Good technicians learn early, to find problems often requires a closer look than the senses provide. Tools and equipment provide this insight. They reveal things we cannot know without their use. One tool, every serious technician will need in their arsenal, is a good dial indicator.
What size dial indicator do I need?
Experienced mechanics may own several styles of indicators over their careers. Many are handy in specific situations, but limited outside their intended use. For general use, a plunger-type indicator, with a back mounting, is a great addition to the tool collection.
A dial indicator is a precision measuring device. They are available in a vast array of ranges and sizes. For automotive use, accuracies of one-thousandth of an inch (.001") is best. More precise instruments are far more fragile and will not find many uses. Lower resolution will not provide the accuracy needed.
The travel an indicator plunger can measure, is the range. Avoid instruments with a long plunger. Longer shafts are very prone to damage and are not useful. A range of one-quarter to one-half inch is best. Things more than 1/4 inch rarely needs measurement, as they are apparent.
Also needed will be a base, to hold the dial indicator. Many specialty bases are available, each with an intended purpose. For general automotive purposes, a magnetic base, with an upright and crossbar will serve most needs. These bases are widely adaptable to most situations.
Buying a name-brand indicator and base is a large plus for serious users. As experience increases, more uses become apparent. Name-brand indicators offer a wide array of accessories, that all interchange, and make the tool far more versatile. I have Starrett brand tools, I have owned for more than 45 years. Accessories for the less expensive tools may not interchange. If the company goes out of business accessories may not be available.
What can I check with a dial indicator?
Mechanics use dial indicators for a wide range of purposes. Very common is the measurement of run out. Run out is variation in the axis of travel of a rotating object. When run out is excessive, vibration often occurs.
Run out may exist in more than one plane. For instance a tire may have a radial run out, which is up and down movement. The same tire could also have a lateral run out, which is side to side in nature. Measuring the run out of a tire and wheel is a common use of a dial indicator. A special tip, with a tiny wheel makes the job easier, but is not mandatory.
Run out is a defect in a product such as a tire. It is surprising to see how much run out may exist. Any amount more than .030" is excessive in a tire. Improper installation may even ruin good tires. Vibration that persists, even after balancing, often results from run out. Balanced and being round is not the same thing. Out of round tires will vibrate, and cause damage, even if perfectly balanced.
Run out can also result from an impact or improper service. A persistent brake shudder is often a result of a damaged hub. Rotors mount to the face of the hub and run out will cause the rotor to wobble. A dial indicator can easily detect this run out with the setup above. Lateral run out greater than one-thousandth of an inch is usually a problem and requires replacement of the hub.
Finding mystery problems
Prying the old brake drums off, easily bends drum brake backing-plates. This can cause irregular braking, drum damage and lockup. Finding a bent backing plate is very difficult without an indicator. With the above setup, rotate the hub or axle. The dial indicator reveals run out of the backing plate. Any amount above five-thousandths of an inch is normally a problem.
Detecting a bent drive shaft is easy with a dial indicator. Test the shaft at each end and in the center, by hand rotating. Run out of more than ten-thousandths of an inch will be a problem and less is better. Drive shaft damage may occur when replacing universal joints and sometimes by running over things. A bent drive shaft most often shakes the whole vehicle, usually around 50 to 55 MPH. Accelerating and decelerating may also cause the vibration to change.
Uses for a dial indicator are almost unlimited. With this set, the run out of a bell housing is checked. Bell housings are sometimes manufactured poorly, or can sustain damage. The locating pins sometime wear the holes in the soft aluminum. Damage to the flexplate and repeated transmission seal failure will occur.
Checking end play
Setting wheel bearing end play is another very useful function. Adjustable wheel bearings must have clearance, to allow for thermal expansion of the rollers. Without adequate end play, the lubricant has no space and the bearing fails. With the magnetic base on the rotor, the indicator tip contacts the spindle. Pulling in and out on the rotor reveals the movement in the bearing.
Obtaining and learning to use a dial indicator is a primary step in becoming a better technician. This one tool can increase diagnostic ability and reveal a new world of variation.