Quote from a post by "Matchless"
"The cambelt sprocket has to be removed to change the seal and because it is made from a sintered material (metal powder pressed into a mould at high temp) it need special care.
You have to bolt a long bar or piece of angle to the sprocket using the aux belt pulley bolt holes. Use this bar whilst removing the centre sprocket bolt (v.v.v.tight). Do not use an air hammer, do not lock it in gear and wallop it etc, you will shear the keyway off the sprocket.
Once the sprocket is off you can lever out the old seal. Now pull out the spacer / sleeve that the seal runs on, turn it back to front and refit it. This gives the new seal an un-worn surface to work on.
Replace the sprocket and fit a new sprocket bolt (it is a stretch bolt). Restrain the sprocket with the angle iron as before and tighten the bolt to 250Nm then a further 60 degrees. (check this...its a while since I did one)
Ideally its a job for two people. "
http://www.omegaowners.com/forum/YaBB.pl?num=1290628831