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