These bearings are 8mm x 3mm with a 3mm central hole. 
The additional height allows for larger, stronger contruction all around but does protrude slightly further from the gearbox shell. 
Some selector plates may require modification to give clearance for the bearing racers. 

These bearings should be installed with a drop of bearing lock (we recommend Loctite 270)  ensuring no liquid enters the bearing. 
The gears should be shimmed prior to packing the bearings with Superlube PTFE grease.