Overdrive 2 Stems for 1-1/4" Steerer Tube

Giant's Overdrive 2 steerer uses a 1-1/4" (31.8mm) diameter at the top of the fork. The standard steerer diameter is 1-1/8" (28.6mm). So, you can't use an adaptor to use a standard threadless stem on an Overdrive steerer; you need to use the matching Overdrive stem.

Giant Contact OD2 Stem

The Giant Contact stem is engineered to offer maximum stiffness at the lowest possible weight. Giant's 1-1/4" OverDrive fork and stem design ensures confident, precise handling with all-day comfort to help you ride farther and faster.
Comes complete with a spacer sleeve so it suits OverDrive1 (1-1/8"; 28.6mm) steerers as well as OverDrive2 (1-1/4; 31.8mm) steerers. Shipping Weight: 300g.

Pro Vibe 7S OD2 Stem 10deg/80mm

Short stem option for Giant road bikes with an Overdrive-2 steerer tube (1-1/4" outer diameter at top).
Puzzleclamp system for optimized front clamp fixation. Triangular body for optimised weight vs rigidity ratio. Alloy 7075 construction. Extensions 80 to 140mm. Angle: -10 degrees. Handlebar Clamp Diameter: 31.8mm. Weight: 123g. Shipping Weight: 175g.