Thickness of brake rotor

beaver5

I have a 2000 GTP.
What is the thickness of the brake rotor when new ?
What is the minimum thickness that you can have to have them machined?
Are there any suggestions on which major brand name to use as a replacement (Delco, etc)?
Any hints on doing a brake job myself?

alcan

Document ID# 698746
2000 Pontiac Grand Prix

Component Specifications
 
Front Brakes:
 
Rotor Thickness (new) 32.2 mm, 1.27 in

Minimum Rotor Machining Thickness 31.7 mm, 1.25 in

Rotor Discard Thickness (see note) 30.7 mm, 1.21 in
 
Maximum Lateral Runout 0.080 mm, 0.003 in
 
Maximum Scoring 1.50 mm, 0.059 in
 
Thickness Variation 0.013 mm, 0.0005 in
 
Important: All brake rotors have a discard dimension cast into them. Replace any rotor that does not meet this specification. After refinishing the rotor, replace any rotor that does not meet the minimum thickness after refinish specifications.

oldharry

I notice that the aceptable depth of scoring is such that attempting to machine out such a score on one side only would reduce the rotor thickness from new to discard. This does not sound good for the manufacturers of brake lathes.

Harry

swschrad

posters like alcan and 0patience have mentioned they use on-vehicle rigs that basically just break the glaze, not refinish the rotors to spec. the pads have to conform to the rotors, not both fitted to each other.

I don't suppose anybody "opens up" brake shoes any more, either. keeps the fiber count under control for more years of heavy breathing, LOL.

alcan

Harry, don't know about you, but I'd have a hard time putting new pads on a rotor with .059" scoring. Yet this is from a current GM manual:
  
Brake Rotor Refinishing
Do not refinish brake rotors when performing routine brake maintenance such as replacing worn disc brake pads. Refinish a rotor only under the following circumstances:

There is a complaint of brake pulsation.

There is scoring greater than 1.5 mm (0.060 in).

All brake rotors have a minimum thickness dimension cast into them. This dimension is the minimum wear dimension and not a refinish dimension.

Important
Do not use a brake rotor that, after refinishing, will not meet the specifications shown on the rotor. Always replace the rotor with a new rotor.

beaver5

I have a suspicion that I am either measuring the wrong way or there is a different rotor used on the GTP. The reason I say this is that the fron rotor is different from the the rear rotor.
The front rotor is two plates with a honeycomb seperating them. It is 1.3125 thick. The two plates are quite thin about .250 to .3125 thick.
 
The rear rotor is a single plate.

I agree about only machining the rotor when they are badly scrored as the new pads will quickly wear to match the shape of the rotor.

beaver5

I have a suspicion that I am either measuring the wrong way or there is a different rotor used on the GTP. The reason I say this is that the fron rotor is different from the the rear rotor.
The front rotor is two plates with a honeycomb seperating them. It is 1.3125 thick. The two plates are quite thin about .250 to .3125 thick.
 
The rear rotor is a single plate.

I agree about only machining the rotor when they are badly scrored as the new pads will quickly wear to match the shape of the rotor.

alcan

The front rotors are vented, rears are solid as is the setup with most 4 wheel disc brakes. The "honeycomb" are actually vents which aid in cooling by providing more surface area for heat dissipation, but more importantly by turning the rotor into a centrifugal air pump. As the rotor spins, air is drawn in at the centre then forced outward through the fins and between the braking surfaces to promote cooling. The splash shield behind the rotor aids in directing cooling air into the hub area.

Grand Prix front rotor specs are the same from 1998 to 2003, as per the G.M. factory manuals:
original thickness = 1.270"
minimum machining thickness = 1.250"
discard thickness = 1.210"
lateral runout = .002" maximum
thickness variation = .0005" maximum

Rotor thickness is measured as overall thickness, about 1" in from the outer edge. Suggest you have your micrometer calibration checked.

swschrad

since few drivers get to speed and have to stop going in reverse, they don't have the weight of the car leaning on the rear rotors. that's what happens in front, and they do most of the braking power on the car. so they will get hotter and need more heft and cooling.

beaver5

How do you measure the thickness of the rotors. the manual says the original thickness is 1.27" thick.
I measure the rotor as 1.3125" including the honeycomb and that after 100,000 KM of use.
Evidently I am measuring it wrong.
How do you measure it so it falls within the factory parameters.
The reason I ask is because when I go and buy a third part rotor I want to make sure it is the correct thickness. I have been caught to many times by buying parts that are supposedly replacement but when I get everything torn down and try to fit the replacement part I find it doesn't fit.

swschrad

now, whether you measure near the hub on the braking surface or near the edge, I do not remember. if it's the edge, you could use a simple sliding dial caliper... but I have seen it done, and I seem to remember it was measured deeper into the rotor than that.

I suspect you also rotate the rotor and measure for minimum thickness, inasmuch as the surface is machined and supposedly flat and plumb, and the minimum useful thickness is the most magic number if you're checking brakes.

if you're chasing pulsing or driveability/brakeability issues, you would use a pin caliper on a frame and look for delta-measurement, aka "runout", which indicates lack of flatness... which would be warpage from overheating in use.

notice I do not claim to be a tech in this field. alcan and oldharry, among others, certainly are. I have learned enough to know when I'm being strung along by somebody who wants my signed check, as opposed to somebody who wants me back for other work for years to come.

oldharry

Well on my own car I put on new OE type rotors rather than try to cut them. While made in China cast rotors were about $23 (cost) I spent $62 (list over $100) to get the same steel centeres cast disk ones that the car had new. I could have had the old ones turned and been just above min.

Another note on cheap parts:

I replaced lower ball joints on a '92 G-20 van (Chevrolet) today. There were replacement ball joints of unknown origin on the vehicle that the bearing portion of the ball had broken into six segments on the right side of the vehicle. The left had chips missing around the upper edge of the ball. Apparently the pin rotated in the ball instead of the ball in the housing, and when there was enough clearence from wear, road shock started breaking the sintered ball. I have never before seen damage like this in a ball joint where the vehicle did not come in on the hook. My customer has ony owned the van for six months, so the source of parts is unknown.

Harry

alcan

Used car lots love white box parts, they last just long enough for the vehicle to pass a safety inspection. Might as well install them with velcro, makes it easier to change them out next time.

beaver5

I went to the Pontiac dealership for a quote on doing a front end brake job.
It seems there are two types of pads for the brakes a "Ceramic" pad at $174 for a set for both wheels ("Ceramic" is evidently original equipment) and a Delco pad which is $90.
When you get a price from the dealership for a brake job they always include the "Delco" set.
I had to pay a additional $36 (the difference in their cost price for the two) to get a upgrade to the "Ceramic".
I would never have known about the two types of pads if I hadn't talked to the parts department before I talked to the service department.

swschrad

and always specify the top-end pad. if I want sponges or Scotch-Brite instead of brake pads, I can go to any number of shops that spring up on a vacant corner and go away in months.

two things I don't want to cheap out on... steering and brakes.

I also find the service advisor is aware of the "upgrade" parts possibilities, for some mystical and unexplainable reason. they always know if there is a "gold" option for a service. darned if I can figure out why............