Dial calipers are also available...
Dial calipers are also available in much larger sizes, with some measuring lengths up to 24 inches. This particular caliper will measure lengths just over 12 inches.
For measuring ODs and shim lengths (within reason, 1 foot and under) and recording general measurements, you should have a set of 6-inch dial calipers. This tool will provide a fast, easy-to-read measurement and it has resolution to the third decimal place. This is probably one of the most versatile measuring tools you can get and it is very reasonable to purchase, ranging in price from $20 to $100 each. In my experience, the units that cost more don't measure any better or with any greater accuracy. A set of "C" micrometers, or "C" mics, will handle a broad range of measurement jobs that would call for a higher level of resolution and accuracy. For the racer, a set of three micrometers with ranges of 0-1, 0-2, and 0-3 inches is probably more than you will ever need. These can be purchased for about $40 to $55 each, and you might even be able to purchase a pre-bundled set for even less per micrometer.
When measuring a drill bit,...
When measuring a drill bit, many new racers and fabricators find out through trial and error that the cutting end is larger than the shank. In the first of this sequence, the measurement comes out to 0.3395 and the diameter on the shank is 0.3365-a difference of 0.003.
Another handy tool to have is a simple dial indicator for measuring surface parallelism on parts that move, such as the discs on your brake system, or measuring endplay on a gearshaft or crankshaft. The dial indicator is reasonably inexpensive, but additional tooling is required to hold the indicator while the measurement is taking place. The cost for the indicator and the associated tooling will be in the $60-to-$100 range.
Measuring into holes to check depth is the job of a depth micrometer. You can also use the tail of a 6-inch caliper to measure hole depths in a pinch, but you will need either a set of depth micrometers or a drop micrometer for a truly accurate measurement. A good depth micrometer can be purchased in a set with up to six rods that are interchangeable and will give you a range of measurement capability from 0 to 1 inch all the way to 0 to 6 inches in depth. The cost ranges from $60 to well over $150.
Not all drill bits have a...
Not all drill bits have a size difference like this, but every standard drill bit is larger on the cutting end than the shank. When drilling a hole that is size critical, this needs to be remembered.
Measuring inside diameters brings up a completely new set of problems. When dealing with holes, there is a larger group of parameters that may or may not be of concern to us, such as roundness, taper, concentricity, and placement in relation to other holes. If all we want to know is the diameter from a general perspective, our set of 6-inch calipers can measure an inside diameter, but it can only tell you the size of the hole from edge to edge. This is fine if you are measuring only to determine the drill bit size required to drill another hole or if you need to determine what bolt size will fit.
In a situation requiring more precision, if we need to check the circularity and the size of the hole, we will use a dial bore. To save money, we could use a set of snap gauges and a dial caliper or a set of "C" mics to determine the hole size and circularity. But this method has some real drawbacks. It takes a long time and the results are dependent on operator skill, not just the robustness of the process. If we are measuring a cylinder bore for a Nextel Cup engine, this would not be the method of choice. To measure this type of ID, you really need a dial-bore gauge because it will tell you how round a bore is as well as the size.
A dial-depth micrometer measures...
A dial-depth micrometer measures parts that can or will be moving. This type of micrometer is great for measuring cam lift. Notice the adapters for holding the micrometer. This includes a magnetic base and some adjusters for making fine adjustments to the setup. With the proper holding tools, it's a very versatile measuring tool.
The problem with a dial bore is that a master dimension is required to set the tool. Usually, this is accomplished with a set of Johannson Blocks, or Jo Blocks. Jo Blocks are ultra-precision steel blocks that are ground flat on all sides, marked for their individual dimensions, and assembled together to establish a known dimension. Once this dimension is set, you then set your dial bore within the confines of the Jo Blocks to establish the "zero point" for your dial bore. Once this is accomplished, the dial bore can be placed in the bore, and any deviation (plus or minus the zero point) gives you the size and the circularity of the hole you are measuring. This works very well for engine blocks and measuring any bore on the engine, cylinder, lifter, main bearing, and cam bearing bores. A dial bore is an expensive tool and even more so when you consider the cost of a set of Jo Blocks. You can spend upwards of $500 very quickly. There are alternatives to Jo Blocks, but they never seem to be as repeatable or as reliable. You can use a "C" micrometer to set a dial bore, but it requires a good deal of experience and isn't the best method.
Using measurement tools is not difficult, but it requires some time to learn the process so you get accurate measurements. Another major consideration is selecting the correct tool for the job. This will make sure you are not trying to use a tool that will give you greater measurement resolution than you really need.
The author can be reached at vahok.hill@cox.net.