The surfaces of {HEAD ~ oxidation films 13a}, {TAIL ~ 13b} are mirror finished if necessary 
The surfaces of { HEAD ~ oxidation films 13a } , { TAIL ~ 13b } are mirror finished if necessary
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH