K75 rear rotor thickness

[Motorhobo]

The Maryland inspector says the rear rotor needs to have the thickness stamped on the rotor itself -- .18", which is supposed to be the service limit stated in the BMW service manual.   But on P. 484 the Clymer says:

Thickness
Standard:  4.0 - 4.04mm (0.157-0.173 in)
Minimum: 3.55mm (.14 in)

0.18 in sounds like a new disk. I have two spares, each with ~.165 in. There is conflicting info on the other forums, like this one from Inge saying the K100 minimum front and rear limits are 3.55mm or .14 in.

http://www.k100-forum.com/t3602-minimum-and-maximum-thickness-of-brake-discs-for1984-non-abs-k100rt

So the only way to get this through inspection is with a goddam new disk? That's absurd -- I ran a rear rotor down to, well I'm not going to tell you how thin because I'd be embarrassed to admit it, but definitely a hell of a lot thinner than 0.157.

Can anybody point to any official BMW info that says this .18 value is bogus? I'll go with the Clymer for myself but I'm not sure the inspector is going to be onboard with that.

 

[Inge K.]

Can anybody point to any official BMW info that says this .18 value is bogus?

From the factory manual:

[Motorhobo]

From the factory manual:

Inge you are my f**king hero yet again -- glad I speak German.

So what's up with this .018 thing? Just a convenient number for selling overpriced brake disks?

[Laitch]

Inge you are my f**king hero yet again -- glad I speak German.

So what's up with this .018 thing? Just a convenient number for selling overpriced brake disks?

You'll probably need to supply DOT a notarized or letterhead verification from an authorized BMW dealer.

[Laitch]

So what's up with this .018 thing? Just a convenient number for selling overpriced brake disks?

From what I'm reading, the front and rear rotors had the same minimum specs until '94 when the rear was changed to the .18" spec using a different style rotor. The manual in Inge's post looks like its from '87. Maybe this was a US-only change. Clymer's should reflect that change but doesn't so the best recourse is—sadly—verification from a BMW service center, unless you're able to baffle and charm the mechano-bureauocrat.

[Laitch]

There doesn't seem to be a Service Bulletin concerning this change in MOTOBRICK.COM's K75/S/RT bulletin archives, either.

[Motorhobo]

So where are you reading that, Laitch? I just talked to a service guy at Hermy's in PA, one of the only old-school dealers left on the planet, and he found it very odd that any US Standard measurement would be stamped on any OEM BMW disk. He also found it odd that the 0.018 would be a minimum, since it's so close to new.

I personally am totally comfortable riding with a rear disk between .173 and .157 as stated in the manual, so I'm going to go try to convince the inspector that this is the correct value and that what's on the disk is the manufacturer's tolerance for a new disk, not the service range of a used one. We'll see how that goes  :dunno2:

[Motorhobo]

Further to this, the early K100 rotor had some cracking issues and there was a recall -- I read something about the disk being made thicker, probably the thickness it is now. So maybe that's the root of the issue. It would be nice to know exactly what's up with this -- I daresay this won't be the last time it comes up, since the Maryland inspection sheet looks really detailed now. Up until 2013 it was basically "starts stops signals horn works = pass" but now there's all these measurements and specs on the sheet and the inspector kept the bike for 45 minutes using his caliper.

Anyway -- off to the inspector -- will report back on whether he wants to bust my nuts or not.

[Laitch]

Anyway -- off to the inspector -- will report back on whether he wants to bust my nuts or not.

Good luck. Pass around the doughnuts.

[kennybobby]

There's a rear brake on a K75?  i never knew what that thang was...

[Inge K.]

Further to this, the early K100 rotor had some cracking issues and there was a recall -- I read something about the disk being made thicker, probably the thickness it is now.

The very early rear disc was slotted and the problem was that they did crack between the slots and
the outer circumference...........same thickness.

From what I'm reading, the front and rear rotors had the same minimum specs until '94 when the rear was changed to the .18" spec using a different style rotor.

This is affecting K1100LT's only.

[Motorhobo]

There's a rear brake on a K75?  i never knew what that thang was...

Yes there is and I used the ch1t out of it trying to coerce the sidecar with the dog to stop when the rest of the bike and me stopped -- the disk was down to .103 -- minimum .140 -- thus proving that there's lots of meat left after even after your teeth hit the bone.

[Laitch]

Yes there is . . .

Many riders in the South have converted the brake pedal into part of a delivery system for Mountain Dew dispensed from a hose attached to the handlebar. kennybobby's bike probably has one of those.

[Motorhobo]

The very early rear disc was slotted and the problem was that they did crack between the slots and
the outer circumference...........same thickness.


This is affecting K1100LT's only.

I still don't understand what this stamp is doing on this disk. In inches, not the actual minimum rating, and on a part with the OEM BMW logo...so not aftermarket and not produced in USA, and definitely original off a K75. I just don't get it -- and sure would like to have some kind of explanation to give to the inspector  :musicboohoo:

[Laitch]

I just don't get it -- and sure would like to have some kind of explanation to give to the inspector  :musicboohoo:

Stand by. I've got a query in at MAX BMW, received an acknowledgement and am waiting for a second response. This lack of clarity is unfair to owners getting inspections and to the people administering them.

[kennybobby]

Yaahoo--mountain dew!  And we have the Moon Pie festival over here in Bell Buckle, TN

@hobo
remove that ABS tooth ring and get yer dremel tool after that stamped ring--soon that incorrect .18 will be gone...
The stamped ring is just riveted on to the brake disc and may refer to itself, not the brake.

[Laitch]

@hobo
remove that ABS tooth ring and get yer dremel tool after that stamped ring--soon that incorrect .18 will be gone...

Great idea, kennybobby! It'll only take a couple of deft flicks with a Dremel to change that 8 to a 3.

[Filmcamera]

+1 to Dremel time...

[Laitch]

Yaahoo--mountain dew!  And we have the Moon Pie festival over here in Bell Buckle, TN

Sun Drop Golden Girl Cola was my drink of choice in Soddy Daisy back in the '70s. Carbonated raisin juice.

[Inge K.]

0,18" is the min. thickness for the K11's........could the disc have been changed for a used one earlier?
Another strange thing BMW ETK doesn't recognize the part#, I have to look through some older discs.

[Laitch]

0,18" is the min. thickness for the K11's........could the disc have been changed for a used one earlier?.Another strange thing BMW ETK doesn't recognize the part#,

Yet another strange—my bike has the same .18" minimum casting—maybe there was an orgy of part swapping at the factory one day, just to relieve the constraint and boredom of orderliness and precision.

[Inge K.]

I have to look through some older ETK discs.

Sorry, didn't find any info on that part#.

Maybe it was a change on the 75's around the same time as the 11LT's..........but it looks like a dark
secret, buried deep in the archives.

[Motorhobo]

0,18" is the min. thickness for the K11's........could the disc have been changed for a used one earlier?
Another strange thing BMW ETK doesn't recognize the part#, I have to look through some older discs.

No -- I have 4 of these disks, they all have the .18 stamp and three of them are guaranteed to have shipped with 94/95 K75s here in the US.

I'm going to the inspector now and will tell him there is a worldwide effort underway to figure this freaking thing out but in the meantime please refer to the German service manual and the Clymer.

[Motorhobo]

I guess the inspector was impressed by the page from the German service manual and the matching Clymer spec -- he accepted the lower, reasonable specs of .157 - .173 and minimum .140.

So the puzzle remains unanswered but that's no longer my problem -- hope the next guy also gets a reasonable inspector with no interest in senseless ball-busting.

[Brad-Man]

Very glad your inspector was a mensch...