A multiple blow percussion drill assembly is specially configured to provide significantly increased maintainability and adjustability in the field, thereby greatly extending its service life and usefulness over a wide variety of drilling conditions. Both spring replacement and impact hammer delay time variations are rapidly accomplished by virtue of unique positioning and interactions of the internal components, without the need for dissassembly of the sealed high impact energy portion of the drill assembly. Advantageously, a first chamber housing the high impact energy transferring components is isolated from a second chamber housing the control components, such that the control components may be rapidly cleaned, adjusted, or replaced via a simple access port.
