Abstract: A method of rendering an aluminum base metal surface resistant to water staining is provided which comprises providing an aluminum base metal surface and applying a stain resistant coating to the aluminum base metal surface, with the stain resistant coating containing a water soluble molybdate salt and water soluble nitrite compound in an amount sufficient to render the aluminum base metal surface resistant to water staining. An aluminum base metal article of manufacture which is resistant to water staining is also provided.

Abstract: A percussion drill having a rounded and tapered contact surface conformation between the inner ends of hardened insert members closely received by cavities in a drill bit. Each insert has a radiused inner end in contacting relation with the bottom wall of its associated cavity. The radius is such that it compensates for angular deviations in the cavity bottom wall due to manufacturing tolerances from a normal relation relative to the cavity longitudinal axis. The radius of the insert inner end is calculated on the basis of a mathematical formula. The arrangement provides a structure which prevents corner load and stress, and insures contact away from the corner to thereby increase bit life.