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