The main port is {HEAD ~ a large central recess} 9 through which {TAIL ~ the belts} of continuous grinding belts can be changed 
The main port is { HEAD ~ a large central recess } 9 through which { TAIL ~ the belts } of continuous grinding belts can be changed
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH