Converting a Non-Posi Rear to a Posi Rear
Mark Giusti -
  1. Drive the car to warm up the gear oil- it will drain easier.
  2. Jack the car and safely support it on four stands so it is level.
  3. With the car level, proper gear oil fill will be achieved.
  4. Lower the jack and let the rear hang down.
  5. Remove the rear cover and drain the gear oil.
  6. With the gear oil draining, remove the rear wheels and brake drums.
  7. Mark the bearing caps. I recommend using a paint pen to mark the caps "L" and "R". I prefer the paint pen instead of a center punch because I feel a punch mark puts a stress riser on the cap which can lead to the possibility of a crack forming.
  8. With trans in neutral, rotate the driveshaft so the bolt head on right side of the differential case is found.
  9. Using a 5/16" six point box wrench, remove the bolt. I find it easier to remove the bolt by putting the wrench on the bolt head and rotating the driveshaft counterclockwise causing the wrench to rest against the top of axle housing, loosening the bolt.
  10. Remove the bolt.
  11. Remove the center pin from the differential.
  12. Push the axles toward the center of car and remove the axle c- clips using a magnet.
  13. Slide the axles out of axle tubes being careful not to nick the seals with the axle splines. Now is a good time to replace the axle bearings and seals.
  14. Check the axles at the seal/bearing area for scores or a rough finish.
  15. Check the splines for being twisted. If the axles appear ok, have them magnafluxed. Replace the axles if damaged. If rules require long wheel studs, GM sells 12mmx3" pieces under part number 22551491.
  16. Clean the gears with a brake cleaner and brush gear marking compound (G.M. #1052351) onto three ring gear teeth and rotate the driveshaft counterclockwise while holding hand pressure on the ring gear to keep it from turning. This gives a tooth contact pattern. The purpose of holding ring gear is to put some drag on the gears for a better picture of the tooth contact pattern.
  17. Remove both caps and reinstall the lower bolts back into the housing.
  18. With a long punch (about 18" long) that fits into the hole the differential pinion pin came out of, carefully pry out the differential using the axle housing as the fulcrum. Careful, the differential is heavy!
  19. Make note of a shim on each side of the carrier between bearing and housing, measure its' thickness and keep them separated left from right.
  20. Check the ring and pinion gears for chipped or broken teeth. Wash out the housing and pinion gear with brake cleaner and blow dry. Wear eye protection.
  21. Remove the ring gear and wash it and the bolts with brake cleaner. An easy way to remove the ring gear is to remove all the ring gear bolts, install three bolts about a turn and then tap on the bolt heads with a hammer until ring gear is free of the carrier.

    Note: ring gear bolts are left hand threads! An electric impact gun works if an air impact gun is not available.

  22. Install new side bearings on the carrier.
  23. Cut the heads off of five used ring gear bolts and cut a screwdriver slot in the ends. A whiz wheel works great.
  24. Install these bolts into every other bolt hole in the ring gear.
  25. Using a ratchet and socket, install the ring gear onto the new posi unit using the cut ring gear bolts and the remaining bolts in the other holes.
  26. Tighten them in a cross pattern with a punch through the pin hole until the ring gear is fully seated. I use an old axle cut to about 18 inches mounted in a strong vise with the side gears sitting engaged on the axle splines. Or, bolt an axle to a wheel and stand it up.
  27. Remove the bolts and discard or cut and slot the other five bolts for an extra set. Save the cut bolts for future use.
  28. Use one drop of red Loctite on new ring gear bolts and step torque to 90 foot pounds in a cris cross pattern. Lube the side bearings with gear oil and place the carrier into the housing with the races on the bearings.
  29. Loosely install the left side cap.
  30. Install the right side shim until fully seated and then loosely install the right side cap.
  31. Remove left side cap and install the left side shim. This shim is a fairly tight fit. Be extremely careful when installing the shims because they are made of cast iron and are brittle and can fracture. I like to tap the shim then rotate the driveshaft so the bearing race can straighten out. Alternate doing this until the shim is fully seated.
  32. Install the left cap and step torque the four cap bolts to 55 foot pounds.

    Hint- hand tighten the cap bolts and then lightly tap the caps with a plastic hammer handle to seat them before torqueing. The bolts and caps can and do break if the cap is not fully seated in the housing- I learned this from experience.

  33. Again, do a tooth contact pattern. It should be the same as before disassembly.
  34. Lube the axle bearings and seals with gear oil.
  35. Reinstall the axles, c-clips, differential pinion pin, bolt, and cover using a new gasket.
  36. Raise rear to ride height and fill with gear oil, synthetic gear oil is recommended.
  37. Install the brake drums and wheels and torque lug nuts to spec.
  38. Put trans in park, lower car and drive.
  39. Check for leaks and noises. Let the rear break in for about 50 miles before hard driving.

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