{HEAD ~ intermediate flat portions} of substantially equal diameter less than {TAIL ~ the diameter} of said corrugations between said corrugations, 
{ HEAD ~ intermediate flat portions } of substantially equal diameter less than { TAIL ~ the diameter } of said corrugations between said corrugations ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH