How to Measure Rotor Runout

How to Measure Rotor Runout


Accurately measuring installed runout


Runout measurement is not being performed on a regular basis so many technicians do not know how to perform it.


The dial indicator is used primary to measure rotor runout. In can also be used to measure arbor runout on the bench lathe, wheel bearing play and hub runout. Accurate measurements depend on proper use of the tool. The following steps outline the proper use of the dial indicator.

Attach flexible mount to vise grips. There are three mounting holes in the mounting block. The end one is usually the best one to use when possible.

Note: Make sure the flexible mount is screwed in tightly to the vise grip mount.

Locate a solid mounting point on the vehicle. Some examples would be the caliper mounting bracket, pad rails, lower strut mounting bolt, steering arm. Do not mount to anything that is not stationary in reference to the rotor (See Figure 23.1).

Figure 23.1 Vise Grip Mounted

On hub style rotors there must not be any bearing play. If excessive play is present adjust bearing before measurement. Hubless rotors have to be secured with spacers and lug nuts. On a 5 lug hub use at least 3 lug nuts. On a 4 lug hub use 2 opposite lugs with spacers. Where possible the lug nuts should be installed backwards with the flat sides against the spacers.

Attach the dial indicator by loosening the clamp screw. Tighten the clamp screw to a point where the dial indicator can be moved around easily.

With the red handle pointing directly in line with the base of the flexible mount , take the slack out of the flexible mount by turning the adjusting screw clockwise (See Figure 23.2).

Figure 23.2 Indicator Clamp

Locate the dial indicator plunger as shown in Figure 23.3. The tip should be about 1/4” from the edge of the rotor. It should be located on a smooth portion of the rotor. If necessary, take the runout reading on the inner surface of the rotor. The plunger should extend out of the housing and the contact tip should be at a slight angle that is in line with the edge of the rotor (See Figure 23.3).

Figure 23.3 Closeup of Needle

While holding the indicator in this position, rotate the red tension handle 180 degrees or until enough tension is present to hold the unit stationary.

Note: Make sure the tip of the indicator is screwed in all the way or vibration will occur during the reading.

Runout can now be taken. Rotate the rotor in the direction the dial indicator tip is pointing.

Watch the needle for the lowest and highest number reached during one revolution of the rotor. The runout is the difference between these two numbers. For example, if the lowest reading was 65 and the highest was 69 the runout is .004” (See Figure 23.4).

Figure 23.4 Reading

If you want to use the zero feature of the indicator rotate the rotor until the lowest reading is obtained. Loosen the set screw and rotate the black dial unit the needle points to zero. Rotate the rotor one complete revolution and note the amount of runout. If done correctly the needle will start and stop at zero without going below it.

Bonus Tip: 

It takes less than 60 seconds to measure rotor runout and KNOW whether the runout will be an issue in that vehicle coming back with a pulsation complaint.

Reoccurring Pulsation

Reoccurring Pulsation

High speed pulsation occurring after a brake job is called reoccurring pulsation. This article will go over the most common causes and solutions. In addition, several tips and tricks are noted through out the article.

Reoccurring Pulsation Cause 1:

One or more of the rotors were installed with excessive runout. Generic specification being used by domestic OEMs is .002”. Excessive runout causes disc thickness variation (DTV) over time. The average for most vehicles is 3,000 to 6,000 miles. This number can vary up or down based on the variables involved.

Cause 2:

Insufficient clearance between inboard and outboard brake pads and rotor’s friction surface. The closer the brake pads are to the rotor’s friction surface the smaller the amount of runout needed to produce disc thickness variation (DTV).

Reoccurring Pulsation Solution 1:

Rotors must be installed with runout less than vehicle’s specifications in order to eliminate runout induced DTV. There are 2 options to perform this.

Option 1:

Use an on-the-car lathe to machine the rotors true to the hub. Make sure to index the rotor to the hub if removing it after the machining process. Make sure to properly torque the wheels using the method described in step 6 below.

