[davidmanze]

  There's been many polls over the years on SDM motors and lens models, yes AFAI remember it was the 16-50mm (I had a dead one) and the DA 50-135mm and 17-70Mmm  which were the most common sufferers and the eldest, but sadly reports started filtering through about the rest, the DA55mm had the least problems being one of the lightest models. 

 

BTW. All the Pentax SDM lenses use the same motors and circuits except the DA 18-270mm which is basically a rebadged Tamron lens, it also uses a Tamron motor.