Firstly, make sure you extend the axle to the maximum point possible by pulling on both ends. Secondly, from the outside of each end, cup face to get your extended measurements. Then push the CV's together to the maximum point possible and measure again. In addition, this is your compressed measurements. The compressed and the extended measurements are among the specifications designers and engineers used to measure the length of an axle. To determine the axle you need to use these specifications.