I've got some photos of the castings mentioned in my original post, I'd love to know peoples thoughts on the cause. The outboard mechanic believes the cause is probably the tin gasket in the 4th photo reacting with the cast aluminium. The new gasket supplied by Mercury, is of a different design, a sandwiched 3 layer type thing, where the tin bit is encased in fibre stuff I think. So I guess they're on to it, but if you've got a Merc with the older type it may be something to keep an eye out for. Its a 2002 motor
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