When Does One-Thousandth Of An Inch Equal a Mile?

When Does One-Thousandth Of An Inch Equal a Mile?

Dave Monyhan, Goodson National Sales ManagerBy Dave Monyhan
Goodson National Sales Manager

One thing I learned a long time ago was if you’re not measuring you’re just guessing.

Today’s tolerances are much closer than they have ever been and that is why exact measurement is necessary. Way back when we mostly had to deal with three places past the decimal and today we’re neck deep in going four places past the decimal. Why you might say, what has changed? Well the engines have changed significantly due to very lightweight components, different methods for sealing gaskets and in some cases no gasket at all. So you have had to up your game when it comes to machining and you have to up your game when it comes to measuring as well.

I was told a long time ago, “Always buy the best tools you can afford and if you can’t afford the best tools then either borrow them or wait until you can afford to purchase the best tools.” Now this statement has both good and bad in it. Good because we all know high quality tools cost money, and bad because as engine builders who are you going to borrow tools from? The guy next door, probably not! You need to have your own tools and tooling to control your quality in every engine job you send out the door. Now I think we can all agree tooling is less costly than buying machines, and even if you are buying a new machine you still need tools and tooling.

So what makes a tool the best tool versus the run of the mill one-use-wonders?

Quality in manufacture, quality in material, and most importantly….support from the company that manufactures or sells it to you. Even if you buy the best tools your hard earned money can afford, you still need the supplier or manufacture to be there for you when you need help. You need them to be able to answer your questions, you need them to be there to take care of any warranty, and you need them to be there to take care of you, the customer. Too many times in this business I hear horror stories of this company or that company that sold somebody a bill of goods and then dropped the ball after the sale. No support whatsoever. It’s too bad, especially in this day and age.

Ok I will get off that band wagon and get back to what I was going to write about this month. Measuring tools and tooling.

Measuring tools are the most critical tools in your shop. They must be of high quality, they must be calibrated, if not you will surely not read a critical dimension correctly and then that engine you just built will be blown up before it gets warmed up. Micrometers, dial bore gauges, setting fixtures, RA surface measuring tools, stem height gauges, dial indicators, calipers, run out gauges, hardness testers. All of these tools just have to be spot on accurate. This is one area you simply cannot compromise. The higher the quality resulting in the higher cost of measuring tools will in fact pay for themselves over time.

Ok, you got the best tools your money can buy, how you take care of them. I am going to focus on the two most popular measuring tools for this article. The micrometer and the dial bore gauge.

What is a Micrometer? The Micrometer is an instrument used for measuring extremely small distances.

How to use a micrometer:

Proper micrometer handling is essential in measuring success.The best start is to bring the micrometer close to the range you are going to measure. Do this by rolling the thimble along your hand or arm – do not ever and I mean never let me see you twirling a micrometer!

When placing the micrometer onto the part, hold it firmly in place with one hand. Close the micrometer using the ratchet until the spindle is nearly on the measured part. This usually can be determined visually. If you hit the part before expected, back off slightly and then slowly and gently close the spindle until the ratchet stop disengages one click.

Note that the procedure requires two hands. If the micrometer is handled with only one hand, the ratchet stop cannot be reached, and reliability will suffer. Some people purchase micrometers without ratchet stops. Don’t be one of them. The micrometer is a contact instrument. That means that there must be positive contact between the part and the micrometer. The amount of contact (all-important feel) is up to you, the machinist. When you are attempting to measure 0.0001” Insuring that the micrometer is repeating the same true reading time after time is critical and directly related to how much pressure you use each time. Because human beings hand pressure can vary so widely this can be a source of serious errors in measurement.

So, how do you know if your micrometer is reading true or may be out of calibration?

Anatomy of a micrometer:

Anatomy of a MicrometerUnderstanding the various components that make up a micrometer is the first step to understanding how to calibrate them. Let’s breakdown the micrometer so we know what all the parts are called and where they are located.

Most all high quality micrometers come with a spanner wrench and a standard. Find the ones that came with your micrometer or order the standard and spanner from your favorite shop supply company.

Ensure the micrometer reads 0 to 0 or you must recalibrateFirst step: measure something, and note the reading. Measure it again and compare the reading. Do they match? Make a note if they do not match. Next, rotate the thimble all the way down until the spindle makes contact with the anvil. Note the position of spindle. It is reading zero to zero? If not then it is time to calibrate. A real indication on whether your micrometer is calibrated or not is to simply measure something of a known size and if you constantly get a different reading, then your micrometer is off and needs calibrating. I will also suggest you practice on this known dimension. Measure this known dimensions several time to insure you have repeated readings on the same known dimension.

How to calibrate your micrometer:

All high quality micrometers can and will need to be calibrated. It is really quite a simple task to perform. First get a micrometer standard for the micrometer you wish to calibrate. In this example I will show the process of calibrating a 1” micrometer.

Calibrating your micrometerPlace the standard between the anvil and the spindle and rotate the spindle until it makes contact with the standard. In this case I am using a 1” standard (photo) the reading on the micrometer is supposed to be exactly 1”. In other words all the corresponding lines will line up perfectly if the micrometer is in fact calibrated. If they don’t, then take the spanner wrench and move the inner sleeve until in lines up perfectly with the lines (photo) 0027, 0028, and 0030

Lock down the adjustment nut securely and repeat rotating the spindle until it makes contact with the anvil. Does it now read zero to zero? If so you have done well young Jedi, if not go back through the procedure again until you get it right.

First and foremost keep your micrometers clean and lightly oiled. Always store your micrometers back in their cases or boxes when not in use. Don’t just toss them on the workbench when you are done. Even when you have the micrometer in use, don’t just lay it down on the workbench. Place a nice cotton towel down first and then place your micrometer on that cloth after you take a reading. Never and I mean never use any type of abrasive or emery to clean your micrometers. Remember to tell your employees that micrometers are not C-clamps. Be careful who you lend your micrometers to. Take time every month and confirm your micrometers are properly calibrated to insure they read accurately.

Sunnen Dial Bore GaugeSo what is a dial bore gauge?

The dial bore gauge (also spelled gage) is a tool that accurately measures the diameter of a given bore. Not a wild boar, or a bore at a cocktail party but a bore in the automotive world is any hole that has been drilled, reamed, machined or honed and are not just limited to just the cylinder bore of the engine, but also those other bores like the; lifter bore, valve guide bore, valve seat counter bore, cam bore and main bore.

Most bore gauges have a three point contact with the main contact being adjustable for varying dimensions. Keep in mind a dial bore gauge is a plus /minus measuring tool. This means when you have the dial bore gauge properly set you will be using it measure the consistency of the cylinder bore you just machined. Most will measure in 6 to 9 different parts of the cylinder bore. This details measurement process will actual tell you a story of whether the cylinder bore is not only on size, but is it also round and straight.

Dial bore gauges are even simple to calibrate.

MMCB-06 Mitutoyo Dial Bore GaugeWhen we say we are going to calibrate a dial bore gauge what we are actually meaning to say is we are setting the dial bore gauge. Dial bore gauges are designed to be used for a wide range of sizes. With each setting you have (X) amount of range from that setting. Most dial bore gauges in the automotive market have a 2” to 6” range. Of course there are other dial bore gauges that are available in various ranges from very small to very large. With all dial bore gauges you change anvils and spacers of certain sizes, according to the bore size you are measuring.

Setting the dial bore gauge is really pretty simple and direct:

First establish what the size is for the bore you are going to measure. I will choose the 4” bore as it the most common size for the Chevy small blocks we all work on. Remove the anvil locking nut and remove any anvils and spacers. Install the correct length anvil, and if needed use the .020” or .050” spacer to get in the range of the bore you are going to measure. (Show the anvils and spacers) The use of these washers will vary from brand to brand. Follow the guide or instructions supplied with your dial bore gauge. Install the corresponding anvil and spacer to get you to that size, in this case 4”. Tighten up the retaining nut. Now we can determine if we have set up the anvil and spacers correctly by using a micrometer or an actual setting fixture.

A dial bore gauge setting fixture is way more than a fancy micrometer. It’s easier to use, much more stable, resulting in more accurate readings and the novice can learn much faster how to succeed in setting up dial bore gauges, versus the balancing act required when just using a micrometer.

One of the best dial bore setting fixtures is made by Sunnen or you can also use what we just talked about……..a micrometer. Now if you are going to use a micrometer I highly recommend you get yourself the “third “hand. The third hand is a soft vise that holds your micrometer while you set your dial bore gauge. To see how to set your Dial Bore Gauge using the "third hand" technique, check out our blog post: Tips & Tricks for Setting a Dial Bore Gauge.

Using the setting fixture:

Sunnen Setting Fixture

Locate the correct standard for the bore size you are going to set. Place the standard into the setting fixture. Zero the spindle on the setting fixture.  Remove the standard, and place the dial bore gauge into the setting fixture.  Now you can rotate the spindle and adjust the dial indicator for zero on the bore gauge to match zero on the setting fixture. 

Now you are ready to measure the cylinder bore using your dial bore gauge as a plus or minus measuring tool. Measure the bore in X and then Y direction at the top, middle and bottom of each cylinder bore. Make sure you get as close to the bottom of the bore as possible for the lower measurement.  Be sure to write these dimensions down. Remember to hit no less than 6 to 9 positions from top to bottom of the cylinder bore. This will determined the size of the bore as well as show any out of roundness or funnel, or hour glass shape of the cylinder.

I got one more thing to say……………if you are working on anything metric……………..measure in metric………….you simply cannot use a 5/16 reamer in a 8mm valve guide and you can’t use a 1/2” socket on 12mm nut, so how do you expect to use a decimal or fractional micrometer to measure metric dimensions? You can’t! Sure you can do the math, and yes you can reference your decimal equivalent chart (that you got from your favorite shop supply company) but you really need to just step up and buy a metric micrometer or a metric dial bore gauge or metric dial indicators to do it right! Got it? Good!

See you in the shop.


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