Oil for Ferret Fluid Coupling?

[LRDG]

Originally Posted by Richard Farrant

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.

Thanks, Richard. That's a bit disappointing. I really wanted to get some thinner oil in there because right now she almost stalls at idle when first or second gear is engaged from neutral. I've got the idle mixture screws set on the lean side so I'll try setting them a bit richer to see if that helps. The idle is around 400 rpm and I can change forward/reverse quietly at that rpm so I'd like to keep it low.

She runs too cool as well. The new thermostat may improve the idle stability.

We have a spare engine and gearbox so I'll take a look just to see where the fluid coupling oil would drain out of the bell housing and to confirm I can't get a drain trough up through the gearbox drain hatch to direct the oil. The oil could be drained quite slowly by rotating the open fill plug hole gradually downwards.

Also I might be able to suck some oil out with small bore tubing through the fill plug.