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