If you find a used Posi at the swap meet or in the junkyard, be aware that GM used different ring-gear flange positions to compensate for the changing diameter of the pinion gear. This photo shows the difference between a three-series on the left and a four-series Posi on the right. The four-series carrier moves the ring gear closer to the smaller diameter pinion gear. Also note the severe scratches on our four-series case. When Tom blueprints a Posi, he polishes the case to remove these potential stress risers.