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 {HEAD ~ a range} of {TAIL ~ 0° to 6°} 
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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 { HEAD ~ a range } of { TAIL ~ 0 ° to 6 ° }
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