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