NOTE: When performing on-the-car machining on vehicles equipped with ABS make sure none of the machining chips accumulate on the magnetic sensor tip. This could trigger false activation of the ABS or fault codes. See Figure 22.1

Lathe Chips

Option 2:

If using a bench lathe to perform off the vehicle machining follow these steps:

1. If resurfacing the rotor(s) use proper machining techniques and make sure the lathe is in good working condition. On hubless rotors make sure to clean the mating surfaces with an appropriate tool. Scratch cut all rotors to ensure accurate setup.

2. Clean the rotor before installing on vehicle to prevent machining dust from contaminating brake pads.

3. Before installing hubless rotors, clean the hub mating surface using an appropriate tool(s). Failure to properly clean this surface can prevent achieving the proper installed runout.

4. When installing new or machined hubless rotors the installed runout should be checked. Using spacers on the studs tighten all lugs to the proper torque using the correct sequence. Measure runout. If runout is not less than manufacturer’s specification index the rotor on the hub to achieve the lowest amount of lateral runout.

5. Before indexing mark the high spot on the rotor and hub using a magic marker or paint stick as shown in Figure 22.2. Indexing involves removing the rotor and rotating it one or two lugs and reinstalling. Repeat this until the installed runout is below specification. If runout is out of specifications and does not change as the rotor is rotated check to see if the high spot moves with the rotor or stays with the hub. This will identify the cause of the runout.

Indexing dial

6. Properly torque the wheel lugs using either a hand torque wrench or torque stick sockets. When torquing wheels use a step torque process. To step torque the lugs tighten all lugs to half the normal torque using proper sequence and then fully torque lugs using same process. Make sure your impact has been calibrated before using torque sticks.

Solution 2

Look closely at Figures 22.3 & 22.4. Both rotors shown have the same runout but what they don’t have is the same gap between the pads and rotor. The gap between the pads and rotors in Figure 22.3 is much smaller than that of Figure 22.4. The smaller the gap the more sensitive the vehicle will be to runout induced DTV. Remember the wear of the high and low spots occurs during both brake apply and release. If something in the caliper assembly is preventing a full or complete release of either the outboard or inboard pad that wheel will be more susceptible to this problem. Improper release of either pad could make even those rotors at or below .002” prone to this problem.

Small Pad to Rotor Gap
Large Pad to Rotor Gap

Pad Release

Effective inboard pad release will be effected by two types of conditions. The first of these involves how well the piston releases. A piston that does not return as far as it should reduces the gap between the inboard pad and rotor. Mileage, brake fluid condition, dust boot seal integrity all can impact the piston’s ability to release properly. The second condition that can effect the release of the inboard pad involves how easy the pad can move in relationship to the caliper mounting bracket, knuckle or slide rails. If the inboard pad binds then complete release will not take place.

Outboard pad release can be impacted by two categories of failures as well. The most common of these is the slide hardware. If the caliper housing is not free to move on its mounting hardware then outboard pad release will be effected. The other category is the same as for inboard pad release, that is anything causing the outboard pad to bind where it contacts the bracket, knuckle or slide rails.

No Short Cuts

Nothing new about what impacts inboard or outboard pad wear but understanding its influence on the issue of runout induced DTV is a fairly new concept. The moral of the story is short cutting quality brake work impacts more than just pad life. The old “pad slap” has more chances to come back and haunt you than what you might have considered. Remember just because the caliper applies and releases doesn’t guarantee its doing it as effectively as it should. Look at the big picture when doing your brake inspection. A few more minutes covering the details up front will pay big dividends in the long run. A saying I like to use sums it all up – “I would rather be paid today for what is wrong with the vehicle than have to give it away tomorrow”.

