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