Tech Tip ~ Gearbox Oil Level

Over the weekend at Gabe’s open house at Sterling Restorations, the “Corvair people” ended up clustered together and discussing things we have done or are planning to do with our cars. For those of you who are new, this is a not something that is mandatory or even planned, but it happens every single time.

I believe in the medical field it is called the MyoCorvairium involuntary muscle. Everyone has it. It just lies dormant until you buy your first Corvair.

While looking over Paul’s beautiful 63 Spyder, the topic of gearbox oil came up. Also, questions on how to actually check gearbox oil on pre-1964 cars (no dipstick) and the proper oil and amount to use. This lead to wanting to share a little about this topic with you in the event that you had some of the same questions.

Here is a past submission to the Pikes Peak Corvair Club Dripline newsletter from noted Corvair guru Steve Goodman.

Occasionally the subject of filling/topping off etc. of gear oil in stick shift Corvair gearboxes is discussed. This is directed towards the 1965 4spd and diff. Please bear in mind that the following is based on my own experiences. Also check the shop manual for your particular year please. I understand too that specs differ by some of the Corvair vendors. Granted the shop manuals have errors but this spec is not one of them. Also one last caveat: I am not saying I am correct, only that it has worked for me for many years.

The 1965 shop manual states 3.6 pints in 4spd and 4.5 pints in diff. Obviously that adds up to 8.1 pints OR rounded off to 4 quarts OR 1 gallon. It was typical for any rear axle (solid axle in other cars) and a 3spd or 4spd in those same ‘other’ cars to be filled to the top.

My own experience happened with my Spyder when I parked with the nose upward in a restaurant parking spot while driving on the highway. When I came out an hour later, there was long trail of black gear oil trickling down the incline. The reason for black color is the additive GM used for posi differentials. I realized that by pointing uphill the input shaft seal was below the level of the gear oil in the diff. Being hot (which means thinner liquid), it easily seeped past the seal and into the bellhousing and out the weep hole, allowing me to mark my spot! This is also the reason you see everything backed into the parking spots here at the shop; the trans end is downhill. I had already noticed how common it was that the side seals of the early Corvairs leaked and realized the oil level was above the bottom of the seal.

I started my own policy of keeping the level between 3/4″-1″ below the fill holes. I closely looked at the level inside the diff in relation to the fill hole and there was more than enough gear oil to keep bearings oiled and guarantee the ‘splash’ of oil from pinion flowing to rear pinion bearing. I also started back then to install 2 input shaft seals inside the release bearing shaft. I have been doing this since 1969 and IF gear oil level is maintained close to that lower lever the parts inside are being lubricated just fine.. The transmission also has more than enough gear oil to keep bearings and gears lubricated.

Getting slightly off subject in 1964-65 GM placed a dipstick on the differential. The decal on the front bulkhead of every 64-5 Corvair regardless of stick or PG was “AXLE CHECK ONLY”.

When the dipstick indicates “ADD” the diff level is darn close to that 1″ below the fill hole.

Needless to say the bottom line is that when allowing the levels to be lower than factory then certainly less than the 8 pints will be used.

Lastly I will repeat the usual caution: Use only gear oil in stick shift Corvairs marked GL-4. DO NOT use any gear oil marked GL-4 –GL-5. The brass synchro rings will suffer badly and quickly if GL-5 is used.

If you have a PG (automatic) Corvair you can use the GL-5 in the diff.

If you want to learn A LOT more about gear oil for our differentials, here is an extensive write-up by Richard Widman.

For those of you with PG/powerglides/automatic transmissions, here is a great tutorial from the great Larry Claypool.

Have a question about your power pack?

Have a story about an experience with your transmission or differential?

Additional tech tip we missed or got wrong?

You know what to do… let us know in the comments.