Precision Measuring For The At-Home Mechanic // Part 1
The world of measuring is so complex by nature that one could get wrapped up writing an entire book on the subject and still not cover it all. This is not my intention with my engine rebuilding book. My aim is to provide you with the principles on how measuring works, what the most important takeaways are on the subject of measuring, and an overview of how to use the tools correctly. Once informed you can then delve further into the intricacies of measuring for your own needs.
There are three terms that are important to the fundamental understanding of measurement. These terms are often mixed up, confused as meaning the same things, or used incorrectly. These terms are accuracy, precision, and resolution. Understanding these three terms will go a long way in ultimately understanding measuring and the capabilities of measurement tools.
Accuracy is how close a given measurement is to the “true” value of an object. For example, if a valve stem was exactly 0.1969” (5.000mm), accuracy would quantify how close the measurement tool was to the true value.
Precision is a measurement of repeatability. For example if an object was measured five times, precision would quantify how close the five measurements are to one another. Another way to think of precision is the finiteness of which a measurement tool can be read repeatedly and reliably.
Resolution is the smallest distinguishable value of a measurement tool. If a ruler is divided up into tenths of an inch then the resolution of the ruler is one tenth of an inch. A micrometer that can be read to one ten thousandth has a resolution of one ten thousandth of an inch. Just because a measurement tool, such as a micrometer, has a very fine resolution doesn’t mean it will be accurate or precise to that resolution. This will be explained more shortly.
Distinguishing the difference between accuracy and precision is most easily done with a set of pictures. Four scenarios can occur when measuring.
- A measurement can be both accurate and precise.
- A measurement can be accurate but not precise.
- A measurement can be precise but not accurate.
- A measurement can be neither accurate nor precise.
Precise and Accurate
Accurate but not Precise
Precise but not Accurate
Neither Precise nor Accurate
Now let's see what happens when the bullseye is taken out of the picture:
Notice that if the bullseye is taken out of the picture there would be no way to reference if the shots were accurate? It would be easy to tell that the “accurate but not precise” and “neither precise nor accurate” scenarios didn’t yield meaningful results. However, the scenario with “precise but not accurate” results could be misleading for someone measuring. This is exactly why it is important to calibrate measurement tools before using them. By calibrating a measurement tool you ensure it is accurate prior to measuring an unknown object.
What Affects Accuracy and Precision?
Now that the key parts of accuracy, precision, and resolution have been outlined how do they apply to measuring tools and measuring? I want to go over three factors that lead to variation in accuracy and precision when working with measurement tools.
1. Type of Tool and Tool Quality
Different types of measurement tools will have different ranges of accuracy. For example a digital 0-6” calipers is usually accurate to 0.001” (0.025mm), however it will have a resolution of 0.0005” (0.0127mm). Just because the resolution is 0.0005” (0.0127mm) doesn’t mean that is how accurate the tool is. For a 0-1” micrometer the accuracy is 0.0001” (0.0025mm) and so is the resolution. Be sure to keep accuracy and resolution in mind when using and shopping for measurement tools. Decent measurement tools should have the manufacturer’s accuracy specified in the tool description.
Using a calipers to measure the bore of a cylinder is a good example of using a measurement tool which is not accurate enough for the task at hand. Most dirt bike cylinder bores have a diametric range in the neighborhood of 0.0006” (0.0152mm), taper limit of around 0.0004” (0.0102mm), and out of round limit of around 0.0004” (0.0102mm). A calipers is off an order of magnitude in accuracy and is not capable of doing the job.
The temperature a measurement is taken at can have a large effect. A standard has been set for the temperature parts are measured and inspected to in the engineering and metrology worlds. That standard temperature is 68°F (20°C), which is what you should strive for when you are precision measuring parts.
The reason temperature is important is due to the concept of thermal expansion. In a nutshell thermal expansion explains how an object’s volume will change as temperature changes. As volume changes so does length, which is what matters in this case. Equations for linear expansion will be used to show the role temperature plays on the diameter of a cylinder bore at two extremes: 32°F (0°C) and 100°F (37.78°C).
Consider a cylinder that has a bore of 3.7795” (96.000mm) - typical of a 450cc engine. At 68°F we will say the cylinder measures exactly 3.7795” (96.000mm). What happens when the same cylinder is measured at 32°F (0°C) and at 100°F (37.78°C)?
Let’s work it out. The formula for linear expansion is:
∆L = α x D x ∆T
∆L = Change in diameter of the cylinder bore
α = coefficient of thermal expansion for aluminum (13.3 x 10^-6) in/in °F
D = original diameter of the cylinder
∆T = Change in temperature (Final Temperature - Initial Temperature)
α = (13.3 x 10^-6) in/in °F
D = 3.7795”
∆T = 32°F - 68°F = -36°F
Now it all gets put into the equation and solved.
∆L = (13.3 x 10^-6) in/in °F x 3.7795” x -36°F = -0.0018”
∆L = -0.0018” (0.046mm)
So the 3.7795 inch cylinder has now shrunk to 3.7777 inches (95.954mm).
A 36°F (20°C) change in temperature has caused the aluminum cylinder bore to change by 0.0018” (0.046mm)! In terms of cylinder measurements this is a big change and illustrates just how important it is to measure engine components in the right environment.
A similar change can be seen when the cylinder is measured at 100°F (37.78°C).
α = (13.3 x 10^-6) in/in °F
D = 3.7795”
∆T = 100°F - 68°F = 32°F
Now it all gets put into the equation and solved.
∆L = (13.3 x 10^-6) in/in °F x 3.7795” x 32°F = 0.0016”
∆L = 0.0016” (0.041mm)
So the 3.7795 inch cylinder has now grown to 3.7811 inches (96.041mm) .
In conclusion if you work in environments that are at one extreme or another on the temperature spectrum you are guaranteed inaccurate measurements. The scenario where measurements are precise but not accurate would be a good illustration for how temperature affects measuring.
3. User Error
Lastly, the person doing the measuring also has an effect on the precision of the measurement. Another example is the most effective way to illustrate my point. Consider a situation where an inexperienced measurer measures a part five times and returns five different measurements. Next, a seasoned measurer measures the same part five times and returns the exact same measurement all five times. Both people measuring used the same measurement tool and carried out the measurements at the same temperature. The only variable that wasn’t the same was the person doing the measuring. This variation of measurement between people measuring isn’t that uncommon and happens all the time. Even two different seasoned professionals who inspect and measure parts daily can end up with different measurements for the same part. It is very likely that the variations in the professionals’ measurements will be much more precise when compared to one another.
How do I know my Measurements are Accurate and Precise?
Alright we’re starting to get into the more interesting - I mean practical, aspects of measuring. Hopefully the last few sections haven’t bored you or deterred you from wanting to measure your own engine components. I assure you, with practice and patience you can become a well versed measuring machine!
I want to touch on some practical ways to determine if measurement tools are working correctly. If you went out and bought a cheap or expensive set of micrometers with measurement capabilities ranging from 0-6” how would you know they are accurate right out of the box? Is the manufacturer responsible for insuring they are accurate? Do they ever lose accuracy? Does the fact they were either cheap or expensive matter? The answer to all these questions is that prior to use, regardless of price or quality, most measurement tools will require calibration.
Calibration is the practice of checking or setting the accuracy of a measurement tool to a known value. For measurement tools such as calipers and 0-1” micrometers simply ensuring the tips of the tool are clean, closing them together, and making sure the tool reads zero may be all that is necessary. This is a fairly easy method of calibration for these two tools, but isn’t possible for measurement tools where the tips don’t close together all the way (ex. 2-3” micrometer) or the tool has tips that extend outwards (ex. dial bore gauge). For these situations measurement gages are necessary for accurate calibration.
Calibration gages (sometimes called standards) come in several varieties depending on tool application. The most common and applicable to engine inspection are gage blocks and ring setting gages. Gage blocks are small blocks of steel (more expensive variants come in other materials) which have been finished to extremely tight tolerances. Gage blocks are toleranced into several different classes. For engine inspection the most accurate measurements that must be made are to 0.0001”(0.025mm). In the measuring world the rule of thumb for calibrating tools is to use a standard which is at the minimum four times as accurate as the tool. So for a micrometer which is accurate to 0.0001” a gage block with an accuracy of at least +/- 0.000025” should be used.
What the measuring world does and what the at home engine builder can do feasibly are two different things. For the majority of us our measuring abilities will get in the way of our accuracy long before the gage block used to calibrate the tool has any effect. For this reason I would suggest that using the standards which come with the tools will be fine and not splitting hairs over not knowing the exact accuracy of the standards.
If you are shopping for a 0-6” set of micrometers most decent sets will come with standards blocks. The standards will usually come in 1, 2, 3, 4, and 5 inch sizes so all the micrometers can be calibrated at any time.
Ring setting gages are similar to gage blocks and are also used for calibrating measurement tools. They are, as the name suggests, rings that have been machined to very fine tolerances. Usually instead of having a tolerance range that the ring falls into the ring will be stamped with the exact diameter it was machined to. A measurement tool such as a dial bore gauge is then calibrated to the exact diameter of the ring setting gauge. Ring setting gages are generally quite expensive as they are challenging to make accurately.
Okay, I Understand the Importance of Calibrating My Measurement Tools. What About Making Precise and Repeatable Measurements?
Precision gets complicated pretty quick because you have to factor in the measurement tool, temperature, and user taking the measurements. These three variables are difficult to separate completely. However, out of the three variables the user is usually the most likely variable to have a large influence on the precision of the measurement tool. In order to make precise and repeatable measurements it is important to do as many things the same as possible. Here are some things I recommend doing to make sure your measurements are as precise as possible.
1. Make sure the temperature in the room you are measuring is at 68°F (20°C).
2. Try to perform measurements of a part or set of mating parts in a short time period. There’s no need to rush the measurements or to measure all the parts in one day. It’s not that critical but, for example don’t measure the diameter of the piston one day and then wait to measure the bore of the cylinder. That doesn’t make sense.
3. When using micrometers use a micrometer stand to secure the micrometer in place. Not only will this make positioning the part easier and the micrometer easier to read it will also keep the heat of your hands from warming the micrometer. Remember thermal expansion? Believe it or not there are actually studies out there detailing how body heat makes a micrometer expand in length. It’s a minuscule amount but nonetheless worth mentioning.
4. A lot of measurement tools are operated by feel. When working with these tools try to be as consistent as possible when turning the handles. The amount of pressure you apply can have a big effect on the final measurement. For example, the difference between a part that drags hard through the tips of the tool versus one that drags but is soft in feel could be several ten thousandths of an inch.
5. Use your fingertips and a light grip. The fingertips are one of the most sensitive parts of the human body. As such they can be utilized to feel subtle variations in measurement.
6. Instead of just taking one measurement take 3-5. This is something you should definitely do when calibrating your measurement tools. By taking multiple measurements you’ll quickly get a feel for how precise your measuring is. If you are all over the board there is a good chance your technique is inconsistent. If you are within a ten thousandth or two each time you are on to something. For important measurements like cylinder bore diameter, taper, and out of roundness take 3-5 measurements. Then take the average of these measurements and use the average as your final measurement.
7. If you are struggling to determine if your measurement tools are working properly compare them to another known good set. If the two sets are not reading close to identical there is probably a problem with the unknown set.
8. Compare your results to those of someone with a lot of measuring experience. If a seasoned machinist can get your measurement tool to repeat to a ten thousandth of an inch it is probably not the tool. If you have the opportunity to get help from a machinist or someone fluent with measuring watch them carefully. Ask them questions, study how they work the tools, and learn from them.
9. Be patient and take your time. Rushing the measurement process is not a good idea and can lead to silly mistakes. You need to be in a state of mind where you don’t feel rushed and don’t mind taking the time to do a thorough job.
10. Write your results down! Write down everything clearly from the calibration measurements, any calculated averages, and measurements of specific parts. By writing things down you can easily work backwards to see if a mistake has been made somewhere.
By keeping in mind accuracy, precision, and resolution you are well on your way to precision measuring like a pro. As I stated earlier, this takes a hefty amount of practice and patience.
In my next post Precision Measuring For The At-Home Mechanic // Part Two, I will be discussing in complete detail how to properly use specific measuring tools. There is a definite finesse to using each of these tools as you rebuild your engine, and as you just learned in this post there are many factors that contribute to a precise measurement. Part Two will be posted to the DIY Moto Fix Website next week and you can have it sent right to your email inbox by subscribing to my eNewsletter by clicking below.
Subscribe To The Moto Fix eNewsletter
Thanks for reading and happy wrenching!
Moto Mind - Empowering and Educating Riders from Garage to Trail