Rate, hear we go. Not sure about the arc on the inside corner but I couldn't be arsed working out the dimensions for it to be an S shape, so for now you'll have to imagine. It actually has an M5 thread so you can use a standard topcap and an M18x1.0 thread so you can use the trialtech topcap thing aswell. You simply screw it into the steerer, then put your stem on it part way and use that to tighten it into the steerer, then drop your stem all the way. I wouldn't use this with any less then 50% of the stems clamp on the actual steerer though, might not offer enough stiffness/stregnth. And if you had any issues such as the steerer tube snapping just below the stem, your warrenty would undoubtably be void. EDIT: Dimensions are very rough at the moment because I can't remember where exactly to measure threads from, so if anyone decides to make it up or something similar, don't take my dimensions as read.