1.5, 36°×45°. For 55mm OD head tube. For 1.1/8" to 1.5" tapered fork. Alloy top cover with reducer cup. CNC machined alloy cups. 1.5" Angular Contact Bearing (black seal).
Best Price: £19.99 from Merlin Cycles
FSA No.55R Semi Integrated 1.5" Step Down Headset – Black / Semi-Integrated / 1.5" / 1 1/8th Step Down Reviews