correct diff oil capacity for my Caterham 1700 Supersprint (de Dion, Sierra diff.)

[tompollock]

The web says 0.9 litres is the correct amount to put in, as does the build manual, (though it doesn't specify which diff this applies to). However, I've put a full litre in and the oil isn't yet up to the filler plug level.  About 1.2 litres came out!  Was it previously overfilled?

Tom.

[Geoff Brown]

Ford Diff Fill to overflowing approximately 1.2 L. 

Filling to overflow with a Sierra Diff is no problem having owned several Sevens with the Ford unit. The problem is with the BMW Diff where the recommended fill level is approximately 0.9L. The original instruction for the BMW fill was also to overflow - approximately 1.1-1.2L but subsequent problems with output seals leaking caused CC to 'create' an easy fix by reducing the fill amount.

Fortunately Sierra Diff output seals are far more robust.

BUT raising a car on axle stands for the Winter does impart strain on the output seals whichever diff though it would be the BMW that would ultimately fail.

[aerobod - near CYYC]

With the BMW diff, it the seals are pushed in all the way against the bearing retaining circlips instead of being flush with the diff face or the diff isn't centred such that the driveshafts can move out when they are fully articulated instead of remaining in full contact with the side bearing, you can get leaks. If the shafts are properly located in the seals, there shouldn't be any leaks - not a single drop from mine in 35,000km, including storing on axle stands in the winter.

This BMW diff with the BMW seals that Caterham fit is fitted to millions of cars, without any leak issue that seems to be documented as a problem.

[John Vine]

#2:

For info, my Haynes Ford Sierra manual gives the following re diff lubrication:

Under "Routine maintenance", it goes on to say:

So, it looks like the correct fill quantity is 0.9L, with the level not more than 10mm below the filler plug (in other words, it's a filler plug and not a level plug).  Note the edict "Do not overfill". 

I'm puzzled as to why Ford didn't simply design the plug to be 10mm lower in the first place.

JV

 

[Geoff Brown]

And good old CC stated in pre BMW diff build manuals to fill to overflowing regardless of what FORD may have recommended!!

[Wrightpayne]

The early 1.3/1.6 sierra's used a 6.5" diff that looks identical to the 7" diff - guess how I know??

This would explain the slightly lower capacity stated in the manual.

[ECR]

I'm struggling to interpret Fords instruction that the level must be"no more than the specified distance (10 mm?) below the filler plug". So if I fill it to a level that is say 15mm below the filler plug I must add more oil?

That being the case, filling till oil pours from the filler plug fulfills the instruction and the filler plug can be used as a level plug also. The only way you could overfill is if the car is on a significant slope (or by bench filling)

?

[John Vine]

I struggled a bit too, Roger.  I'd agree it could have been made clearer, but I think what it means is this:

So if I fill it to a level that is say 15mm below the filler plug I must add more oil?

Yes, but not so much that the level is less than the specified distance (10mm) from the filler hole.

...filling till oil pours from the filler plug fulfills the instruction...

Ah, but it doesn't because the level has to be the specified distance (10mm) below the filler hole, as measured with that high-precision instrument, the piece of bent wire.  Anything less than 10mm constitutes over-filling.

Of course, all this is Haynes' wording and guidance, but is (presumably) based on Ford's own.

JV

[tompollock]

In the end I filled up with 1 litre, slightly over the Haynes manual recommended amount but well under the 1.2 litres that came out and recommended by some. 

There was already a slight misting of diff oil visible on the de dion tube and I can't see any benefit in making matters worse by submerging the seals in the stuff. There are enough gears splashing around through the oil in the diff anyway to guarantee adequate lubrication everywhere.

[Wrightpayne]

I have a PDF copy of the Ford overhaul instructions - unfortunately it doesn't specify fill capacity / filling instructions.

If anyone wants a copy just drop me a PM with your email address.