The presence of the gap 602 and the absence of a gap 601 indicates that {HEAD ~ replacement} of {TAIL ~ the consumable part} 605 is necessary 
The presence of the gap 602 and the absence of a gap 601 indicates that { HEAD ~ replacement } of { TAIL ~ the consumable part } 605 is necessary
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH