On that note I would like to supply the Shep fix first.
On one of my CX I cleaned around the Tach blade with some Carb/Brake cleaner and a cotton bud.Then squeezed some RTV Gasket sealer in but not enough to impede the cable when fitted as previous owner of the Tach had done.
Let it set good for at least a hour.It's been like that for quite a few thousand miles.Why make work I say?
That's the easy way. But if you want to do it the right way and actually replace the seal then there are two other methods. Beware in some parts of the globe the seal isn't available anymore.
There are two methods for removing the seal itself.
Method 1 Larry Style
When you get the cover off, put it in a vice to hold it in. Then, heat up the aluminum really hot with a propane torch until the rubber seal starts to smoke for about 30 seconds. Then grab the tack drive shaft with a pair of pliers and simply pull it out.
Method 2 Kathy Style
Take a self tapping screw and screw it between the inside and outside of the seal. Eventually it should grip the seal and pull it forward.
