Changing the oil on any gearbox can be tricky. It’s usually the filling up part that tends to be the stumbling block as they never really have a filler cap like engines do. This has a lot to do with the fact that it shouldn’t really need changing that often (if at all) but the 126 gearbox is a leaky one and an oil change helps prelong the life and keep those gear changes nice and smooth
it’s actually quite easy if you use some ingenuity and can easily be done in half an hour. This guide is on a BIS but can in most cases be applied to the air cooled one too.
To make life easy it’s best to get the back end in the air about 6-8 inches
Once you’ve scrambled underneath look towards the starter motor and you will see a square nut sticking out the side of the gearbox. This is the filling point. I find it best to remove this FIRST as the last thing you want is to drain the old oil out and find you can’t fill it back up. It’s a funny size so I found a 1/2 socket end to be the perfect size so tap it on and use a 15mm socket on the other end. Here it is removed.
With the filler plug out and making sure you have a Suitable oil container you can now undo the drain plug. It’s a standard Allen key so no drama there. It may be tight.once out let it drip for a bit.
Once all the oil is out refit the drain plug. Now it’s time to refill. If you can, use a length of petrol hose - about 10mm wide - and push it in the filler hole. Push the other end up past the stater motor, then fit the biggest funnel you can in the other end. Because I have a BIS I tied the funnel to one of the engine lid levers. The oil choice is up to you but I used 20w50 mineral oil and it takes 1.1 litres.
With this done pull the hose out then refit the filler plug. Job done