In 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 {TAIL ~ its opposing surface portion} 
In 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 { TAIL ~ its opposing surface portion }
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 OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL