There are three main types of calipers. The vernier type (top) is usually the least expensive but most difficult to read, the dial calipers (middle) are the most common and have the advantage of not requiring a battery, and the digital (bottom) is the easiest to read but does require a battery.
Precision measuring tools have an important role to play in quality maintenance, just as they do in quality construction. Precision means that the instrument is capable of measuring accurately to .001 inch or less. This leaves out tools such as the machinist’s rule. For this article we will take a look at calipers, micrometers, dial indicators, and the bore gauge. To get precise measurements with all of these tools, the instrument must be in good condition and it must be properly used.
Accuracy
The accuracy you need for any job depends on what you are measuring. Checking the diameter of a bolt obviously doesn’t require great accuracy, certainly +/- .001 inch is fine. It is the same for measuring the thickness of a piece of aluminum sheet metal. You rarely care if the thickness of a sheet of .025-inch-thick metal is .02495 or .02505. Thus, in most cases a tool that can accurately measure to within .001 inch is all you need. However, if you are measuring the diameter of a crankshaft journal, accuracy to .0001 inch is needed to do quality work. Or if you are measuring a cylinder bore as part of an overhaul, you may be looking for such precision.
It is one thing to have a tool that will measure to .001 inch. It is another thing to actually know that it is that accurate. For that you will need some gauge blocks. These are precision ground steel blocks that have been measured to extremely fine tolerances. They come in various sizes and grades, and are priced accordingly. A reasonably priced set that is plenty accurate enough for our needs can be purchased from Amazon for about $70. They even have a 1x2x3-inch block accurate to +/-.0002 inch for about $16, which will meet the needs of most aviation maintenance technicians. The point is to pay for only what you need, but get something that is accurate enough and in the size range you will be using. For example, if most of your measurements are one inch or less, a one-inch gauge block is what you need.
Once you have some gauge blocks, check your measuring tools to make sure they are reading correctly. If they are not, you will need to adjust them if possible, get a new tool, or at least note that it is not suitable for precise measurements. Proper care and handling of precision tools will keep them in good working order for years, but a drop onto a concrete floor can easily knock a micrometer or set of calipers out of tolerance permanently. The best practice is to check the accuracy of your tool before every use if high precision is required.
Positioning the measuring tool correctly will make for precise measurements, but a poorly positioned tool will yield worthless numbers. It is very important to hold the tool exactly square with the object being measured. When measuring the outside diameter of larger round objects, positioning the tool so it is not only square but at the exact largest diameter point is crucial. The tool should slide all the way to the largest point without being too loose. Similarly, when measuring a large diameter hole, it is vital to be sure you are measuring the largest possible diameter. Move the tool around to be sure you are at the correct point, then double-check to be sure you are square with the object being measured.
Parts of the caliper: (1) lower jaws for outside measurements, (2) upper jaws for inside measurements, (3) scale, (4) dial with indicator needle, (5) bezel, (6) clamp screw, (7) beam, (8) depth rod.
Calipers
Calipers are probably the most common precision measuring tools used by amateur airplane builders. They are very handy for verifying the diameter of a bolt or the thickness of a piece of metal. They are also handy to check the thickness of brake pads. They come in two basic styles—dial and vernier. Dial calipers are almost always the preferred choice because they are easier to read. But the budget-minded builder may use less expensive vernier calipers and still achieve the same level of accuracy.
The depth rod of a dial caliper is used to check the thickness of a brake pad. This new pad measures .240 inches thick. When it gets down to .125 inch it will need to be replaced.
Note that the larger jaws of calipers are best suited for measuring outside diameters, whereas the smaller points opposite the large jaws are preferred for measuring inside diameters. Calipers are particularly handy for measuring the diameter of small drills. Often the size marking on a small drill will be missing or very difficult to read. Calipers can tell you the diameter in a few seconds.
You can use the depth gauge feature of the calipers to measure the thickness of a brake pad to see if there is enough material left to go to the next scheduled inspection. Extend the depth gauge beyond the expected dimension and position the end against the brake caliper body. Then slide the calipers down to meet the surface of the brake pad, all the time keeping the calipers square with the face of the brake pad. When the calipers bottom out, read the thickness of the pad. Then check the manufacturer’s recommended thickness tolerance to decide whether or not the pad needs to be replaced. The depth gauge feature is also handy to measure the length of a bolt.
Gauge blocks are an important part of your precision measuring toolbox. Here a dial caliper is checked with a one-inch gauge block. Always check your measuring tools before using them for precise measurements. A drop onto a hard surface can throw them out of adjustment.
Micrometers
A micrometer is the best tool for measuring the wear on a brake rotor. It can be done with calipers, but the micrometer will allow you to get to the point of least thickness if it is not on the edge of the rotor, where it seldom is. The shortcoming of micrometers is that they only measure a range of one inch. Thus, if you have some items to measure that are less than an inch thick, you will need a zero-to-one-inch micrometer. If you then need to measure something that is, say, 1.5 inches in diameter, you will need another micrometer that covers that size range.
Micrometers come in sizes with one-inch increments. To measure the diameter or thickness of something that is about 2.5 inches, you will need a two- to three-inch micrometer. Shown here are zero- to one-inch and two- to three-inch micrometers.
Where a micrometer shines is in measuring the diameter or thickness of something to great precision. Many micrometers are calibrated to .0001 inch (one ten-thousandth of an inch). This kind of precision is needed to evaluate the condition of internal engine parts such as crankshaft journals. Calipers that measure to .001 inch are not adequate for this task. A micrometer can also be used to set up a bore gauge, which we will address shortly. For example, the diameter of the main crankshaft journal on a Lycoming O-360 engine is 2.375 inches. Acceptable dimensions range from 2.3745 to 2.376 inch. It takes a precision instrument used with care to take such precise measurements. This is what a micrometer is made for.
