Angles θ1a, θ2a, defined respectively between a diametrical plane P1 and {HEAD ~ a first ridge plane} 1a facing {TAIL ~ the small diameter portion A} and between a diametrical plane P2 and a second ridge plane 2a facing the small diameter portion, are in a range of 0° to 6° 
facing
Angles θ1a , θ2a , defined respectively between a diametrical plane P1 and { HEAD ~ a first ridge plane } 1a facing { TAIL ~ the small diameter portion A } and between a diametrical plane P2 and a second ridge plane 2a facing the small diameter portion , are in a range of 0 ° to 6 °
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL 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