More Info: The fact that excessive runout is the leading cause of reoccurring pulsation should not be anything new. The principal is easy, too much runout on fixed bearing vehicles causes the high and low spot of the rotor to scrape against the pads. This scraping occurs during both non-braking and braking. Over time the high and low spots are worn thinner than the other portions of the rotor. This difference in thickness is known as disc thickness variation or DTV (See Figure 22.5). It is also called parallelism. The two friction surfaces of a rotor are supposed to be parallel to one another to within a certain tolerance. The average value for this specification is .0005”. As little as 5 ten thousands of an inch or another way to say it would be ó of one thousandth of an inch tolerance is all that is allowed.

Disc Thickness Variation DTV Views


If DTV is in excess of acceptable limits the thin spots pass between the pads during brake apply causing the caliper piston must move out to take up the gap. The thick portions of the rotor push the piston back. The in and out movement of the piston causes the pedal to pulsate especially at speeds above 35mph. What has been stressed is the importance of getting the rotor’s installed runout to acceptable levels. Most domestic OEMs are agreeing that this amount is .002” or less. While this is definitely the most important aspect of curing the reoccurring pulsation it is not the only thing to consider. There are other factors that will contribute to the vehicle’s sensitivity to this.

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Rotor & Drum Specification Accuracy

Rotor & Drum Specification Accuracy

Unfortunately, not all specifications are reliable. As a result, it can sometimes be tricky to get a quality job done. Because consolidated specification sources often make mistakes, it is important to make sure you find the right numbers. This article aims to help you learn how to verify rotor and drum specifications in order to complete a job. In particular, the example below will help illustrate a potential problem, and how to fix it.

The Specification Solution:

Take a close look the following chart, you’ll notice that the 1998 Skylark has different “machine to” specifications. As you can see, points “A” “B” and “C” are all different. Because of the differences, issues can occur. For instance, using the wrong specifications could result in the machining of a rotor that should have been replaced, instead of serviced.

Rotor Specification Chart

As a mechanic, you are making some pretty important decisions. When it comes to rotors and drums, you need to ensure that your numbers are accurate. For example, try to find two specifications from different sources. For the most accurate information, look to the rotor itself, information providers, and parts manufacturers.

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Rotor & Drum Specification Issues

Rotor & Drum Specification Issues

Machining rotors and drums can be a relatively straightforward task. You look up specifications and follow what they say, right? What about when there are not specifications listed? To help you out, this article will show you what to do when you can’t find the numbers you need to get the job done. Likewise, there will be various tips as you read. 


No Machine to Specification Listed

Many vehicles no longer provide a machine to specification. However, this does not mean the rotor is not serviceable. In fact, most of these rotors are allowed to be machined to the discard specification. Though possible, it is not a practical procedure in a shop environment. This is especially true in the states where there are safety inspections performed. For instance, it is possible to perform a brake job and have the vehicle fail a state inspection only a couple of months later.

The chart below shows an example of this. In the image no machine to specification is given. Instead “NS“ is listed. The “NS” is short for “Not supplied by manufacturer”.

Table showing various machine to specifications

In this case, the machine to is calculated by adding .015” to the discard specification provided. In the case adding .015” to this would yield a machine to thickness of .995”.

No Discard Specification Provided

In the chart below, no discard specification is given. Instead a footnote number is listed. In particular, the footnote listed is number 2 which can be found at the end of the make section. Footnote 2 says:

“Discard when thickness is smaller than the minimum machining specification”

Brake specifications guide

As a result, the discard and machine to specification are the same number. This would not be practical for the customer. For example, if the rotor were machined down it would go beyond service in a very short period of time. Consequently, another brake inspection would result in the rotor being under specifications. Because of this the industry has adopted creating a revised machine to specification by adding .015” to the number provided. In the case of the 2001-99 Cougar the machine to listed is .874”. So, adding .015” to it will bring the revised machine to up to .889”. This is number you would use to determine whether the rotor can be machined or not. The .015” allows for normal rotor wear during the life of the friction material.

NOTE: Although it is best to use the aftermarket approach, it is acceptable to machine these rotors down to the machine to specification listed. The invoice should be marked “rotors at minimum machining/discard” to avoid future problems.

Brake Drum

The same holds true for brake drums. There is no drum discard specification provided for the vehicles in the highlighted box. Instead, a footnote number 3 is listed and that footnote says:

“Discard when diameter is greater than the maximum machining specification”

This means the maximum machining diameter and discard diameter are the same for these vehicles. If the drum were machined up to the machine to it would go beyond service in a very short period of time. Another brake inspection or a state inspection would result in the drum being over specifications after just being serviced. The industry has adopted the position of creating a revised machine to specification by subtracting .015” from the number provided. In the case of the 2001-99 Cougar the machine to listed is 9.040”. Subtracting .015” from this value would result in a machine to specification of 9.025”. This is the number you would use to determine whether to machine the drum or not.

Bonus Tip:  ‘NS” does NOT mean “Not Serviceable” as some technicians have been led to believe. It means “Not Supplied”.

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How To Determine if A Rotor Can Be Serviced

How To Determine if A Rotor Can Be Serviced

The determining factors in whether or not a rotor can be serviced are both dimensions and condition. Because of this it’s important to know when to service or replace the rotor in order to avoid incorrect information that leads to over or under selling.The dimensions determine whether it can be machined, replaced or in some cases left alone. Not understanding the meaning of the dimensions involved in making these decisions can lead to a rotor being machined when it shouldn’t be. Likewise, its possible a new rotor sells when the old rotor was still serviceable.

Rotor Solution

Understanding all of the critical dimensions of a rotor will allow accurate determination of the part condition and the type of recommendation that should be made. In the rest of this article, you will learn all of the important dimensions necessary.

Nominal Thickness:

Nominal thickness is the thickness of a new rotor. While this thickness is not normally used, it is listed in most specification books.

Machine to Thickness:

This is the thinnest point that a rotor can be machined to and still be put back into service. The “machine to thickness” is there to provide enough rotor material to last the life of one set of pads. Essentially, the idea is to make sure that the pad will wear through before the rotor does. To help you visualize, the average difference between nominal and machine to thickness is .050“ to .060“. The typical difference between machine to and discard (explained below) is .015“. See the image below:

Rotor Dimensions

One common myth, called throw away motors, says that late model vehicles have no meat on them and are thus not meant to be machined. This is not true. Again, the average rotor provides .050“ to .060“ with some giving as much as .100“ between nominal and machine to thickness. With the average machining taking off .010“ to .015“ there is usually room for at least on machining. The only true throwaways are marked like the one in Images below. The footnote (3) on this BMW denotes the rotor should be replaced not machined. True throwaway rotors are marked in this fashion.

Throwaway Rotor Footnote
Throw away rotor footnote

Rotor Discard Thickness

The thickness at which a rotor should be replaced is called the discard thickness. One common understanding is that the discard thickness is a heat related dimension. For example, if a rotor is at or below discard thickness it cannot dissipate the heat generated, however, this is not correct. Discard thickness is the thinnest a rotor can wear to. If the brake pads wear to nothing the caliper piston won‘t fall out of the caliper housing. It has nothing to do with heat. See the image below.

Discard thickness diagram

Many vehicles with a rotor at or below discard stop without any issues. The only sure way to know that a part is below discard is to measure the it and compare the measurement to the specifications.

Parallesism (Disc Thickness Variation, DTV)

The two friction surfaces of a rotor are designed to be parallel to one another within a certain specification. The allowable tolerance is known as parallelism. It is also known as the rotor‘s disc thickness variation or DTV. See Below.

DTV view diagram

Other Rotor Factors

A common question presented about rotors involves the thickness of one friction disc to the other on vented rotors as shown in the image below. There is not a published specification for the allowable tolerance for the difference in the thickness of one disc to the other. Some rotors are actually designed intentionally with different thickness discs. It is generally accepted that small differences are acceptable and are either intentional or a result of manufacturing variations.

Friction Discs

When machining, the thickness of one friction disc to the other should be taken into consideration. For example, if a rotor is gouged on one friction surface due to metal to metal contact with the brake pad but not the other, it could be possible to machine the gouge out and the rotor still be over machine to thickness. The problem would be that the gouged friction disc would be considerably thinner than the opposite disc. When faced with this situation common sense is the only guide. If the gouge is deep enough to result in having to remove a considerable amount of material it is probably best to replace the rotor.

Corrosion and rust can also have an impact on the rotor‘s ability to function correctly. The first image below shows clogged vent holes due to rust buildup. This part is thick enough to machine but the rust will prevent proper cooling. Likewise, the second image below has experience enough corrosion to actually weaken its structural integrity.

Plugged Fins

The rust has eaten away at the thickness of the cooling fins. This condition could result in the two halves of the part separating resulting in a catastrophic brake failure. Always perform a thorough visual inspection of the rotor to determine its condition and serviceability.

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What To Do About Brake Noise

What To Do About Brake Noise

Both customers and technicians hate dealing with brake noise. As you can imagine, customers find brake noise annoying and embarrassing. On the other hand,Technicians find brake noise frustrating and unproductive. Unfortunately, both groups find brake noise all too common. Most shops that perform brake service will tell you that brake noise represents the most common reason for brake comebacks.

Brake noise is the layman’s description of the symptom. In fact, brake noise is actually vibration that is occurring at a frequency that is audible to the human ear. All brakes make noise or vibrate. The difference is that most brakes operate at a frequency that we can’t hear.

The Solution to Brake Noise

To prevent or fix brake noise all you have to do is make sure the brakes operate (or vibrate) at the right frequency. In theory it really is that simple, but in real life there is often much more to it than meets the eye. There are many variables involved in the equation that can result in brake noise. Understanding these variables is the starting point of preventing brake noise complaints. Notice the word “prevent” not fix. This should be our primary goal. By getting a handle on how to prevent noise complaints from occurring, you will be better able to handle any comebacks.

Because most brake noise complaints involve disc brakes, we are going to focus our attention on them. As the brake pedal is applied and the pads are squeezed against the rotor, vibration is produced. For example, the points where the vibration takes place are pictured in the image below. Vibration can occur between the:

1. Pads and rotor

2. Pads and Caliper

3. Caliper and knuckle or mounting bracket

4. Knuckle and pads or mounting bracket

5. Caliper and mounting hardware

Diagram of brakes

The number of points where the vibration occurs will vary with the type of disc brake used. Likewise, the type of brake will also determine which causes will be more common. As you can see from the list, the prevention of most brake noise complaints will boil down to two things: performing quality brake service and using quality parts. Through the rest of this article you will learn about each potential problem spot, and how to prevent noise coming from each of them.

Potential Brake Noise Problem Spots

Rotor Finish

The rotors smoothness on its friction surface is important in preventing brake noise complaints. This is especially true right after a brake job is performed. The smoother the surface at the start of a brake job the lower than chances for brake noise.

Friction Material

Many people think that the brake pad is the leading culprit in creating brake noise. While that can be the case, it isn’t always true. If you have access to Mitchell On Demand or All Data you might want to take some time and look through some of the TSB’s (Technical Service Bulletins) dealing with brake noise. Many of these bulletins will tell you that a change in material solves the problem.

If you take a closer look, you would see that it is not the only method used. How does this relate to you and the friction material you are using? If you use a quality product from a reputable supplier then you should not have noise problems with MOST vehicles. Quality pads should be designed after the OEM pad. This means they should have shims, tapered edges or dust slots if the OEM pads were equipped with these features.

Pad Shims

Pad shims are used as an insulator between the pads and caliper. This serves to change the frequency of the vibration so brake noise is eliminated. The shims could be attached to the brake pad or they could be supplied as separate parts. Make sure your replacement pads are shimmed to match the OE pads and when servicing vehicles with separate shims either reuse the shims if in good condition or replace them. Do not reuse shims where the protective coating has worn off or if the shims are rusting and flaking. Do yourself a favor and add the cost of replacing the shims into your estimate now instead of giving them away later.

Pad Hardwear

Brake drum

It used to be that imports were the only vehicles using pad anti-rattle hardware but now disc brake assemblies similar to that shown in the image above have become the standard on many domestic vehicles. The pads “snap” into the caliper mounting bracket and are held in place by various clips. The caliper straddles the assembly and serves only to provide the squeezing of the pads against the rotor. 

The caliper mounting bracket absorbs all of the braking force and maintains the pad’s position. The anti-rattle hardware applies tension to the pads so they fit snugly against the mounting bracket. These clips are essentially small springs made of stainless steel. They are subjected to the same heating and cooling as rear brake hardware. How many of you are strong believers that the brake shoe hardware should be changed with each set of shoes? What’s the difference with the front hardware? Simply cleaning and reinstalling it won’t be good enough in many cases. 

Some add an extra step of “re-tensioning” the clips by bending the tabs slightly. This may work for a short time but won’t provide a long-term solution. Suggesting the replacement of this hardware with each pad replacement is something you should consider. The hardware can be pricey and you should explain to the customer why you are suggesting it. If they decline and have a noise problem later then you are in a better position than if you said nothing about the hardware.

Caliper Hardware

Rubber bussing worn down

Most vehicles use floating calipers. The rubber bushings or sleeves in these calipers allow the caliper to move freely on brake apply and release. They wear over time and can increase the chances for brake noise. Careful inspection should be done and replacement should be suggested or required based on the findings. If the bushings or sleeves are too loose fitting the caliper will be allowed to move too much (See Image Above). This will increase the chances for a brake noise problem especially if other problems are present. Again, if the customer declines the service, mark the invoice accordingly. If a noise comeback occurs, you will have set the groundwork for what needs to be done. It’s a better position to be in then having nothing to fall back on.

Cleaning & Lubricating

Cleaning & lubing – The amount and type of cleaning you will have to do will vary with the type of vehicle being serviced and the area of the country you live in. Some vehicles have only a few key areas that need to be cleaned when servicing the disc brakes while others require considerable more effort. If you work in the south or out west then you won’t have to deal with too much rust like those of us in the snow belt. For more information on cleaning and lubing see our blog post.

Pad Staking

Caliper off
Stake On Caliper

Some outboard pads are designed to be “staked” to the caliper. This staking holds the pad firmly in place and prevents vibration. Older domestic RWD vehicles use outboard pads that stake to the caliper using small tabs as shown in the first image above. This style of pad is best staked off of the caliper. Bend the tabs on the pad until a press fit is needed to install it. Use a pair or channel locks to press it onto the caliper using a piece of cardboard to protect the friction material. 

Some General Motors vehicles require a different procedure to stake the outboard pad. The second pads pictured are first installed on the vehicle. Next a chisel is driven between the hat of the rotor and the base of the pad to hold the pad against the caliper. Now, have an assistant apply and hold the brake pedal. This will hold the pads in place while you perform the staking. The tabs should be bent over at about a 45-degree angle. Release the brakes and check the tightness. No movement should be felt between the pad and caliper.

Pad Break In

Starting the mating of the pads to the rotor and getting the brake job off on the right foot is critical to the long term success of the brakes. See our blog post for more information on the proper break-in procedures.

Final Thoughts

If you apply the information above and the information contained in the related articles mentioned, you will eliminate 8 of 10 brake noise comebacks. If that is the case what do you do with the other 2? This is when you consider the friction material as the possible culprit. If involved in a brake noise comeback, first look at whether any of the steps above have been skipped or not performed properly, in other words, check to see if a quality brake job was performed. If the answer is yes, then consider an alternate pad.

Brake pad noise is one of those things that annoys everybody so it is better to do the best job possible to prevent it.

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