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