Box One Center Clamp 31.8mm Stem 1-1/8"

Product Overview

The Box One Center Clamp is a front-load stem designed for BMX racing with 31.8mm oversize handlebars. Cold forged from 6061-T6 aluminum with a dual mid-bolt wedge mounting system, it eliminates rear pinch bolts to reduce weight and knee hazard while maintaining strength. Features an open skeletal design and reversible 5-degree rise/drop configuration.

Key Features

• Cold forged 6061-T6 aluminum construction – Forging process compresses aluminum grain structure for maximum strength-to-weight ratio, creating the lightest oversize BMX stem available

• Center clamp mounting system – Dual mid-bolt wedge design replaces traditional rear pinch bolts, reducing weight and eliminating potential knee contact points during crashes or sharp turns

• Open skeletal design – Material removed in non-structural areas minimizes weight while maintaining stiffness at handlebar and steerer clamp interfaces

• Reversible rise/drop – Can be installed with 5-degree rise or flipped for 5-degree drop to adjust handlebar height and rider position

• 31.8mm oversize handlebar clamp – Larger clamp diameter distributes clamping force over increased surface area for secure handlebar retention with reduced stress concentration

Technical Specifications

• Material: 6061-T6 aluminum
• Steerer tube clamp: 1-1/8"
• Handlebar clamp diameter: 31.8mm (oversize)
• Clamp type: Front-load
• Mounting system: Dual mid-bolt wedge (center clamp)
• Rise/drop: ±5 degrees (reversible)
• Extension lengths: 53mm or 60mm
• Weight:

• 53mm: 244 g (8.6 oz)
• 60mm: 246 g (8.8 oz)
  • Stack bolt: M6×20L (P1.0)
  • Clamp bolt: M8×20L (P1.25)
  • Minimum exposed steerer required: 40mm
  • Carbon steerer compatibility: Not compatible

Compatibility and Ideal Use Cases

Designed for BMX race bikes with 1-1/8" aluminum or chromoly steerer tubes and 31.8mm oversize handlebars. Requires minimum 40mm of exposed steerer tube above the headset top cap for proper wedge engagement. Best suited for competitive racing where lightweight components and clean cockpit design are prioritized. The center clamp design eliminates rear pinch bolts that can contact rider's knees during crashes or aggressive cornering. Available in 53mm (shorter reach, more upright position) or 60mm (longer reach, lower position) extensions to accommodate different rider preferences and frame geometries. Not compatible with carbon fiber steerer tubes or standard 22.2mm handlebars—verify fork steerer material and handlebar diameter before ordering.

Note: US orders ship direct from manufacturer and may arrive separately from other items. Available in 60mm length in black color only.