It will be seen that the primary inner shell 30′ has an external diameter D1 while {HEAD ~ the secondary inner shell} has {TAIL ~ an internal diameter} D2 
has
It will be seen that the primary inner shell 30′ has an external diameter D1 while { HEAD ~ the secondary inner shell } has { TAIL ~ an internal diameter } D2
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH