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      05-28-2021, 07:32 PM   #1
ggggbmw
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Measure the whole disc face!

I swap pads so often for track/summer/winter that I don't have any correlation between pad wear and disc wear. At 50K miles, I knew my discs were getting worn, but the faces still looked good.

Obviously, the way to determine disc wear to is measure the thickness. I've been using my standard digital caliper, and last fall I measured 29.02 and 28.96 on my front discs. That was down from 29.4mm the year before. Definitely some wear, but well above the 28.4mm minimum thickness. Considering the new thickness of 30.1mm, I still had at least a third of the thickness left. Not great for track, but no real worries.

The problem is that I was only measuring the outer edge of the disc. (Not the wear ridge, but just inside that on the worn surface.) My standard caliper isn't really the right tool, and since I'm a tool nerd, I decided to buy a cheap digital disc caliper. For $40, I got a tool that isn't super accurate, but easily measures to 0.1 mm accuracy, and more importantly can read multiple places across the disc face. And the longer arms make it WAY easier to use the caliper and read the values.

So, measured the discs with my fancy new tool, and I got a 28.9+mm reading on the outer edge. About the same as my good caliper. Than I measured the inner edge of the wear face. 27.9mm. Uh-oh. I just went from having some comfortable meat on the discs, to being WAY under minimum spec. Both discs were about the same, and definitely needed immediate replacement.

So, I thought I would post this here to remind everyone that the disc wear may not be even from the inside to outside edge. And that maybe a cheapish tool would be a good investment to learn how much wear your discs really have.

Got the new discs in the mail today. On the car, I've never really noticed how big these things are, but handling them as separate pieces, wow, they are MASSIVE. Way bigger and heavier than any other discs I've ever replaced on my family sedans/wagons. They are weighing in at about 22.8 lbs. And these are the original BMW aluminum hat models. Hate to see what the cheaper full iron discs weigh.
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      05-28-2021, 10:48 PM   #2
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Quote:
Originally Posted by ggggbmw View Post
I swap pads so often for track/summer/winter that I don't have any correlation between pad wear and disc wear. At 50K miles, I knew my discs were getting worn, but the faces still looked good.

Obviously, the way to determine disc wear to is measure the thickness. I've been using my standard digital caliper, and last fall I measured 29.02 and 28.96 on my front discs. That was down from 29.4mm the year before. Definitely some wear, but well above the 28.4mm minimum thickness. Considering the new thickness of 30.1mm, I still had at least a third of the thickness left. Not great for track, but no real worries.

The problem is that I was only measuring the outer edge of the disc. (Not the wear ridge, but just inside that on the worn surface.) My standard caliper isn't really the right tool, and since I'm a tool nerd, I decided to buy a cheap digital disc caliper. For $40, I got a tool that isn't super accurate, but easily measures to 0.1 mm accuracy, and more importantly can read multiple places across the disc face. And the longer arms make it WAY easier to use the caliper and read the values.

So, measured the discs with my fancy new tool, and I got a 28.9+mm reading on the outer edge. About the same as my good caliper. Than I measured the inner edge of the wear face. 27.9mm. Uh-oh. I just went from having some comfortable meat on the discs, to being WAY under minimum spec. Both discs were about the same, and definitely needed immediate replacement.

So, I thought I would post this here to remind everyone that the disc wear may not be even from the inside to outside edge. And that maybe a cheapish tool would be a good investment to learn how much wear your discs really have.

Got the new discs in the mail today. On the car, I've never really noticed how big these things are, but handling them as separate pieces, wow, they are MASSIVE. Way bigger and heavier than any other discs I've ever replaced on my family sedans/wagons. They are weighing in at about 22.8 lbs. And these are the original BMW aluminum hat models. Hate to see what the cheaper full iron discs weigh.
IIRC, TIS specifies measuring rotor thickness on the outside, middle, and inside of the wear surface, doing so in three equidistant places around the disc. That would give you nine measurements in total and a pretty good idea of how much rotor thickness you have left.

