Newb Brake Rotor Question

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676
Location
TX south plains
When I had some work done on my 1994 Jag XJS they wanted to do rear brake work. I decided to do it myself. I ordered pads but didn't order rotors. I plan to check the rotors to see if they look like they need to be replaced. There is no sign in braking that there is any vibration. However, in reading online, most seem to say that you always have to replace or turn the rotors. Do you? Caveats: 1. I have never done brake work before. 2. if the rotors look worn I'll obviously replace them. 3. it has outboard brakes and not the inboard from earlier models. TIA, Tony
 

Astro14

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13,259
Location
Virginia Beach
There are two things to consider here: 1. rotor thickness...there is a minimum, you need a micrometer to measure...if the rotor is below that number, it must be replaced. 2. rotor surface. To get a good bed-in of the pads (a thin transfer layer of pad material on the rotors) you need a truly fresh surface, achievable with new rotors, turning of rotors or otherwise finishing the surfaces to remove the previous pad material and allow the bed in. Part of this is to level the surface as well, so that any grooves that would cause poor pad-rotor contact are removed...the old pads wore those grooves and so were mirror images...but the new pads are flat...so when placed on a non-flat surface, they lack good contact and have poor friction/stopping power. As a practical matter, many new cars specify rotors that have a minimum thickness near the new thickness...and so, it's easier to just change the rotors and pads each time. My Volvo is like this...can't say whether your Jag is or not, but I would at least have the rotors mic'd and then either replaced or turned...
 
Last edited:
Messages
19,528
Location
Lake Forest, CA
Originally Posted By: Astro14
As a practical matter, many new cars specify rotors that have a minimum thickness near the new thickness...and so, it's easier to just change the rotors and pads each time. My Volvo is like this...can't say whether your Jag is or not, but I would at least have the rotors mic'd and then either replaced or turned...
My E430 is the same, min thickness is 2mm less than nominal thickness, therefore no resurface is allowed.
 

cmf

Messages
415
Location
Florida
Astro14 pretty much nailed it. What is really important to note is that you either have to turn them, or replace them. You cannot leave the old rotors on. Sometimes the rotors wear unevenly; as in they wear more on the outside of the pad than the inside. This will require them to be turned multiple times to come up with a usable surface. Also, make sure that you're measuring from part of the area the brake pads contact. The brake pads often do not contact the outer edge of the rotor and if you measure the outer edge it's going to obviously be to spec since it was never worn.
 
Messages
25,987
Location
Upstate NY
Better to measure the rotor with a vernier caliper since it has wider jaws than a micrometer and will measure over a wider area of the rotor giving you a better idea. You can see if the jaws are hitting the rotor straight on or not.
 

cadfaeltex

Thread starter
Messages
676
Location
TX south plains
Given my lack of knowledge of the service history of this car, I believe if I pull it off I'll just replace it. Probably an online order part anyway. I'm thinking of replacing the solid rotor with a vented. Looked at EBC and they have a solid OEM style and a vented, slotted, and dimpled version for about twice as much. Any other ones to consider?
 
Messages
3,431
Location
USA
Check for rotors on Amazon. Rotors are very heavy, so the best deals come from places with free shipping. I recently bought rotors from Amazon. Rock Auto had a similar base price, but then shipping was more than $20, if I remember correctly.
 
Messages
19,479
Location
Chicago Area
Many cars nowadays have new cheap rotors available. So costs are close for cutting them vs. new ones. But in your case, new parts may be silly high. So get them cut with your new pads.
 
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