Every time you service the brakes, usually at condition inspection, be sure to check the thickness of the rotors. A micrometer works best for this because it can more easily measure worn spots in the center of the rotor
To take such a measurement, you will need a two- to three-inch micrometer graduated in ten-thousandths of an inch (.0001). When closing the micrometer onto the journal, be sure to use the small knob on the end of the micrometer to avoid overtightening the instrument and thus making the reading inaccurate. Check more than one place on the journal to make sure it is not out of round or varying from one point to the next.
Vernier Scales
The most basic calipers and micrometers use vernier scales. They can be a bit difficult to read, but with practice and a logical approach, they can be mastered (see Figure 1 for an example).
Figure 1: Some calipers and micrometers have vernier scales rather than dials. These tend to be less expensive, but they can still be quite accurate. Here is how to read this instrument: The zero on the bottom jaw is the pointer (blue arrow). Note that it is past the 1.4-inch mark and also past the first small .025-inch mark. Thus the dimension must be at least 1.425 inch. To see how much past, we go to the vernier scale and see which mark lines up best with the larger scale. In this case it looks like the 16 (red arrow) lines up best, so we add .016 to 1.425 to get the exact measurement of 1.441 inch.
From the scale we can see that the measurement shown is something over 1.4 inches, since the large divisions are obviously one inch with 1/10-inch increments in between. These divisions are divided further into .025-inch increments. Since the zero indicator on the moveable jaw is past the first .025 mark, we have a measurement that is at least 1.425 inch. Now looking at the vernier scale, we see that the 16 line best lines up with a mark on the main scale, so we add 16/1000th to the 1.425 we have from before. That gives us a total measurement of 1.441 inch. Vernier micrometer scales work in a similar manner, but the vernier scale is usually used to determine .0001-inch increments.
Another good example of how to use a tool with a vernier scale is a “Hints for Homebuilders” video available from EAA.
Dial Indicators
Dial indicators always need to be attached to some sort of mount, depending on the measuring task at hand. Many times a magnetic base will do the job, but at other times a custom mount will be needed. The trick is to get a mount that is steady and solid so there is no deflection in the mount as measurements are being taken. This can require real creativity in some cases.
Dial indicators generally come in two types—those that measure small deflections and those that can handle a larger range, sometimes up to several inches. If there is concern about a bent crankshaft after a prop strike, a small displacement dial indicator can be set up to measure the runout of the crankshaft flange. While this alone is not a sufficient test for verifying the condition of an engine after such a mishap, it can give you a quick indication of how much damage was done to the straightness of the crankshaft.
Here a dial indicator mounted to a magnetic base is used to measure valve wobble in a Lycoming cylinder. Typically this test is performed with the cylinder on the engine. Lycoming Service Bulletin 388C describes the test in detail.
Another use of a dial indicator is to measure the wobble of the valve stems in a Lycoming engine. Service Bulletin 338 C shows an alternate method of measuring valve clearance using a dial indicator. Of course, you can make your own mount instead of buying Lycoming’s tool.
Dial indicators also have many uses for machining processes, but that is beyond the scope of ordinary airplane maintenance.
Bore Gauge
The bore gauge is designed to measure the inside diameter of cylinders or other round objects. It is typically used in conjunction with a micrometer, which is needed to set it up. It is designed to measure small deviations from a preset dimension, for example the recommended inside diameter of an engine cylinder. The scale is usually graduated in .0001 inch. The nominal dimension is set on the bore gauge, and then any deviation from that dimension will be shown on the bore gauge as a variation from that dimension.
A Jabiru cylinder gets checked with a bore gauge to detect wear. The bore gauge is zeroed at a nominal dimension. The dial then reads deviation from that nominal dimension. Some care must be taken to keep the bore gauge square with the walls of the cylinder to get an accurate reading.
In the case of a cylinder for a Lycoming O-360 engine, the factory only lists a maximum dimension of 5.1305 inches. You would thus set up the bore gauge to that dimension and reject the cylinder if at any point the maximum dimension was exceeded. Of course, the cylinder could potentially be bored out to an oversized dimension and overhauled as per Lycoming standards if it was otherwise in good condition. Needless to say, you would need a five- to six-inch micrometer to set up the bore gauge and a five- or six-inch gauge block to verify its accuracy. If you needed to make such measurements often, you could consider having a ring machined to an inside diameter of 5.1305 inches and use it to set up your bore gauge.
The bore gauge is a little tricky to use because you must be sure you are exactly square with the bore every time you take a reading. It works best to gently rock the gauge back and forth until you get the minimum reading. Then check another spot the same way. Check the top, middle, and bottom of each cylinder, and then take some more measurements with the gauge perpendicular to the earlier readings. That way you can tell if the bore is out of round.
Unless you do serious engine work, you may never use a bore gauge. Here is a bore gauge set with different length rods for different sized holes.
Digital Measuring Tools
Just as with everything else, electronic digital measuring tools are becoming more popular. They have the great advantage of being easy to read, especially compared to vernier instruments. On the other hand, as a corollary of Murphy’s Law, the batteries that power these tools are bound to fail at the worst possible time. To avoid this, you may want to hang onto the old-school mechanical measuring tools…or you could just keep some extra batteries on hand.
As builders, many of you probably invested in a good set of calipers and perhaps a micrometer. To that you should add gauge blocks appropriate for the sizes of things you are likely to measure. Then take care of these tools to keep them accurate. Other precision tools may be acquired as they are needed. With some good care these tools can last a lifetime.
