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