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5,364 Posts
Discussion Starter · #1 ·
I have noticed there is some confusion among members when it comes to measurement standards, conversion of units and how to interpret decimal places in measurements, so I will be sharing my limited knowledge with you all. I will also be touching base regarding thread sizes and how to read threads when you go to the store to pick out a bolt or screw.

Additionally I will cover some basic ground regarding some of the basic tools utilized to measure parts, tolerances, gauges and threads, etc.

If you see something that is incorrect or would like to add something please indicate to me in the thread or via PM and I will gladly update the first post to reflect the most complete information possible.


There are two types of measurements that we encounter when dealing with our automobiles.
1. US / Imperial System: Length standards are as follows:
  • 12 inches = 1 foot
  • 3 feet = 1 yard
  • 220 yards = 1 furlong
  • 8 furlongs = 1 mile
  • 5280 feet = 1 mile
  • 1760 yards = 1 mile

2. Metric / SI (System International) System: Length standards are as follows:
  • 10 millimeters = 1 centimeter
  • 10 centimeters = 1 decimeter
  • 10 decimeters = 1 meter
  • 10 meters = 1 decameter
  • 10 decameters = 1 hectometer
  • 10 hectometers = 1 kilometer
  • 1000 meters = 1 kilometer

There are also standards for area, volume, weight, mass and capacities but I will not be going into those as there is currently no need for them. For a very complete, up to date guide on measurements standards and conversions check out this link:




1 inch is typically divided into 64 parts for measurement purposes. You can find a list of all the fractions for Inches listed right here: http://mdmetric.com/tech/cvtcht.htm

You will also see an immediate conversion to millimeters right next to it (mm)

Typically, inches are interpreted with 4 decimal places. The amount of decimal places on a part/print directly correlates to the tolerance that has to be met for that specific part. For example, engine internals will need to be made using extremely tight tolerances while suspension components will have more relaxed tolerances.

Additionally, the tolerance displayed will also have to correlate with the type of machinery that will be used for manufacturing. For example, a metal mold used for plastic injection will be able to hold 3 or 4 decimals while a Swiss turning machine (lathe) will easily hold 5 or 6 decimal places.

How are decimal places read?

1.0000 = 1
0.1000 = 100 thousands
0.0100 = 10 thousands
0.0010 = 1 thousand (also called out as “1 mil” in machine shops/fabrication shops)
0.0001 = 1 thousandth (1 tenth of a thousand)

Therefore, with a number like 1.1595

One would read: One inch, one hundred and fifty nine thousands and five thousandths of an inch.

It would NOT read: One inch, One thousand, five hundred and ninety five thousands.

Very seldom will a measurement on a common automotive accessory be shown with 4 decimal places but sometimes they are, and people misinterpret the actual measurement which may lead to confusion later on.


Since millimeters are based on the decimal metric scale, they are never measured in fractions but instead always in decimals:

Most common measurement instruments do not display more than 3 decimal places. According to our conversion chart 0.001mm = 0.000039 therefore trying to attain a 3rd decimal place in mm is very hard. (I’ve never tried it… yet)

Online Units Converter:
This is probably the best unit converter I have found on the internet. I have it bookmarked and I recommend you bookmark it also.


Mainly, remember that 1” = 25.40mm


Let’s talk about threads. There are two types of thread denominations, USA threads and METRIC threads.

Threads: USA Threads are broken down into many series but we are not going to delve into any of them because we can make a whole forum section dedicated to them. Among some of them is the NC (National Coarse), NF (National Fine), NEF (National Extra Fine), UC (Unified Coarse Thread) UNF, UN, UNS and UNM and there’s many more that I don’t look forward to remembering as I have no use for them.

The following chart shows you the most comm
only used USA threads.

There are others in between the ones listed but they are very rare and almost never used therefore they are not shown.

Thread Size:The thread size column on the left is consists of two numbers. The first one is the diameter of the thread and the second one is the number of threads per inch. #2, #4, #5, #6, #8 and #10 are nominal sizes and are assigned numbers to eliminate the need to use fractions.

Basic Major Diameter: This measurement is the O.D. (outside diameter) of the thread itself.

Size:The size column shows the drill size to use when making holes for this type of thread. I will post a drill chart with the standard drill sizes.

Metric Threads:
Metric threads are just that, metric threads. There are no variations or different types or styles.

Thread Size:
There are two numbers on the thread size column which stand for Diameter and Thread Pitch (not threads per inch)

Basic Major Diameter:
This column shows the size of the threads themselves.

Drill Size Column:
This column shows the right drill size/number to use when making holes for the type of thread listed.


5,364 Posts
Discussion Starter · #2 · (Edited)

Let’s skim over drills. Drills are assigned both a diameter and either a number or a letter. These are all the drill sizes available. There are bigger and smaller diameters but for brevity’s sake I will post these charts only.


Now that I have shown you how to read dimensions I will give you a quick overview of tools that you can use to measure.

Caliper: A Caliper is probably one of the most useful measurement tools for the regular Joe blow weekend mechanic. Better than a ruler and more than precise enough for what you need. While I am a hardcore dial-caliper user I recommend you acquire a digital caliper as it will allow you to convert inches to mm with the touch of 1 button. Also, as a newbie at reading decimal figures, it will give you less chances for error.

I personally recommend Mitutoyo or Starret calipers as they are name brands but you are free to get the caliper that fits your lifestyle and budget the best. Calipers are sold from 6” to 30”. A 6” or 8” should be more than enough for you.

This is what a digital caliper looks like:

Note: As with all measurement instruments, it is a very delicate tool and should be handled with extreme care. Dropping a caliper is unacceptable and if you do, you might as well throw it away as it will never ever be the same and will never give you an accurate measurement again. Also, calipers must be calibrated before their first use, and recalibrated throughout their lifetime.

Micrometer: A micrometer is a more specialized tool and is better suited for giving you tighter measurements. (4th, 5th and/or 6th decimal places)

This is what a digital micrometer looks like. Also, note that this is the correct way to hold a micrometer. Holding it with two hands will make you look like a retard among people who know. So look at the picture and learn!

Feeler Gauge: Feeler Gauge is used to measure distances between two parts. This is what a feeler gauge tool looks like.

Thread Gage: A thread gauge is used to check thread sizes. This is what it looks like:

Pitch Gauge: It is used to determine pitch. This is what a pitch gauge tool looks like.

Ruler: The simplest measurement instrument available that one should never overlook. Make sure you get one with both inches and mm (dual scale)

That is all I have for you today. I hope you have learned something from all this information and I hope that this clears any doubts in your mind about measurements or dimensions.If you have any questions feel free to ask. If you think I have something mistaken please bring it up to me as I’m not perfect and I may be mistaken.


3,418 Posts
Thumbs up for a useless thread I'll never use!

Reserved too... so I can make fun of you when this flops!

1,295 Posts
Uh good thread, I guess, probably more useful in a forum for math but ok Ill go for it. Im a little confused though, I have taken too many math class not to argue your method for reading decimal points. .1 is usually refered to as a tenth, .01 is a hundreths, and .001 is a thousandth. I guess the way you described it is if you are reading it as some huge number, as you explained. I dunno thats just my two cents, could just be the way i was taught.

1,523 Posts
Great! I really hope something comes out good from this research
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