On checking out the water pump on a recently aquired 76 GL1000 engine, I found the water pump cover corrosion you see in the pictures.
The water pump itself was worn out with the bearings completely gone. But even a good pump would have a hard time circulating coolant with that amount of corrosion in the cover, due to the leakage.
On top of the corrosion, the wobble in the bearings had caused the impellor to contact the cover and wear it away so that the area where the corrosion is was quite thin. It may even have been the wearing away of the surface that started the rot in the first place.
The impellor was the old Bakelite one, with the edges of the blades worn away from the contact. Maybe if it had been the later metal one, the resulting noise would have made the previous owner get it fixed!
The other possible cause of the rot being in this one place is the fact that, by the time it came to me, the engine had been sitting for quite some time out of the frame with just a small amount of water, rather than coolant, in it.
Anyway, in spite of this site's policy of renovating rather than scrapping, this cover is for the bin, as is the pump.
On a previous engine, the cover leaked badly.
When it was removed and checked, the distortion was quite evident.
The remedy for this was quick and cheap.
An old mirror (because it is optically flat) was used with some 600 emery glued down. I just used a light spray of trim adhesive on the back of a sheet.
The cover was then rubbed down to remove the high spots. Keep turning it round so that you even up the metal removal.
Eventually, you will have a nice even grey colour over the whole of the mating surface.
Use a torque wrench when replacing it so that you don't end up having to do this all over again!