How to Use a Dial Indicator on a Car to Measure Runout

What runout is and why it matters

Runout is how far a rotating part wanders from a true circle or a flat plane as it turns. A brake rotor with too much lateral runout wobbles slightly side to side, which can cause pedal pulsation and uneven pad wear. A bent axle flange, a flywheel or a driveshaft can show the same kind of error.

A dial indicator turns that wobble into a number you can read. The plunger tip rides on the surface, and the needle moves as the surface moves in or out. One full turn of the part tells you the whole story.

What you need before you start

For automotive work, a 0 to 1 in plunger indicator with 0.001 in graduations is the usual choice. That means each mark on the dial is one thousandth of an inch, which is fine enough for rotor and hub checks. If you are still choosing gear, our best dial indicator comparison lists range, graduation, base strength and reach for several sets side by side.

• A dial indicator with a magnetic base, fine-adjustment knob and an on and off switch
• A flat contact tip (the sets we compare include up to 22 interchangeable tips)
• Your vehicle's service manual for the runout limit and torque specs
• Jack stands, a wheel chock and a torque wrench
• Brake cleaner and a clean rag for the surface you will touch
• Safety glasses

Prepare the car and the part

Park on level ground, chock the wheels that stay on the ground and lift the car with a jack. Put jack stands under the car before you go near the wheel. Never rely on a jack alone, and never work under or beside a car held up only by a jack.

Remove the wheel. Clean any rust and scale off the hub face and the rotor. Rust trapped between the hub and rotor is a common cause of false runout readings, so a quick cleaning can save you a lot of confusion.

If the rotor is held on only by the wheel, reinstall a few lug nuts with flat washers or the proper hardware so the rotor sits snug against the hub. Tighten them evenly to the torque in your manual. A loose rotor will give a misleading reading.

Mount the indicator and measure rotor runout

Choose a solid, stationary spot for the base. The steering knuckle, strut body or a clean section of the caliper bracket all work well. The base needs a clean, flat steel surface to grip. Switch the magnet on and give the base a gentle tug to confirm it will not slip. Magnetic bases in the sets we compare list holding forces of 130 to 140 lb where stated, but the real grip depends on how thick and clean the metal is.

Now follow these steps:

• Swing the arm so the plunger tip touches the rotor face about a quarter to half an inch inside the outer edge, clear of any lip or chamfer.
• Make sure the plunger moves straight in and out, square to the rotor face, not at an angle. A tilted plunger reads smaller than the true movement.
• Push the indicator in until the needle has moved about a quarter turn. This preload lets the needle travel both ways as the rotor moves.
• Use the fine-adjustment knob and the rotating bezel to set the dial to zero.
• Turn the rotor slowly by hand through one full revolution. Watch the needle.
• Note the highest and lowest readings. Their difference is your lateral runout.
• Repeat on the other side of the rotor, then compare the numbers with the limit in your manual.

Other runout checks on a car

The same method works on other rotating parts. Only the mounting point and the surface you touch change. Always zero the dial with some preload, and turn the part by hand rather than with the engine.

If you have a base with a V-groove or a rotating arm, you may find it easier to reach tight spots. Our dial indicator comparison notes which sets offer a long or 360 degree arm.

Reading the result and what to do next

Every vehicle has its own runout limit, and it is usually listed in the service manual for that model. This applies to Ford vehicles as well as any other make, so look up the figure for your exact year and model instead of relying on a general number from a forum.

If the reading is over the limit, take the rotor off, clean the hub again and refit it. You can also try moving the rotor one bolt position at a time and measuring again. If the runout follows the rotor, the rotor is the problem. If it stays at the same spot on the hub, the hub face is the likely cause. A hub that is out of spec usually needs repair or replacement rather than a quick fix.

Write down each reading as you go. A simple note of position, reading and any changes you made makes it much easier to see what helped.

A note on shaft alignment

Shaft alignment on machinery, such as a pump and motor pair, uses the same tool in a different way. You mount the indicator on one shaft or coupling half and rotate both while reading the other. That job calls for the maker's procedure and usually a stiffer setup. For car work, runout checks like the ones above are the common use.

Safety notes

Brake parts need care. Never blow brake dust off with compressed air, because the dust can be harmful to breathe. Use brake cleaner and a rag instead, and wear a dust mask if you are working in a dusty area.

Wear safety glasses whenever you work under a car or around spinning parts. Keep fingers and loose clothing away from the hub while you turn it. Do not start the engine or put the car in gear while an indicator is mounted near a moving part.

Magnetic bases can pinch skin when they snap onto steel, so switch them on with your hands clear. Keep them away from pacemakers and electronics as well.

Frequently asked questions

How much preload should I use on the plunger?

A quarter turn of the needle or a little more is enough for most jobs. The goal is to keep the needle moving freely in both directions during a full turn.

Can I measure runout with the wheel still on the car?

You can measure the tire or wheel, but that is a different check. For brake rotor runout, remove the wheel so you can touch the rotor face directly.

Why does my reading change every time I turn the rotor?

Common causes are a loose base, a tilted plunger, rust under the rotor or uneven lug torque. Recheck each of those before you trust the number.

Does a longer arm give more accurate readings?

Not by itself. A long arm helps you reach the spot, but it can flex. Keep the arm as short and stiff as the job allows.