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