A = 22,4 mm
B = 30,0 mm
C = 27,0 mm
SHI: standardized Headset Identification System
EC30 / 25.4 | EC30/26
This is a classic tax rate (upper and lower shell) with an inner diameter of the head tube of 30mm (top and bottom), a 25,4 mm measured Steerer (1 ") and a Gabelschaftsmaß of 27mm to the brace.