I started looking - surprisingly, the BS-7, 50-60 Sport, Homer and 90-100 manuals show no torque specs for the head nuts. The Tora/Chibi manual (60 CC) similar construction shows "Tighten the head nuts progressively and evenly to 125 inch lbs. of torque." That value is right at the top of the torque range for a 6mm bolt, so should be good.