Differential Oil Leak

The right outer seal of the differential has been leaking for quite a while.  Not a lot, but enough to require a piece of cardboard under the wheel and to oil soak the brake pads.  I've been putting off doing anything about this since eventually I need to replace a noisy bearing inside the differential.  However, that project is on long term hold now that I have the PF coupe to work on.

So I decided to just replace the oil seal that was leaking.  Here you can see the splined axle shaft with the bearing just outside of that.  Then just outside of the bearing, you can see the oil seal where the arrow points.  This seal rides on the outer portion of the rear hub (red arrow in picture below).  There is no seal to prevent oil from seeping through the splines into the inside of the hub and eventually through the holes to the splines where the wheel mounts.  It will also get on the inside of the knock off.  Perhaps this is a self-oiling feature?  There is a felt seal around the axle inside the differential tube.  I think that this is designed to allow only a certain amount of oil to seep out to the bearing.

In any case, the oil seal was about as hard as a rock and obviously not doing its job any more.  However, it is on the inside of the flange you can see bolted to the differential tube with the eight bolts above.  Ferrari did design a fail safe if this seal does leak a little.  On the inside, there is a cupped surface on the hub with three holes (blue arrows) in it.  These holes lead to the outside of the hub away from the brake disk.

If there is just leakage while the car is moving, the centrifugal force will cause the oil to run through the holes and be flung on the inside of the wheel.  Not a nice clean up job, but better than having an accident because the brakes got oily.  In my case, it was leaking all of the time.  So the oil would just drip down the hub and onto the disk, wheel and tire.

After unbolting the flange, replacement of the seal was pretty easy.  I used the old seal to drive the new one into place as I didn't have anything quite the right size.  I also put a thin layer of silicone gasket maker on the inner edge of the flange where it fits to the differential tube.  As there is oil on the inside there and no gasket, I wanted to make sure that it wouldn't leak.

The last problem is getting a cotter pin back into the castellated axle nut.  There are holes in the hub, but they don't line up with the hole in the axle.  Since there are only eight splines, one moves the holes 1/8 of a turn.  On the front wheel, the hub turns on the axle, so it's easy to line up the holes.  I'm not quite sure why they didn't line up on the rear, but in any event, I had to work around the problem.  I ended up putting a bend into the cotter pin (left picture).  This enabled me to just fit it through the hole in the hub and then through the axle hole.  After some playing around I got everything to slide into place (right picture).

You will notice a line drawn on the end of the axle.  I always draw these when working with cotter pin holes.  That way you can look and see exactly where to leave the castellated nut so the slots align with the hole.

Of course, the last thing to do was bleed the rear brakes.  Unlike the front where the caliper is on a flexible tube, the rear caliper brake lines are solid and attached to the differential tubes.  So you have to undo the brake line to remove a caliper.  Then one has to bleed them after putting everything back together.

One thing I forgot to mention was the lubricant that I used.  The original 330 GT owner's manual specifies Shell Spirax EP 250 (1.8 liters).  The addendum for the 5 speed model specifies Shell S 6721 A (2.5 liters) with no mention of the weight.  I used Penrite Transoil 250 which I bought from Classic Auto Lubes in Tyler, Texas.  The 5 liter jug was about $50 with shipping.  Penrite is an Australian company with local dealers in various countries.

Since then, another late 330 GT owner tried to obtain more information about the S 6721 A oil.  He tried several sources and eventually Ferrari Customer Service replied with:

Thank you for contacting us.

As regards your kind request, we would like to inform you that after carrying out our relevant checks with our competent department, Ferrari Classiche, we can confirm you that the lubricant originally recommended for the gear was Shell Spirax EP 90. At the present time Shell Spirax W 80 or else Agip Rotra SAE 80-90 are used in substitution.

Moreover, the lubrificant [SIC] used for transmission is SPIRAX EP140.

We remain at your complete disposal for any further information or assistance you may need.

Kind regards,

Ferrari Customer Care

I think that 90wt oil would leak out of most of the differentials without a rebuild, so I'm going to stick with the 250wt.  It was interesting getting this oil (almost thin grease) into the differential.  I measured out the right quantity into a gallon jug.  I found a funnel shaped cap from another oil jug fit onto the gallon jug.  Then I fit a tube tightly over the funnel and upended the jug with the tube running into the filler hole.  I cut a hole in the jug to let air in and left the whole thing to drain overnight.

BTW, the owner's manual's for both 4 speed/OD and 5 speed transmissions specified 90wt which conflicts with the reply above.

Additional information posted by Dyke Ridgley on tomyang.net:

In the early 1970s, I researched Shell S6721A. It was a "factory fill" made up for Ferrari that contained Spirex 90 (which is a EP Hypoid oil) and a Limited-Slip additive. This was developed for the 275 transaxle and saved the factory from having to purchase and add a limited-slip additive separately.