by Tim Palmer
When was the last time you checked your differential lube level? We know of three cases in the last month where the lube level was allowed to low and caused the catastrophic failure of a differential. In the worst case scenario this type of failure could cause a serous accident, under the best case scenario it hurts your pocket book.
How do you check the lube? Remove the pipe plug located on the driver's side of the differential. Stick your finger in. The lube level should be level with bottom of the hole. It you have manual transmission, now is the time to check it also. To check it remove the hex plug on the driver's side of the transmission. Stick your finger in. The lube level should also be level with the bottom of the hole. If you have a 64 or 65 with an axle dipstick, you should still check the lube level. We know of one case in the past where the dipstick showed full but there wasn't any lube in the differential. You may be tempted to check just the transmission because the manual transmission and differential share lube. We have found flow between them to be minimal. In all the cases of failure in the last month the manual transmission's still had lube in them. I know the differential filler plug is difficult to get to and is messy to fill but it is a necessary evil.
What lube should you use? According to the shop manual SAE 80 is required for early models and SAE 80-90 is required for late models. There is no real advantage to going to higher viscosity gear lubes. Using higher viscosity gear lubes on manual transmissions can cause hard shifting in cold weather.
Two main causes have been identified as causing the leaks. The first is the side adjusters. The large o-ring used to seal them was a square ring until 63 or 64 when it changed to a round o-ring. With age the square rings leak. The second area of leakage is the seal where the yokes insert into the side gears. The leak is caused by the siding action of the yoke in and out with the suspension movement. Normally this doesn't cause any problem, however when the splines become worn the yoke will start to have a side to side motion and woller out the seal causing leakage. The late model Corvairs don't seem to have the problem because their yokes are bolted to the side gear, so there is no siding motion to wear the splines. All the failures were on early models. It would be a wise precaution to replace the side adjuster o-rings and seals, especially if you have a early model.