From what i can work out, Top ring gap = 0.0157" or (0.39mm) 2nd ring gap = 0.0186" or (0.47mm) So if my understanding is correct. Thats 74.5 x 0.03937 to give you the imperial bore diameter of 2.933065. Top ring, you times that by the first figure. 2.933065 x 0.004 = 0.01173226 Im assuming Its boosted? Its under 0.015 gap so add 0.004 To give you 0.0157" rounded up. You then repeat that for the 2nd ring but the gap multiplication is now by 0.005