{HEAD ~ The threading action} of {TAIL ~ the mandrel} into the expander draws the seal against the tubesheet to seal the testing apparatus and allows the rib plug to be in situ pressure tested 
of
{ HEAD ~ The threading action } of { TAIL ~ the mandrel } into the expander draws the seal against the tubesheet to seal the testing apparatus and allows the rib plug to be in situ pressure tested
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH