Another advantage to the 9-inch is that the axles are easier to install and remove. Ford designed the factory 28- and 31-spline axles to be removable by yanking four bolts per axle flange, which allows the axle to slide out. The axles have pressed-on bearings and retainer plates that bolt to the housing flanges. There are no C-clips inside the rearend holding them in place like on a 12-bolt. The plus here for the 9-inch is that if you break an axle inside the housing, the axle retainer will keep the axle in place. On a 12-bolt, breaking an axle means it slides right out of the housing, causing massive sheetmetal damage along the way. You can convert a 12-bolt to the 9-inch style of axle retention, but it requires a C-clip eliminator kit and modifications to the housing.