Was doing a second scrubbing of my s4 t2 rotors and found these spots. I have read on here that rust on the combustion surface is acceptable but I still wanted to double check as this is near the apex seal groove. Thanks for your help.

 

Are you talking about the casting marks in the pocket? I wouldn't be concerned with those..

 

Not the casting marks but the areas at the corners near the apex seal grooves

 

you're not cynical enough! compare the size of the pitting to the size of the casting grain on the combustion recess. which one is bigger?

 

Those rotors are in phenomenal shape so long as the grooves are correct/clearanced for the seals you're using.

 

I see absolutely nothing wrong with the surface finish.
However, it may be a trick of the camera, but it looks like the apex seal groove is opened up/no longer parallel. Check the clearance with new seals, it should be no more than.004" anywhere, and preferably .0025".

 

Can you help clear up some confusion that I have about checking the groove clearance? The Haynes manual pretty clearly explains that you should insert the feeler gauge fully (bottomed out) when checking the clearance. However, the FSM doesn't have this pretty specific instruction. Is there a commonly understood method of checking the groove clearance? I have searched for the answer to my question on this forum with no luck so far.

 

That is for verifying that there is ENOUGH clearance, so the seal doesn't bind.
It doesn't check for excessive clearance, which generally only happens at the tip, as the seal beats the tip side to side, I've had rotors that had .003" clearance at the bottom and .020" (not a typo) clearance at the tip.