So... Viscous fan coupling... FYI this is not a "how to"! This is just a "how it turned out"...!
I read up on as many posts as I could on taking these apart and ended up reasonably brain fried...
Dismantled it and found some rather brown looking silicone fluid and not a huge amount of it. Left it to drain, warmed it up, soaked it in brake cleaner, agitated it, soaked it in WD40, more brake cleaner... more draining... Seemingly endless amounts of residue... until finally it was clean...
Bought some 10000 cst silicone oil for RC cars as seems to be (one of ) the norms. Probably too thick for the UK but this truck will eventually get to Africa and will likely see Spain/Portugal/North Africa so it'll be slightly over-cooled if the stuff works and I haven't wrecked the coupling altogether...!
The M6 philips head screws were a nightmare to get out, (no impact screwdriver - couldn't be bothered getting one for just one job and waiting for it to turn up from someone on fleabay). Eventually cracked them all loose. Seal popped out... (so much regret, but it was very grubby). I've since replaced the screws with M6 stainless, allen key head machine screws. 16mm thread length perfect with no washer. May include washers later. Not sure. They're all threadlocked anyway but old washer was captive on old screws.
Much consternation that rubber seal had stretched and I'd have to buy a whole new unit cos I was dim. Tried boiling it, microwaving it, freezing it... Nothing... couldn't get it into the groove without a massive loop of rubber sticking out.
Didn't have much to loose so had some rubber grease from the brake refurb and gave that a go... works perfectly (hopefully no contamination of silicone - but you don't need 42 fingers and a thumb to get it in and stay in) the tackiness of the grease holds the seal in place and you can work it around and compress it ever so slightly as you go which reduces the excess loop until it pops in and stays in... Well chuffed...!
Filled it up, had a 60ml bottle of silicone, used probably about 40ml ish until the outer shell was filled to the bottom of the 4 oval holes. No idea if there is too much... too little... whether it'll last... how it'll perform... but it's back together!
As I said at the beginning... definitely not a how to guide... as it's not proven to work... If anyone has done this refurb on the 100 series in-place of replacement and has testimony that it works I'd be very grateful for some reassurance... or if you see that something is fairly majorly wrong... Please say! I'd prefer to strip it again now rather than later!
Cheers Graham