I bought, but have yet to use, the BMW tool for performing these measurements:






The best price I found for the measurement caliper was at Germain BMW Parts: https://parts.germainbmw.com/oem-par...10-83300495451.

Rotors for performance cars certainly have quite a bit of mass, don't they! The dimpled and slotted units I got as part of an M Performance Brake Kit came in at:

Front: 370X30 26.50#
.Rear: 345X45 19.25# vs. 19.50# for the 34206797605 plain rotors
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      05-29-2021, 09:44 AM   #3
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Quote:
Originally Posted by dradernh View Post
Quote:
Originally Posted by ggggbmw View Post
I swap pads so often for track/summer/winter that I don't have any correlation between pad wear and disc wear. At 50K miles, I knew my discs were getting worn, but the faces still looked good.

Obviously, the way to determine disc wear to is measure the thickness. I've been using my standard digital caliper, and last fall I measured 29.02 and 28.96 on my front discs. That was down from 29.4mm the year before. Definitely some wear, but well above the 28.4mm minimum thickness. Considering the new thickness of 30.1mm, I still had at least a third of the thickness left. Not great for track, but no real worries.

The problem is that I was only measuring the outer edge of the disc. (Not the wear ridge, but just inside that on the worn surface.) My standard caliper isn't really the right tool, and since I'm a tool nerd, I decided to buy a cheap digital disc caliper. For $40, I got a tool that isn't super accurate, but easily measures to 0.1 mm accuracy, and more importantly can read multiple places across the disc face. And the longer arms make it WAY easier to use the caliper and read the values.

So, measured the discs with my fancy new tool, and I got a 28.9+mm reading on the outer edge. About the same as my good caliper. Than I measured the inner edge of the wear face. 27.9mm. Uh-oh. I just went from having some comfortable meat on the discs, to being WAY under minimum spec. Both discs were about the same, and definitely needed immediate replacement.

So, I thought I would post this here to remind everyone that the disc wear may not be even from the inside to outside edge. And that maybe a cheapish tool would be a good investment to learn how much wear your discs really have.

Got the new discs in the mail today. On the car, I've never really noticed how big these things are, but handling them as separate pieces, wow, they are MASSIVE. Way bigger and heavier than any other discs I've ever replaced on my family sedans/wagons. They are weighing in at about 22.8 lbs. And these are the original BMW aluminum hat models. Hate to see what the cheaper full iron discs weigh.
IIRC, TIS specifies measuring rotor thickness on the outside, middle, and inside of the wear surface, doing so in three equidistant places around the disc. That would give you nine measurements in total and a pretty good idea of how much rotor thickness you have left.

I bought, but have yet to use, the BMW tool for performing these measurements:






The best price I found for the measurement caliper was at Germain BMW Parts: https://parts.germainbmw.com/oem-parts/bmw-template-813010-83300495451.

Rotors for performance cars certainly have quite a bit of mass, don't they! The dimpled and slotted units I got as part of an M Performance Brake Kit came in at:

Front: 370X30 26.50#
.Rear: 345X45 19.25# vs. 19.50# for the 34206797605 plain rotors
Quote:
Originally Posted by dradernh View Post
Quote:
Originally Posted by ggggbmw View Post
I swap pads so often for track/summer/winter that I don't have any correlation between pad wear and disc wear. At 50K miles, I knew my discs were getting worn, but the faces still looked good.

Obviously, the way to determine disc wear to is measure the thickness. I've been using my standard digital caliper, and last fall I measured 29.02 and 28.96 on my front discs. That was down from 29.4mm the year before. Definitely some wear, but well above the 28.4mm minimum thickness. Considering the new thickness of 30.1mm, I still had at least a third of the thickness left. Not great for track, but no real worries.

The problem is that I was only measuring the outer edge of the disc. (Not the wear ridge, but just inside that on the worn surface.) My standard caliper isn't really the right tool, and since I'm a tool nerd, I decided to buy a cheap digital disc caliper. For $40, I got a tool that isn't super accurate, but easily measures to 0.1 mm accuracy, and more importantly can read multiple places across the disc face. And the longer arms make it WAY easier to use the caliper and read the values.

So, measured the discs with my fancy new tool, and I got a 28.9+mm reading on the outer edge. About the same as my good caliper. Than I measured the inner edge of the wear face. 27.9mm. Uh-oh. I just went from having some comfortable meat on the discs, to being WAY under minimum spec. Both discs were about the same, and definitely needed immediate replacement.

So, I thought I would post this here to remind everyone that the disc wear may not be even from the inside to outside edge. And that maybe a cheapish tool would be a good investment to learn how much wear your discs really have.

Got the new discs in the mail today. On the car, I've never really noticed how big these things are, but handling them as separate pieces, wow, they are MASSIVE. Way bigger and heavier than any other discs I've ever replaced on my family sedans/wagons. They are weighing in at about 22.8 lbs. And these are the original BMW aluminum hat models. Hate to see what the cheaper full iron discs weigh.
IIRC, TIS specifies measuring rotor thickness on the outside, middle, and inside of the wear surface, doing so in three equidistant places around the disc. That would give you nine measurements in total and a pretty good idea of how much rotor thickness you have left.

I bought, but have yet to use, the BMW tool for performing these measurements:

[IMG]https://u.cubeupload.com/EdWinter377...icknessMea.jpg[/IMG]


[IMG]https://u.cubeupload.com/EdWinter377...icknessMea.jpg[/IMG]

The best price I found for the measurement caliper was at Germain BMW Parts: https://parts.germainbmw.com/oem-par...10-83300495451.

Rotors for performance cars certainly have quite a bit of mass, don't they! The dimpled and slotted units I got as part of an M Performnce Brake Kit came in at:

Front: 370X30 26.50#
.Rear: 345X45 19.25# vs. 19.50# for the 34206797605 plain rotors
Are these mperformance dimpled and slotted one piece? they appear heavier than the mperformance 2 piece mentioned above by a material amount.
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      05-29-2021, 10:19 AM   #4
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Quote:
Originally Posted by TonM3 View Post
Are these mperformance dimpled and slotted one piece? they appear heavier than the mperformance 2 piece mentioned above by a material amount.
That's due to the fact that dradernh's front rotors are 370mm diameter (3/4-series Mperformance size) vs the standard Mperformance 340mm diameter on the 2-series.
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      05-29-2021, 01:00 PM   #5
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Quote:
Originally Posted by TonM3 View Post
Are these mperformance dimpled and slotted one piece? they appear heavier than the mperformance 2 piece mentioned above by a material amount.
They are: Front – 34106797603, and Rear – 34206797598.

You can see what they look like here: https://www.realoem.com/bmw/enUS/sho...diagId=03_0104, and here: https://www.realoem.com/bmw/enUS/sho...diagId=03_0103;

as well as here:


and here:


They look to me like they're made of two pieces riveted together, so not really a two-piece arrangement in the manner of race brakes like this AP Racing disc with hat:

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Last edited by dradernh; 05-29-2021 at 01:22 PM..
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      05-29-2021, 02:47 PM   #6
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28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...

"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).

Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"

Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4

Yes you need to measure at 3 spots and even even on a few different position on the disc...

I got this for $25 https://www.harborfreight.com/6-in-l...per-63714.html
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      05-29-2021, 05:33 PM   #7
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Quote:
Originally Posted by pikcachu View Post
28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...

"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).

Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"

Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4

Yes you need to measure at 3 spots and even even on a few different position on the disc...

I got this for $25 https://www.harborfreight.com/6-in-l...per-63714.html
Thanks for the link to the caliper. They've got one in stock locally to me. I'll pick that up before I replace my pads. I've only got 35K on my original pads and rotors, but I'm going to replace the pads w/ Brembo ceramic. I wanted to check the rotor thickness, but didn't want to spend $$$ on a tool I won't use often.
$27 is perfect.
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      05-29-2021, 05:36 PM   #8
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Quote:
Originally Posted by dradernh View Post
IIRC, TIS specifies measuring rotor thickness on the outside, middle, and inside of the wear surface, doing so in three equidistant places around the disc. That would give you nine measurements in total and a pretty good idea of how much rotor thickness you have left.
I bought, but have yet to use, the BMW tool for performing these measurements: ...
Rotors for performance cars certainly have quite a bit of mass, don't they! The dimpled and slotted units I got as part of an M Performance Brake Kit came in at:
Front: 370X30 26.50#
.Rear: 345X45 19.25# vs. 19.50# for the 34206797605 plain rotors
Those rotors sure are pretty.
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      05-29-2021, 07:02 PM   #9
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Quote:
Originally Posted by PeterWT View Post
Quote:
Originally Posted by pikcachu View Post
28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...

"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).

Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"

Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4

Yes you need to measure at 3 spots and even even on a few different position on the disc...

I got this for $25 https://www.harborfreight.com/6-in-l...per-63714.html
Thanks for the link to the caliper. They've got one in stock locally to me. I'll pick that up before I replace my pads. I've only got 35K on my original pads and rotors, but I'm going to replace the pads w/ Brembo ceramic. I wanted to check the rotor thickness, but didn't want to spend $$$ on a tool I won't use often.
$27 is perfect.
Check the EBC red stuff if you don't want dust.
Else EBC yellow stuff are my choice for sport street and even moderate track
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      05-30-2021, 07:33 AM   #10
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Quote:
Originally Posted by pikcachu View Post
Check the EBC red stuff if you don't want dust.
Else EBC yellow stuff are my choice for sport street and even moderate track
I already bought the Brembo, but if I don't like them, I'll consider swapping them for the EBC red or yellow. Thanks for the heads up.
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      06-01-2021, 10:18 PM   #11
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Quote:
Originally Posted by pikcachu View Post
28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...

"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).

Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"

Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4

Yes you need to measure at 3 spots and even even on a few different position on the disc...

I got this for $25 https://www.harborfreight.com/6-in-l...per-63714.html
That's interesting information on 'pad replacement' vs. 'disc replacement'. Where did you find that information?

Measured the rear discs this past weekend while putting the track pads on. (BIR this weekend, yay!) Just as much variance from inside to outside as the fronts. But the inside minimum is still comfortably above the listed 22.4mm minimum. Likely replace them soon.

That Harbor Freight unit looks identical to the Vinca unit I bought on Amazon. Not a super high precision tool, but more than accurate enough for this purpose.
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      06-01-2021, 10:30 PM   #12
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Quote:
Originally Posted by ggggbmw View Post
Quote:
Originally Posted by pikcachu View Post
28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...

"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).

Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"

Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4

Yes you need to measure at 3 spots and even even on a few different position on the disc...

I got this for $25 https://www.harborfreight.com/6-in-l...per-63714.html
That's interesting information on 'pad replacement' vs. 'disc replacement'. Where did you find that information?

Measured the rear discs this past weekend while putting the track pads on. (BIR this weekend, yay!) Just as much variance from inside to outside as the fronts. But the inside minimum is still comfortably above the listed 22.4mm minimum. Likely replace them soon.

That Harbor Freight unit looks identical to the Vinca unit I bought on Amazon. Not a super high precision tool, but more than accurate enough for this purpose.
Info came from the bmwtis
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      07-25-2021, 12:03 AM   #13
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Anyone know the minimum for the rears? I know they come brand new at 24mm.
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      07-25-2021, 09:53 PM   #14
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Quote:
Originally Posted by da jemster View Post
Anyone know the minimum for the rears? I know they come brand new at 24mm.
The stated minimum thickness is 22.4 mm
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      07-31-2021, 11:20 AM   #15
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Quote:
Originally Posted by ggggbmw View Post
The stated minimum thickness is 22.4 mm
Thx!
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