Brake Lathe Arbor – Checking & Correcting Runout

Brake Lathe Arbor – Checking & Correcting Runout

If you’ve ever worked on brakes, you’ve likely used, or at least seen a brake lathe. This handy tool helps mechanics to machine rotors to the right specifications ensuring a smooth braking experience. However, there are times where the the brake lathe can end up causing more problems instead of solving them. This article will go over some of the issue that come up and how to prevent/solve them.

Causes of Brake Lathe Runout

Typically, there are three main reasons that brake lathe with cause runout:

1. Arbor runout

2. Adapter

3. Not verifying accuracy of setup

The good news? Each of these issues can be helped via the steps below.

Measuring Arbor Runout

Essentially, proper care mixed with periodic measurement will eliminate the arbor as a cause of brake lathe runout. For example, follow the steps below to check and correct arbor runout.

1.Witness Marks

Lathes using tapered arbors use witness marks to indicate the most accurate fit for the arbor to spindle. These marks are made at the factory after the arbor is matched to the lathe. For example, verify that the marks are aligned as shown in the image below.

Witness marks on a brake lathe

2. Position the Dial

Next, you’ll need to set up the dial indicator . For example, see the image below.

Dial indicator attached to brake lathe

3. Rotate and Measure

Finally, use the draw bar nut to rotate the spindle while also watching the dial. It is best to use a socket and ratchet when performing this step. Use a smooth 360 degree rotation for the most accurate measurement. If arbor runout is within specifications (typically nor more than .002 with the closer to zero the better.) No service is needed. If outside of specifications follow the steps below.

Correcting Excessive Arbor Runout

Once the issue has been found, steps should be take in order to correct it. The following section will go over the necessary steps to correcting the arbor runout.

1. Inspecting the Arbor(s)

Begin by removing the arbor. Inspect the arbor and spindle taper(s) for chips or rust build up. (See below) The tapered surfaces of the spindle should be cleaned using a fine to medium steel wool. The arbor’s tapered surface(s) can be cleaned using a wire wheel.

Rusted Taper

Note: Do NOT use sandpaper, emery cloth, or anything that could remove metal. The taper surfaces determine the arbor’s fit and changing them in any way could result in permanent damage to the accuracy of these surfaces.

2. Clean

Use a clean rag to wipe all mating surfaces before installing the arbor. Put a light coat of WD40 or similar treatment on the tapered surface(s) before installing. This will prevent rusting.

3.Witness Marks

Next, align the witness marks and tighten the arbor nut or draw bar to specification. Typically, it will be around 50-60ft lbs.

4. Repeat

Finally, repeat the steps found under “Measuring Arbor Runout”. If the measurement is now less than .002″ you are finished. Otherwise, follow the steps below to determine the cause.

Determining the Source

Even after completing all of the steps above, there are times that the specifications don’t match up. When this happens, it is important to figure out the cause, so that it can be fixed.

1. Greatest Runout

Rotate the arbor until the dial indicator is at its highest reading. This is the point of greatest runout. Use a magic marker to mark the point on the arbor as shown below. This mark will be used to determine if the arbor is the source of the excessive runout.

arbor high spot

2. Loosen

Next, loosen the arbor and rotate it 1/8 of a turn clockwise then tighten the drawbar. Continue by measuring the arbor runout.

3. If Less Than .002"

If runout is less than .002″, make a mark on the arbor at the spindle witness mark.

4. If More Than .002"

If the runout is more than .002″, loosen the drawbar and rotate the arbor 1/8th of a turn, then retighten th drawbar.

5. Re-Check

Now you are ready to re-check the runout.

6. Still Greater Than .002"

At this point, if the runout is still more than .002″ repeat steps 4 and 5 until the runout is less than .002″. Next mark the arbor to the spindle witness mark.

7. Check Mark

Finally, if the runout is still greater than .002″ after the arbor has been rotated 360 degrees, rotate the arbor until the maximum reading is shown. Next, check to see if the magic marker reference mark from step 1 is aligned with the dial indicator plunger. If the two are aligned, the arbor is bent and will have to be replaced.

Note: Some older lathes may not have a witness mark. Dial indicate the arbor and mark both the arbor and spindle.

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How to Diagnose Excessive Disc Thickness Variation (DTV)

How to Diagnose Excessive Disc Thickness Variation (DTV)

What is DTV?

When a rotors thickness varies at different locations, it is called disc thickness variation, or DTV for short. Typically, the average specification for DTV is .0005″. On the other hand, the maximum allowable variation in thickness on a rotor is 5 ten thousandths of an inch. Because this measurement is so small, it is not measurable at the shop level.

Disc Thickness Variation (DTV) Illustration
Image Credit:

Measuring the DTV

Most publications give instructions for measuring the DTV. Normally, the steps involve making 6-8 measurements at a specified distance from the edge of the rotor. Next, it’ll typically have a chart for you to compare the measurements with each other. In some cases, the measurements will show more than .0005″. Because of this, we know that the rotor has excessive DTV. Because the measurement is so small, it can only be observed in a laboratory environment.

Determining DTV

Since measuring the rotor in shop isn’t an option, it’s best to identify it via symptoms. For example, excessive DTV is the leading cause of high speed pulsation complaints. Likewise, DTV is typically more pronounced when stopping from speeds above 35mph. This is caused by the changing thickness of the rotor thus causing the caliper piston to move in and out. Similarly, the movement of the caliper piston causes fluid movement in the hydraulic system that is felt in the brake pedal.


When a vehicle is exhibiting high speed pulsation, then one or more of the rotors has excessive DTV. For instance, if the vehicle is disc/drum, then both front rotors should be serviced according to their thickness condition. In another example, if the vehicle is equipped with 4 wheel disc brakes, use the location of the vibration to help pinpoint whether it comes from the front or rear. Typically, if the steering wheel, dashboard, and/or brake pedal shake, the problem will be related to the front rotors. Otherwise, when the vibration comes from the seat of floorboard, it is likely to be a rear rotor issue. As a rule, DTV is more common on front disc brakes than rear.

Bonus Tip: You CANNOT measure thickness variation at the shop level. Because of this, you must rely on the symptom it produces to in order to know whether you have it or not.

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How to Measure Rotor Runout

How to Measure Rotor Runout

The rotor runout dial indicator is used primarily to measure rotor runout. Likewise, it may be used to measure arbor runout on the bench lathe, wheel bearing play, and hub runout. Even so, the tool must be used to correctly in order to get an accurate reading. Because of this, runout measurement is often not performed correctly and yields inaccurate results.As an example, this article will go over the correct steps to completing the process.

How to perform a Rotor Runout Measurement

1. Attach Vise Grips:

To begin, attach the flexible mount to vise grips. As you’ll see, there should be three mounting holes in the mounting block. Typically, the end one will be the best option, use it when possible.

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

In order to keep everything stable, be sure to locate a solid mounting point on the vehicle. For example, consider the caliper mounting bracket, pad rails, lower strut mounting bolt, or steering arm. On the other hand, be sure to mount on a stationary location that is not attached to the rotor.

Vice Grips Attached to Bolt

In addition, there must not be any bearing play on hub style rotors. If excessive play is present, adjust the bearing before you measure. On the other hand, hubless rotors have to be secured with spacers and lug nuts. For example, a 5 lug hub should use 3 lug nuts. Likewise, on a 4 lug hub, use 2 opposite lugs with spacers. Likewise, the lug nuts should be installed backwards (when possible) with the flat sides against the spacers.

2. Attach Dial Indicator

To attach the dial indicator, loosen the clamp screw and tighten it to a point that the indicator can move freely. Next, make sure that the red handle points directly in line with the base of the flexible mount. To do this, turn the adjusting screw clockwise. (see below)

Indicator Clamp

3. Set the Plunger

Next, locate the dial indicator plunger as shown below. The tip should be about 1/4″ from the edge of the rotor. In addition, the plunger should be placed on a smooth portion of the rotor. If necessary, take the rotor runout reading on the inner surface of the rotor. Likewise, the plunger should extend out of the housing and the contact tip should be at a slight angle in line with the edge of the rotor. (see below)

Close Up of Needle

4. Add Tension to the Rotor Runout

While holding the indication in position, rotate the red tension handle 189 degrees or until there is enough tension to hold the unit stationary.

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

5. Take the Rotor Runout Reading

Finally, the rotor runout reading can be taken. To do this, rotate the rotor in the direction that the dial indicator tip is pointing. Next, watch the needle for the lowest and highest number reached during one revolution. The runout number will be the difference between these two numbers. For example, if the lowest reading was 65 and the highest was 69 the runout is .004″ (see below).

Runout reading on dial meter

To use the “zero” feature rotate the rotor until the lowest reading is obtained. Next, loosen the set screw and rotate the black dial until the needle point to zero. Finally, rotate the rotor a complete revolution and note the amount of runout. As a result, 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 rotor runout will be an issue in a vehicle that comes back with pulsation.

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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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