{TAIL ~ A new replaceable cutter shell} may be inserted in place of {HEAD ~ the worn replaceable cutter shell} 49 and the earth boring operation continued 
{ TAIL ~ A new replaceable cutter shell } may be inserted in place of { HEAD ~ the worn replaceable cutter shell } 49 and the earth boring operation continued
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH