'99 XJ size of front brake rotor - JeepForum.com
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post #1 of 14 Old 11-08-2019, 03:43 PM Thread Starter
cyberdyke
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'99 XJ size of front brake rotor

Happy Friday,

How do I know if my rotor is 82 or 78mm?
'99 XJ 4.0 auto straight 6

Thanks!

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post #2 of 14 Old 11-08-2019, 04:19 PM
CJ7-Tim
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What does 78mm or 82mm measure ?

Have you checked out the Tech/FAQ links and TSBs and read this thread about 1999 brake rotor measurements and unit hub differences ? - www.jeepforum.com/forum/f11/any-difference-hubs-360959/

The answers to your brake rotor questions are there.

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post #3 of 14 Old 11-09-2019, 05:38 AM
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What is the build date? Mine is April and I have the 2000 disks.
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post #4 of 14 Old 11-09-2019, 10:31 AM
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Quote:
Originally Posted by cyberdyke View Post
Happy Friday,

How do I know if my rotor is 82 or 78mm?
'99 XJ 4.0 auto straight 6

Thanks!
3.22" or 3.07".

Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/BM brakes
99' XJ /4.0L-AW4/ 5.5" lift / Rancho shocks/33" DC Extreme country MTs. Jeep #18
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post #5 of 14 Old 11-09-2019, 08:09 PM Thread Starter
cyberdyke
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How do I find the build date?

What do you mean by 3.22" or 3.07"?
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post #6 of 14 Old 11-09-2019, 10:03 PM
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Quote:
Originally Posted by cyberdyke View Post
How do I find the build date?

What do you mean by 3.22" or 3.07"?
https://www.jeepforum.com/forum/f11/...e-hubs-360959/

The problem lies in there being composite and full cast rotors used in the same year as well as two different heights for the different unitbearings.

The composite rotor has a very thin hat shell, so if you measure the overall height, a taller inside depth early rotor can get very close to the same overall height of the later full cast with an overall shorter depth but with a much thicker hat.

The only dimension that matters is the depth from the inside of the hat to the inboard face of the rotor so it will match the unitbearing height.
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Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/BM brakes
99' XJ /4.0L-AW4/ 5.5" lift / Rancho shocks/33" DC Extreme country MTs. Jeep #18
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post #7 of 14 Old 11-09-2019, 10:16 PM
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Quote:
Originally Posted by cyberdyke View Post
How do I find the build date?
Top left corner.
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Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/BM brakes
99' XJ /4.0L-AW4/ 5.5" lift / Rancho shocks/33" DC Extreme country MTs. Jeep #18
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post #8 of 14 Old 11-10-2019, 07:16 AM
CJ7-Tim
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I am going to suppose that 82mm is about 3 3/16 inches and that 78mm is about 3 1/16 inches.


Read the info at the link provided above.


The build date is an unreliable fact for which hubs or rotors to buy because the exact changeover date is unknown, and because previous owners or a repair shops may have installed different hubs and/or rotors than the factory installed. It is also possible to have one of each older type vs. newer type hubs and rotors if Bubba was broke when he needed replacement parts.

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post #9 of 14 Old 11-10-2019, 05:28 PM Thread Starter
cyberdyke
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Interesting article, this is a daily driver and I don't relish the idea of ripping the brakes apart just to measure them and have to put everything back together while I wait to get parts. There has to be a BETTER way to find out this information.
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post #10 of 14 Old 11-10-2019, 06:16 PM
CJ7-Tim
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Go to the auto parts store and buy one set of each kind of rotors. When the brake job is done, go all the way back to the parts store and return the unnecessary parts. This will take just about as long as it takes to pop off the rotors and measure, or ID the unit hubs. YMMV.
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post #11 of 14 Old 01-18-2020, 02:13 PM
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The earlier rotors (1999 and older) are deep rotors and will fit on 2000, 2001 year models but it will rub on the backing plate and make un wanted noise.

You can eliminate the sound or rubbing by adding washers on the wheel studs just the amount to make the rotor off the backing plate.
(you have to remove the rotor to put the washers).

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post #12 of 14 Old 01-18-2020, 04:54 PM
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Quote:
Originally Posted by Hamid View Post
The earlier rotors (1999 and older) are deep rotors and will fit on 2000, 2001 year models but it will rub on the backing plate and make un wanted noise.

You can eliminate the sound or rubbing by adding washers on the wheel studs just the amount to make the rotor off the backing plate.
(you have to remove the rotor to put the washers).
Or by buying the correct rotors and simply installing them. I've had several 99's and never had to guess at which rotors I needed.

p.s. You're wrong about the 99' and older rotors. 99' is the transitional year and had two different versions.
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Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
03' Rubicon/Zone 4" coils with 1" spacers up front/Rancho shocks/5.13 gears-35" MTZs/BM brakes
99' XJ /4.0L-AW4/ 5.5" lift / Rancho shocks/33" DC Extreme country MTs. Jeep #18
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post #13 of 14 Old 01-18-2020, 05:00 PM
CJ7-Tim
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Quote:
Originally Posted by Hamid View Post

You can eliminate the sound or rubbing by adding washers on the wheel studs just the amount to make the rotor off the backing plate.

I think this will get you the auto parts store for the correct parts, or to work and back home until the correct parts arrive, but I would not consider washers a safe or long term solution.
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post #14 of 14 Old 01-19-2020, 01:08 AM
Hamid
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Sure it is not a solution for a long term. The wheel nuts tend to come loose after a long drive. I drove my XJ 1K miles before I put the correct rotor.

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