In {TAIL ~ a spiral groove} bearing a gap between a surface portion which has grooves thereon and its opposing surface portion is smaller than the gap between any other surface portion which does not have {HEAD ~ grooves} thereon and its opposing surface portion 
In { TAIL ~ a spiral groove } bearing a gap between a surface portion which has grooves thereon and its opposing surface portion is smaller than the gap between any other surface portion which does not have { HEAD ~ grooves } thereon and its opposing surface portion
OTH TAIL 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 OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH