As described, the testing apparatus rod and mandrel threads into and pulls on {TAIL ~ the expander member} of {HEAD ~ the ribbed plug} 
As described , the testing apparatus rod and mandrel threads into and pulls on { TAIL ~ the expander member } of { HEAD ~ the ribbed plug }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD