Doing routine TLC maintenance on Saturday. Checked the bevel box - oil level perfect. Checked the gearbox - no oil! Poured a full litre of EP80-90 into the box with no sight of the level coming up to anywhere near the filler. Since the box takes 850cc, it's obviously going somewhere else.
So what's the story? Is the oil escaping into the shaft housing? Is this a 'take the box out, strip it and replace the oil seals' job, or can I change the seal by just taking the shaft off? Let me guess - I'll have to strip everything but the handlebars and dissect a gearbox full of tensioned springs just waiting to fire essential ball bearings into the dark recesses of my shed.
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The gearbox oil level is checked with the c-spanner for adjusting the rear suspension preload. On the original spanner there is a line marked near the tip of the handle (7mm from the end). You use the handle of the spanner as a dipstick to check the oil level through the filler. The oil level is 116mm ( 4 5/8 inches ) down from the top edge of the filler hole. If you don't have the original c-spanner, mark two lines 116mm apart on a piece of metal or wire or whatever you have handy that will fit through the filler, and line the top one up with the top edge of the filler hole. That should give you the correct oil level. If you have put in a litre of oil you will probably find that the level is too high, and will have to drain some of it out. Once you have the correct level, keep an eye on it over a few weeks to see if it drops, before you go dismantling anything. If there is a bad leak there it will show up quite soon.

Good one Avenger, I missed the part where Paul didn't use a dipstick.

I use a plastic chopstick with the marks cut into it. Easier than dragging out the c spanner every time I need to check the level.

Probably the quickest and easiest thing to do would be to drain the whole lot and put 850cc in.

The pressure of too much gear oil will eventually seep (in a rush) past any sloppy seal if it's left overfilled and ridden any distance. It's a large case, a litre could 'hide' very easily in there if looking at the filler hole for evidence of oil is all that's done. Time for a drain and re-fill to 850cc as Blakey suggests.

It also could put too much pressure on the seal of the clutch operating rod and disappear in the clutch area after having made your clutchdisk useless........Or burst the rubber on the clutch actuating arm and spoil the rear tyre. Which way do you think Murphy'll go....

I listened to the advice and dropped the oil from the gearbox to refill it properly using a dipstick. Good job I did - the old oil came out as mayonnaise. Fresh oil now back in the box and measured at 116mm below the top of the filler opening.
Thanks everyone.

A few months back I drained the old stuff out and filled to correct level with an Synthetic equivalent found on fleabay, gearbox is lovely and smooth.