Down reaming apparatus

A cutterhead for a rock boring apparatus comprising:

• a cutterhead body;
• a plurality of arms radially disposed on said cutterhead body, said arms having cutter assemblies thereon, said plurality of arms including a first arm having a given length and additional arms, said additional arms each of a different length that is less than the given length of said first arm, said additional arms being oriented on said cutterhead body such that the lengths of said additional arms are decreased by the same amount from each of said additional arms to another of said additional arms.
• a plurality of arm extenders having cutter assemblies thereon, said arm extenders being oriented in a first position in which each of said arm extenders is attached to one of said arms, said arm extenders each having a length such that the combined length of each of said arms and attached arm extender is substantially equal;
• and
• spacer means having a cutter assembly, said spacer means being adapted to be attached to at least one of said arms to increase the diameter of said cutterhead in conjunction with relocation of each of said arm extenders from said first position to a second position on another of said arms such that the combined length of each of said arms and attached arm extender in said second position is substantially equal and is greater than the combined length of each of said arms and attached arm extender in said first position.