‘Built Axles’ are for BMW customers with a ring gear which is friction welded to the OEM differential, commonly on BMWs manufactured past 2007. To upgrade to a Wavetrac, or any other aftermarket differential this friction welded ring gear and differential assembly needs to be separated. Once separated, the ring gear can be machined and threaded for use with a Wavetrac LSD. In the past, this would result in a number of days of your vehicle being off the road waiting for the machine shop but today, Wavetrac Europe stocks a range of the most popular pre-built and Wavetrac equipped axles so you can install in a matter of hours rather than days.