A word of caution. A special holding tool is required to slacken the large steel nut holding the plastic fan to get to its spacer shims when you adjust belt tension. Without the tool it is very easy to damage the non metal fan. If the fan fails or the belt lets go the engine will overheat and seize. This happened to a local weight shift flyer a few years ago; very costly exercise indeed !
Personally I would drop the Rotax engine out and then the back of it is easy to get at. When the cord start housing and the black plastic belt cover is removed all is revealed and easily worked at. A new belt is the best answer and inspires confidence for future use.
The new cooling fan belt and holding tool, plus a few spare shims I was given were taken to the strip yesterday where I carefully removed the whole engine/prop as one piece - quite easily. The fan pulley nut was so tight I wondered if I could ever prevent the holding tool from jumping off the fan side whilst I tried to hold the engine from moving and undo the nut all at the same time. Eventually I used a mole wrench to lightly clamp the tool to the fan face, reaching it round a convenient gap between the blades. Then I thought my troubles were over, as Graham had explained that after that the fan just came off by hand. The parts drawing shows about 6 shims, those not in use go behind the metal pulley half as they'd get damaged if under the securing nut's spring washer, but technically that does mean the pulley centre line moves a fraction depending where the shims are - I assume the belt is flexible enough sideways to accommodate that small offset.
Not so in my case; I eventually had to hammer it off with a block of wood from behind. The reason turned out on inspection that a previous owner had possibly forced the fan on without taking care to align the slot with the key on the shaft, so it effectively cut its own tight groove, making it almost impossible to get off. A couple of fan blades were damaged in the process. T
The fan damage could have been more subtly caused than simply by being forced on with the key way not in line with the woodruff key on the shaft, because the key on the shaft doesn't project much at all so that holding the fan back with the tool whilst tightening that big shaft nut would put all the resisting torque through a shallow key via the softish nylon pulley slot. Logic says it's prone to be forced round the shaft in the direction I found it had actually ended up, so careful assembly isn't enough, I used just tight rather than tryto retorque that hyuge nut 'to the book' it's worked well for years now.
An alternative might be to raise or make a taller key for more positive engagement in the nylon pulley groove ?
While the black plastic fan cover was off a small triangular 'flap' was cut off to allow cautious finger access to feel the belt tension on pre-flight inspections.
But I did put the grille on again as it should prevent loose paper or rag from getting sucked into the fan. In any case having tackled the job once I found it fairly easy to lift the engine out for good access.
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