ZIPP SL Sprint Carbon Stem

The Zipp SL Sprint Stem is massive. For good reason. They set out to make the stiffest stem while still keeping it light. With a reasonably light 165g, the stem actually weighs in at 1.8g per Newton meter of stiffness. This means watt monsters like Mark Cavendish gravitate to it.
                       
The bigger your arms, the more upper body oomph you can apply to your handlebars, the more you want a super-stiff stem. No reason to let that energy go to waste. And just to make sure they're still keeping in line with their aerodynamic ethos, the aluminum handlebar cap is shaped to flow smoothly into the stem.
 
One difference you probably see is the bolt heads. It's a small change from the norm, but in many respects, a big deal. Zipp has opted for Torx bolts for a few reasons. They won't easily strip, when a Torx bit is used in a torque wrench, they readings are more accurate, and both these things mean that the bolt heads are more durable. Over-torque an Allen-head bolt, and you could start to strip the interior walls of that bolt. Use a Torx instead and you won't face the same risk if you over-torque. And then, if you choose to use a torque wrench, the wrench's readings will be more accurate with the Torx bolt in there. The particular size, T25 was chosen because it is seeing increasing use with disc brakes, chain ring bolts, derailleurs, and shifters. It' also on more and more multi-tools and is easy to find in hardware stores. But, just to be safe, Zipp includes a tool with this stem.