I like to have both the transmission in neutral, and the wheels unblocked. That way you can rotate the flex disc once it is in place with the first bolt and it's easier to get to the bolt heads if you can position them where you want them to be. To keep the assembly from rotating, when tightening the bolts, I have used a large screwdriver inserted in a u-joint.

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Tomas, Sweden
1966 Mercedes Benz 230S with OM617.912, automatic. Disk brakes from W108
1983 Mercedes Benz 300TD grey, OM617.912 and 5-speed manual
1983 Mercedes Benz 300TD blue 7-seater, OM617.912 and 5-speed manual