Box Two Centre clamp Hollow stems are cold forged from the highest quality 6061-T6 aluminium available. Featuring an open skeletal design and a dual mid-bolt wedge mounting system, they are lightweight yet maintain a very high level of strength. A 53mm extension stem weighs just 218 grams/7.7 ounces. By removing the rear pinch bolts and moving the mounting bolts to the middle of the stem, we reduced the weight and size and eliminated a serious hazard to the riderâÂÂs knees.