zgeezer Posted November 17, 2009 Share Posted November 17, 2009 (edited) I replaced the pads on my 2004 MazdaSpeed Mx5 twelve months ago. The rotors were "turned" very slightly and Mazda OEM pads installed. Today, I noticed that each of my rotors have about 100 small cracks that run the width of the swept area at a 45 degree angle tangent to the inner swept diameter. The rotors are not "blued" and have not been abused, other than one or two "slow downs" from 110 to 80 mph. The cracks are visible, but the rotors feel smooth when I run my finger nails across them. There is no noise, no shuddering, and no sound of anything dragging. These are the original stock Mazda rotors and they are not drilled or slotted. Am I driving an unsafe car? Is this a condition that I should immediately replace all four rotors? My initial sense is that the car is "unsafe at any speed". What is the most likely cause for this condition? Any comments please. G Edited November 17, 2009 by zgeezer Added to post Quote Link to comment Share on other sites More sharing options...
johnc Posted November 17, 2009 Share Posted November 17, 2009 my rotors have about 100 small cracks that run the width of the swept area at a 45 degree angle tangent to the inner swept diameter. If one of the cracks spans the entire width of the swept area then replace the rotor. If one of the cracks opens up to more then about .015 then replace the rotor. If they are a bunch of short, narrow cracks then you're fine. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 17, 2009 Share Posted November 17, 2009 Very common on Porsche rotors. Take John's advice and you'll be OK. Quote Link to comment Share on other sites More sharing options...
zgeezer Posted November 17, 2009 Author Share Posted November 17, 2009 Thanks for the quick replies and peace of mind. No need to confront my mechanic, rob the wife's house improvement kitty, or fear driving my DD. Just another item to check on my daily walk around before cranking up. Again thanks. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted November 17, 2009 Administrators Share Posted November 17, 2009 ... No need to confront my mechanic, rob the wife's house improvement kitty... :lmao: ...Just another item to check on my daily walk around before cranking up. Again thanks. A good pilot always performs a thorough preflight inspection of the craft he is flying. Quote Link to comment Share on other sites More sharing options...
260DET Posted November 20, 2009 Share Posted November 20, 2009 What actually happens is that a sudden application of heat results in tension between the hot outer layer of the rotor and the cooler inner. Resulting in cracks. To avoid this warm the brakes up before doing heavy applications, works every time. Including with Xdrilled rotors that are often blamed when it is driver error. Quote Link to comment Share on other sites More sharing options...
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