A vent hole in the wrap of {HEAD ~ the orbiting scroll member} cyclically moves over {TAIL ~ the two grooves} 
A vent hole in the wrap of { HEAD ~ the orbiting scroll member } cyclically moves over { TAIL ~ the two grooves }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL