Preferably, {HEAD ~ the annular chamber} is defined by {TAIL ~ an annular groove} in the first hub collar 
is defined by
Preferably , { HEAD ~ the annular chamber } is defined by { TAIL ~ an annular groove } in the first hub collar
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH