# Engine Building Fundamentals: Imperial and Metric Units

## Imperial and Metric Units

### 05.24

00:00 | - Since engine building and engine blueprinting requires extensive use of very precise measurements, we come to the dilemma of how to make and express these measurements. |

00:11 | Of course we have the option of using the imperial measurement system that uses feet and inches, or the metric measurement system where we use centimetres and millimetres. |

00:21 | But which is the correct option, and which should you use? The correct choice for you will depend to a degree on where you are in the world and what type of engines you're working on. |

00:33 | For example, in the U.S. market the imperial system is very much dominant, while in the rest of the world we see a preference towards the metric system. |

00:43 | The engine building industry is actually slightly unique in my experience, and so much as even in countries where the metric system is embraced, often engine builders and machinists will still favour the use of imperial measurements. |

00:59 | This means that you need to make a choice, particularly as many of the measuring tools that you'll need, such as micrometres and dial gauges, will either be sold in metric or imperial format. |

01:12 | This isn't a choice that I can make for you and you need to decide based on your location what engines you're likely to deal with and what units you're most comfortable with. |

01:23 | Of course, you can easily convert from one unit to another and the majority of engine specifications will include both metric and imperial units, so all is not lost. |

01:36 | Despite living in New Zealand we will use the metric system. |

01:40 | I actually tend to work, and think, in imperial units when assembling engines. |

01:46 | Many of the components that we deal with in the after market, such as cams for example, originate from U.S. manufacturers and hence many of the specifications are in imperial units; so even if you intend to work in metric units it's essential to have an understanding of imperial units and the conversion factors so you can swap easily between the two. |

02:11 | For the remainder of this course I'll be using both imperial and metric units so that you can get familiar with both. |

02:19 | It is easy to swap between the two if you can just remember that there are 25.4 millimetres in one inch. |

02:28 | This means that if we're looking at a measurement in inches and we want to convert to millimetres we just need to multiply the measurement by 25.4. |

02:39 | Likewise, if we have a measurement that's in metric we can swap to imperial by dividing by 25.4. |

02:48 | Regardless which method you're using we need to understand that often we'll be talking about very small measurements or clearances and it's very common to hear engine builders using terms such as thou or hundredths. |

03:02 | For example, when we're talking about a bearing clearance, it may be expressed as two and a half thou, or 2.5 thou. |

03:11 | What this actually means is that the oil clearance in this case is 0.0025 of an inch. |

03:20 | To understand how this works we need to look at where the decimal point is. |

03:25 | The first number to the left of the decimal point represents a whole inch. |

03:31 | As we move to the right of the decimal point though, we're expressing some fraction of an inch. |

03:37 | For example, the first digit to the right of a decimal point represents a tenth of an inch. |

03:43 | This second, represents a hundredth of an inch, and finally, the third number represents thousandths of an inch. |

03:52 | So, let's use that understanding to look at the value 0.0025 inches. |

03:59 | Here, the digit two, is the third decimal place which we know is thousandths. |

04:06 | Since we'll be commonly using thousandths, or thou, in reference to these measurements we represent this value as 2.5 thou. |

04:17 | Likewise, if we're working in the metric system it's common to speak in hundredths of a millimetre. |

04:23 | Using our same imperial oil clearance of 2.5 thou we can convert this to metric by multiplying by 25.4. |

04:33 | This gives us a metric value of 0.064 millimetres. |

04:40 | This would be expressed as 6.4 hundredths since the digit six is the second decimal place, which represents hundredths. |

04:50 | You'll see more examples of these measurements as we move through the course, but for now I want you to understand that you may use either measurement system and that it's being official to able to understand and work in either. |

05:03 | Remember, that we can convert from imperial inches to metric millimetres by multiplying by 25.4, since there are 25.4 millimetres in an inch. |

05:15 | Likewise, we can convert from millimetres to inches by dividing by 25.4. |