Dial indicators are one of the primary measuring tools used in precision engine building. They are typically used to measure deck clearances, crankshaft thrust and straightness, lifter travel and other measurements that involve the distance between two surfaces or small amounts of component travel. The most commonly used type of dial indicator reads in .001-inch increments over a range of one inch. For everything except cam checking, many builders use an indicator with only .250-inch total travel which is sufficient for most precision measurements.
Dial indicator face with adjustable scale
Standard dial with .001-inch marks
The dial indicator uses a calibrated stem that transfers linear movement to an easy to read dial face. Most dial faces are graduated in .001-inch hash marks so that one full revolution of the indicator needle equals one tenth of an inch or .100-inch travel. A much smaller calibrated dial incorporated into the primary dial face indicates hundredths or .100-inch travel. It has markings for every tenth of an inch so you don’t have to visually count the number of full revolutions if your measurement is larger than one full rotation or .100-inch. Sometimes there are exceptions. The accompanying photo (click to enlarge) shows a dial indicator on one of ARP’s rod bolt stretch gauges. Note the nomenclature on this specialty dial is different than a standard gauge. It indicates a range of 0 to 1 inch and, more importantly, it specifies 0.0005 inch between each hash mark. So the 5 equals .005-inch, the 10, .010inch and so on. This gauge has not been zeroed, but if it had been the indicated travel is showing just a tick over .003-inch. Most gauges indicate .100-inch of travel per full revolution. This one only indicates .050-inch per revolution and because it is a rod bolt stretch gauge, almost all of your reading will fall between the 5 (.005-inch) and 10 (.010-inch). With this gauge do not make the mistake of thinking each hash mark equals .001-inch. Always note the spec on the gauge
face to be certain you read the hash marks correctly.
Dial Indicator magnetic mounting base
Deck Bridge equipped with dial indicator
Accurate use of a dial indicator requires rigid mounting and absolute stability. If the indicator moves all while the measurement is being taken, the measurement is compromised. Hence dial indicators are almost always used with an adjustable magnetic base that can be clamped firmly to a nearby surface. The magnetic base can be adjusted to position the dial indicator so that the stem can read movement in the same direction as the part being measured. Depending on how you position the indicator, it can read travel in both directions. Position the indicator so the stem is parallel to the direction of component motion. Then adjust it so the stem is positioned about midway in its range of travel against the part being measure. The most important part now is to zero the indicator prior to every measurement. You do this by rotating the outer dial to position the zero mark directly opposite the needle regardless of where it is pointing. Once the indicator is zeroed you can take the measurement by moving the part in question. The indicator will show movement either positive or negative.
With the exception of lifter travel and total valve lift, most measurements you take will be less than .050-inch. That’s why many builders only use an indicator that reads .250-inch or less. For instructions on taking piston deck measurement and other uses for a dial indicator see How to Use a Deck Bridge. For unusual applications you can also purchase an inexpensive kit with various extensions for measuring hard to reach components. Dial indicators take precision measurements, but some builders just toss them in the tool box with all the other tools. That’s hardly the way to ensure accuracy. If you’re a serious builder, chances are you will own more than one dial indicator. For consistent and reliable results, keep them clean and store them in their own cabinet or a separate tool box with other precision instruments.