shaft splines - they are smaller than the R230 diff in the TT and they are typically what fails in high-torque conditions (aka: launching at 6KRPM with drag slicks). I dont really plan to go nuts with drag racing - it really isn't my gig, but if the rear fails, I will definately be purchasing a complete TT rear end and going with the 4.10 Leader gear set. I like this gearing ratio FAR too much to give it up. :)