Forum

Garage => Mechanicals => Topic started by: raueda1 on 01 August 2019, 03:44 PM

Title: Brake questions
Post by: raueda1 on 01 August 2019, 03:44 PM
In the course of doing a brake job I encountered a couple issues.  Any contribution from the collective hive-mind would be welcome and helpful.

1.  Rotor cracks:  The rear rotors had many tiny sub-hairline cracks.  They weren't even visible till they got sprayed with brake cleaner.  The dirty fluid revealed them.  You can't feel them with your fingernail but there were a lot so I decided to replace the rotors.  Anyway, they're surprisingly cheap, so why not?  Now I just noticed that the front rotors, which seem to be in otherwise fine shape, also have a few of these miniscule cracks.  There wasn't any judder or any other problem till my caliper self-destructed.  Thoughts on replacing them too?  I avoided because of fear of setting bearing runout.

2.  New rear rotors fit perfectly EXCEPT.....   they had an extra hole for a small flat top hex screw that locks the rotor to the axle flange.  However, my axle flange doesn't have the hole for this screw so I can't use it as somebody intended.  The screws weigh 5gm.  I'm concerned that this small amount of weight - the weight that would otherwise be filling the hole in the disk - might be enough to cause a vibration cause the rotor is slightly out of balance.  My plan is to grind the thread off the screw, put it in the hole and keep it there with the wheel.  Does this make sense or am I overthinking it?

Thanks,
Title: Re: Brake questions
Post by: tcj on 02 August 2019, 01:00 AM
Hi Rueda,
my 2ct:
1) replace it. Brakes are the most important things on a 6.9.
The cracks will not get smaller or go away.

2) Vibrations are greater the farther they are from the center. So you will not feel this small weight near the centre

regrads,
Thomas
Title: Re: Brake questions
Post by: daantjie on 02 August 2019, 07:24 AM
Indeed, safety first ;)
Title: Re: Brake questions
Post by: raueda1 on 02 August 2019, 08:58 AM
Yeah, good counsel.  I need to resist my irresistable urge to cut corners.  New rotors now on order.  As for the little screws, I'm going to put them in anyway on the theory that it can't hurt and brings the assembly a wee bit closer to ideal balance, even if insignificant.  Cheers,
Title: Re: Brake questions
Post by: ptashek on 02 August 2019, 09:26 AM
This doc from Brembo is pretty old, but a good reference when it comes to diagnosing rotors:
https://www.bremboparts.com/en/pages/documents/8b6fd884-e07b-4e99-96b2-16c493862e44

Sometimes what appears as minute cracks is just the cast iron structure, but it's impossible to tell exactly without a microscope :)
Title: Re: Brake questions
Post by: daantjie on 02 August 2019, 09:37 AM
This doc from Brembo is pretty old, but a good reference when it comes to diagnosing rotors:


Thanks for this info  always good to learn some more about arguably the most important component on a car ;)
Title: Re: Brake questions
Post by: raueda1 on 02 August 2019, 12:22 PM
This doc from Brembo is pretty old, but a good reference when it comes to diagnosing rotors:
https://www.bremboparts.com/en/pages/documents/8b6fd884-e07b-4e99-96b2-16c493862e44

Sometimes what appears as minute cracks is just the cast iron structure, but it's impossible to tell exactly without a microscope :)
Nice reference, thanks for that.
Title: Re: Brake questions
Post by: s class on 03 August 2019, 04:04 PM
If you have rear rotors with the additional screw hole, they are the updated A126.... version.  I've never worried about the extra screw hole.