Down hole pressure intensifier and drilling assembly and method

A jet assisted drill system which uses a high pressure intensifier, positioned in a down hole drill assembly. Drill fluid from a drill stem is directed into the drill assembly, and in one mode this drill fluid is passed through the pressure intensifier to cause a piston assembly of the intensifier to reciprocate, with high pressure pistons of the piston assembly delivering high pressure drill fluid to a discharge jet in the drill bit assembly. The lower pressure fluid which drives the low pressure pistons is discharged into a downstream annular passageway and to the drill bit assembly. In a by-pass mode, a selector valve directs drill fluid from an upstream main passageway portion directly to a downstream main passageway portion to pass out the drill bit assembly. This flushes out debris which is carried upwardly out of the drill hole. There is a control valve to operate the piston assembly and a trigger valve operated by the piston assembly to direct fluids selectively to the control valve.