I used a dial indicator and a woodworking lathe to check my driveshaft.
I first hand rotated it to see if I had any run out and then ran it at low speed. It kept exceeding my expectations. By the time I was done, I spun it up to 2500RPM.
I made a custom mounting plate for the flex disk end pretty quickly and bolted it on and used a live center for the other end. I wouldn't do the project for someone else but my results have been good for quite a while.

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2001 x5 3.0 (sold with broken motor), 1990 e30 M3, 1991 318is, 2002 325i, 2008 335i Dinan stage 3 6 speed,