Cal, I believe the 1.6mm spacer is required when using the new motor mount as they've shaved 1.6mm off of the boss where the spur gear shaft goes into the motor plate (on the aluminum version). The old motor mount and the plastic one both seem to have that extra bit of material built into them.

BTW, I continue to have the odd problem of the car coming off the track at the end of a race, making a lot of noise in the gearbox. If I loosen and retighten the motor screws a few turns, it silences the problem. I'm beginning to wonder if heat buildup is making things move around, or if minor bumps on the track are throwing it off. I had a little knock while practicing last week and it began making a racket immediately. So, loosen, tighten, quiet. Weird.