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