SteererD. 28.6 (1 1/8")
There are two critical measurements for a stem: (1)Steerer tube and (2)Handlebar diameter. The steerer tube is almost always 1 1/8 (28.6) right from the beginning of the clamp on stems in 1990. Therefore many stems in the catalog only specify the handlebar diameter. The handlebar diameter has changed from the 1990's. It used to be 1" (25.4mm), but now many handlebar diameters are 31.8.
Some stems specify both key measurements:
Clamp Diameter (Handlebar Clamp Diameter)
Steerer Diameter (Fork Diameter)