Don, the bigbest concern with any bearing is that plating would break away from the steel backing with the bearing in use in the engine. One of the components that I had quality engineering oversight responsibilities for back in my working days was engine bearings. I worked with the Mahle bearing plant in Atlantic Iowa, and the Federal Mogul bearing plants in Greenville MI and Mexico. The standard tests for bearing overlay ( the plated materials ) adhesion were:
* Bend Test, one would physically distort the bearing until it snapped, the plated material would need to break with the steel, and not flake off
* Gouge Test, one would go at the plating edges and body with a sharp scapel, the material may dent and cut in but not break free.
* Rub Test, a bar of smooth steel with a similar diameter to the bearing would be rubbed with force against the plated surface, the plating may roughen, but not break free.
as you can see all of these tests are all destructive testing.
It is impossible to know with confidence from a photograph but what you may have is Metal Migration in plated materials. That is a failure mode that can occur, but we never saw it as the bearing were made and put into use. Even service bearing were not around for more than a few years.
Metal Migration is something that takes years and years and years for it to happen. The plating substrate in the bearing are layered on in an order to prevent or stop that process.
The only way to know with confidence is to have an scanning electron microscope study done of the surface and see if copper is present. I doubt if you want to do that.
This article has information on Metal Migration.
https://materion.com/-/media/files/Allo ... oating.pdf
The discoloration may also be some simple corrosion of the tin plate top layer. Since it is on both of the half sides of the thrust bearing but not the other mains, it would be an interesting study to know why. If packaged together they should of had similar lives in storage. Are the other mains old, but not as old as the thrust bearing? Does the thrust bearing actually have a plating defect allowing Metal Migration?
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