Oil for Ferret Fluid Coupling?

[Richard Farrant]

Originally Posted by LRDG

Chris, That's the link that says I should be using ISO 10 or 15 viscosity grade oil in the fluid coupling, which is a very thin, quite specialized oil for high speed spindle bearings. It's MUCH thinner than the ATF that's in there now, which has 30 grade viscosity.

I did some more searching and the consensus, especially over on HVMF, is that OM-13 spec oil is indeed ISO 10 or 15 viscosity grade.

I suspect the heavier ATF is contributing to the quite high drag I get from the fluid coupling, especially when engaging 1st and 2nd gear at a standstill.

Anybody know how I can drain the fluid coupling without having to split the engine and gearbox?

You are correct, ISO10 is one of the thinnest hydraulic oils. The common grade for plant machines, cranes, etc. is ISO32, so you can see there is a difference.
I spent many years working on British Army military vehicles (41 actually) and have dealt with fluid flywheels in everything from the 1940 Daimler Dingo to the CVR(W) Fox. The Dingo originally had engine oil in the flywheel but postwar this was changed to OM13 (ISO10), there would have been drag I suspect with the engine oil, but they would not have had anything more suitable in their inventory of oils at the time.
If you really want to drain the oil, it will make one wholly mess, the oil capacity is about 9 pints and that will flood out all through the base of the vehicle before any of it could be drained from the hull. Better to wait until you have to pull the gearbox, then it can be drained in to a container, without too much mess.