Brake Pad Thickness Variation Questions

For the first time ever, I checked the thickness variation of my 1991 Civic's
front brake rotors (NON-abs). I used a micrometer whose calibration I trust. It
actually reads to the nearest 0.0001 inch, but I have rounded to the nearest
0.001 inch below. The minimum thickness required on these rotors is 0.75 inch.

On one of the rotors, the readings were

0.812 inch
0.814
0.811
0.809

So this rotor's maximum thickness variation is 0.005 inch.

The spec is to have a thickness variation of no more than 0.0006 inch. This is
the AllData spec for the 91 Civic. Chilton's gives a spec of 0.0028 inch(!).)

There is a groove in this rotor, too. Chilton's says some scoring is normal, as
long as it's not deeper than about 0.0152 inch. I have not yet checked the
depth, but the groove is not all that easy to feel. It's easier to see. So I'm
not too concerned.

The other rotor shows no thickness variation.

The pads are a little over 2 years old and are about 5/16 inch thick. In other
words, there is plenty of pad left.

My brakes feel and work fine; no pulsing.

I can't see a good reason for resurfacing or replacing the rotor right away. At
some point I figure in the next couple of years, I'll have to replace the pads.
Then maybe I'll revisit the rotor issue.

Can anyone point out any detriment to the operation of this car's brakes or
wheels and tires by ignoring this thickness variation on the rotor for a year or
more?

I'm figuring on keeping the car for another five years or so (when I think the
rust from northern driving may take its toll on the body and other parts), so I
buy new stuff for it with only a five-year time frame in mind.

This is, as is common for many of my posts these days, more of an academic
inquiry. Again, the car drives fine. I'm just curious about the notion of
thickness variation.

Caroline wrote:
> For the first time ever, I checked the thickness variation of my 1991 Civic's
> front brake rotors (NON-abs). I used a micrometer whose calibration I trust. It
> actually reads to the nearest 0.0001 inch, but I have rounded to the nearest
> 0.001 inch below. The minimum thickness required on these rotors is 0.75 inch.
>
> On one of the rotors, the readings were
>
> 0.812 inch
> 0.814
> 0.811
> 0.809
>
> So this rotor's maximum thickness variation is 0.005 inch.
>
> The spec is to have a thickness variation of no more than 0.0006 inch. This is
> the AllData spec for the 91 Civic. Chilton's gives a spec of 0.0028 inch(!).)
>
> There is a groove in this rotor, too. Chilton's says some scoring is normal, as
> long as it's not deeper than about 0.0152 inch. I have not yet checked the
> depth, but the groove is not all that easy to feel. It's easier to see. So I'm
> not too concerned.
>
> The other rotor shows no thickness variation.
>
> The pads are a little over 2 years old and are about 5/16 inch thick. In other
> words, there is plenty of pad left.
>
> My brakes feel and work fine; no pulsing.
>
> I can't see a good reason for resurfacing or replacing the rotor right away.

unless you can feel pulsing, or notice the brake to be grabbing in any
way, i'd ignore it. chances are, thickness variation of this magnitude
is going to cause less variation in braking force than any variable
elastic expansion in the brake lines, so i wouldn't bother doing any
resurfacing.

> At
> some point I figure in the next couple of years, I'll have to replace the pads.
> Then maybe I'll revisit the rotor issue.
>
> Can anyone point out any detriment to the operation of this car's brakes or
> wheels and tires by ignoring this thickness variation on the rotor for a year or
> more?
>
> I'm figuring on keeping the car for another five years or so (when I think the
> rust from northern driving may take its toll on the body and other parts), so I
> buy new stuff for it with only a five-year time frame in mind.
>
> This is, as is common for many of my posts these days, more of an academic
> inquiry. Again, the car drives fine. I'm just curious about the notion of
> thickness variation.
>
>

Caroline wrote:
> I can't see a good reason for resurfacing or replacing the rotor right away. At
> some point I figure in the next couple of years, I'll have to replace the pads.
> Then maybe I'll revisit the rotor issue.

only if the thickness is below spec. then replace. ive never turned or
replaced a rotor on any of the cars ive done brake jobs on. including a
77 accord with grooves i could feel with my fingernail.

On Sat, 09 Oct 2004 08:50:56 GMT, SoCalMike
> wrote:

>> I can't see a good reason for resurfacing or replacing the rotor right away. At
>> some point I figure in the next couple of years, I'll have to replace the pads.
>> Then maybe I'll revisit the rotor issue.
>
>only if the thickness is below spec. then replace. ive never turned or
>replaced a rotor on any of the cars ive done brake jobs on. including a
>77 accord with grooves i could feel with my fingernail.

I second the motion! NEVER turn Honda rotors!

"John Ings" > wrote
> > wrote:
>
> >> I can't see a good reason for resurfacing or replacing the rotor right
away. At
> >> some point I figure in the next couple of years, I'll have to replace the
pads.
> >> Then maybe I'll revisit the rotor issue.
> >
> >only if the thickness is below spec. then replace. ive never turned or
> >replaced a rotor on any of the cars ive done brake jobs on. including a
> >77 accord with grooves i could feel with my fingernail.
>
> I second the motion! NEVER turn Honda rotors!

Thank you, Jim, Mike, and John.

I note the original (91 Civic) rotor thickness was 0.83 inches. So it's taken 13
years to wear down about 0.02 inches. At this rate, unless I put on some non-OEM
pads that are unusually hard or have some other weird mishap (like a major
gouge), the rotors are unlikely to hit the minimum thickness spec of 0.75 inch
before the engine itself dies.

I theorize whatever caused the groove may have thrown the thicknesses out of
whack on the rotor.

Anyway, per your suggestions, I am not going to touch it again until I hear the
wear indicators or happen to be rotating the tires and see the pads are about
down to the min. specified thickness.

Caroline wrote:

> I theorize whatever caused the groove may have thrown the thicknesses out of
> whack on the rotor.

thats called "runout", right? ive never checked for it, or measured disc
thickness. youre definately detail-oriented, and love learning! much
props for that
>
> Anyway, per your suggestions, I am not going to touch it again until I hear the
> wear indicators or happen to be rotating the tires and see the pads are about
> down to the min. specified thickness.

weird thing about the indicators... sometimes, if youre on the bleeding
edge of them making noise, and have to do a panic stop, or just brake
hard? theyll break off. or they break off some other way-im just
surmising how it happened on the couple cars ive done brakes on its
happened to.

by the time i did thebrake jobs on those, it was definately making a
grinding sound, and the indicators were broken off. i wont rule out
stupidity or neglect, either none were my cars.

anyhoo... on the 3 that were metal on metal of the 20 or so ive done, i
just slapped new pads on em and let em bed in. no pulsing, no warping,
nothing weird happened. i imagine they "self-turned".

the last one i did a few months ago was a 92 GMC 1500 pickup of my
brothers. the passenger side rotor looked pretty chewed. slapped new
pads on, told him to drive it a while and let me know if there was any
pulsing or anything. he says it brakes normally.

of course, the friggin brake fluid is coffee colored, but if i stressed
out about everyones vehicle i ever worked on and did them to my personal
standards, id have no free time to post to usenet. besides, hes pretty
broke (student), so even getting him to spring for the raybestos
"lifetime" pads took convincing.

"SoCalMike" > wrote
> Caroline wrote:
>
> > I theorize whatever caused the groove may have thrown the thicknesses out of
> > whack on the rotor.
>
> thats called "runout", right?

I'm taking a brakes community college course, and this all came up this past
week.

Runout and thickness variation are different. "Runout" is the rotor's wobble.
It's measured with a dial indicator affixed to the car such that it stays fixed
while someone can turn the rotor. Out of spec. runout will cause thickness
variation, so they're definitely related.

The instructor said something about how runout specs typically are one order of
magnitude higher than thickness variation specs. (This is indeed so for Hondas.)
He had a reason for this but I missed it.

> ive never checked for it, or measured disc
> thickness. youre definately detail-oriented, and love learning! much
> props for that

Thanks. I never had enough hands-on before going into (really kinda unsatisfying
defense industry) engineering. I love this (engine etc. repair) and wish I could
make a living from it. But alas, it's a tough work environment.

I do think technicians are underpaid, do more for society, are smarter, and have
to have more integrity compared to medical doctors and lawyers.

> > Anyway, per your suggestions, I am not going to touch it again until I hear
the
> > wear indicators or happen to be rotating the tires and see the pads are
about
> > down to the min. specified thickness.
>
> weird thing about the indicators... sometimes, if youre on the bleeding
> edge of them making noise, and have to do a panic stop, or just brake
> hard? theyll break off. or they break off some other way-im just
> surmising how it happened on the couple cars ive done brakes on its
> happened to.
>
> by the time i did thebrake jobs on those, it was definately making a
> grinding sound, and the indicators were broken off. i wont rule out
> stupidity or neglect, either none were my cars.

> anyhoo... on the 3 that were metal on metal of the 20 or so ive done, i
> just slapped new pads on em and let em bed in. no pulsing, no warping,
> nothing weird happened. i imagine they "self-turned".

Interesting. I drove a lab-donated car for my class the other day with one
metal-to-metal pad. The grinding sound and feel was dramatic. But the rotor was
thoroughly thin on that side, too, so it had probably been driven this way a
while.

> the last one i did a few months ago was a 92 GMC 1500 pickup of my
> brothers. the passenger side rotor looked pretty chewed. slapped new
> pads on, told him to drive it a while and let me know if there was any
> pulsing or anything. he says it brakes normally.

Good to know. :-)

> of course, the friggin brake fluid is coffee colored, but if i stressed
> out about everyones vehicle i ever worked on and did them to my personal
> standards, id have no free time to post to usenet. besides, hes pretty
> broke (student), so even getting him to spring for the raybestos
> "lifetime" pads took convincing.

Hey, those pads cost as much as OEM!

BTW, was it you who mentioned that lifetime warranty on the Raybestos pads a few
months ago, when I said I currently had them on my car?

Someone in my brakes class who works in parts said Raybestos did indeed for some
time warrant its pads for their life, even for normal wear and tear. Raybestos
has since changed its policy, and normal wear and tear no longer counts.

My pad hardware is original or nearly so and all rusty-looking. I look forward
to replacing the hardware sooner rather than later but will wait until I need
new pads, so I can buy a full kit. Those tiny little clips at the top of each
pad go for over $4 each at Majestic!

Caroline wrote:
>
>
> Thanks. I never had enough hands-on before going into (really kinda
> unsatisfying defense industry) engineering. I love this (engine etc.
> repair) and wish I could make a living from it. But alas, it's a tough
> work environment.

Indeed...

> My pad hardware is original or nearly so and all rusty-looking. I look
> forward to replacing the hardware sooner rather than later but will wait
> until I need new pads, so I can buy a full kit. Those tiny little clips
> at the top of each pad go for over $4 each at Majestic!

You can often just take a wire brush to the spring clips to clean them up.
By the way, Honda's silencer grease works about the best that I've come
across (a thin coat of the silencer grease goes on the backing plate's ears,
the back side caliper contacting surfaces, and on the silencer shims), and
by all means, stay away from that red silicone goop or spray silencer stuff.

Eric

On Sat, 09 Oct 2004 05:34:58 -0700, John Ings >
wrote:
>>only if the thickness is below spec. then replace. ive never turned or
>>replaced a rotor on any of the cars ive done brake jobs on. including a
>>77 accord with grooves i could feel with my fingernail.
>
>I second the motion! NEVER turn Honda rotors!

So that dealer twelve years ago took me, huh? Thought so.

J.

JXStern wrote:

> On Sat, 09 Oct 2004 05:34:58 -0700, John Ings >
> wrote:
>
>>>only if the thickness is below spec. then replace. ive never turned or
>>>replaced a rotor on any of the cars ive done brake jobs on. including a
>>>77 accord with grooves i could feel with my fingernail.
>>
>>I second the motion! NEVER turn Honda rotors!
>
>
> So that dealer twelve years ago took me, huh? Thought so.

they do it to be on the "safe side". if the brakes make some noise or
pulse a bit, youll be back for a freebie. they dont want that.

ive personally never had a problem with any of those problems myself,
just slapping new pads on and taking it easy until they "bed in".

if i DID discover a problem that didnt go away after a week or so, its
easy enough for me to just throw some new discs on and still be ahead,
money-wise.