring washer (15) having an annular relief (25) depending from in its top surface (24). The relief (25) is disposed adjacent the circumferential wall (27) forming a centrally disposed opening (18) for the washer (15), the circumferential wall (27) not physically engaging the shank (37) of a fastener (39) when mounted and torqued thereon. The washer (15) includes a series of radially-extending serrations (23), each beginning adjacent the relief (25) and extending outwardly through the span (21) of the annular body (17) forming washer (15). In its mounting to a fastener (39) to be applied to a bearing surface (45), the serrations (23) face the undersurface (40) of the fastener's head (42), and the head (42) is torqued to a proper setting. The serrations (23) efficaciously bite into the head's undersurface (40), providing a tight engagement of fastener (39) to the bearing surface (45), and one which eliminates looseness in the fastener (39) upon a thermal expansion of the metal of the elements.