I replaced both the rear discs and pads on my parent's 97 850 about 6 months ago and the other day when I was over at their house I notice that one of the discs has a small dimple in it. I'm thinking that it could be an air pocket in the casting, but it looks too round and almost like a tool mark like a drill spot. I haven't driven the car so I don't know if it is effecting the braking at all. The rotors are Brembo rotors that I bought from fcp groton. I also forgot to check to see if there was a matching dimple of the other side. Thoughts?

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dimple is at depth of minimum rotor thickness. If you can machine the rotor and still have some evidence of the dimble being there, they're thick enough to use after turning.

 

That dimple will not effect braking and is likely a wear indicator. Just think about slotted rotors and cross drilled rotors. Not a problem.

 

Thanks guys. That was another thought I had about it being a wear indicator.