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Practical Engine Building: Imperial and Metric Units

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Imperial and Metric Units


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 had the option of using the imperial measurement system that uses feet and inches, or the metric measurement system where we use centimetres and millimetres.
00:22 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.
00:34 For example in the US market the imperial system is very much dominant, while in the rest of the world we may see a preference towards the metric system.
00:44 The engine building industry is actually slightly unique in my experience in so much as even in countries where the metric system is embraced, often the engine builders and machinists will still favour the use of imperial measurements.
01:00 This means that you'll need to make a choice, particularly as many of the measuring tools that you'll need such as micrometers and dial gauges will either be sold in metric or imperial format.
01:13 This isn't a choice 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.
01:25 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 where we use the metric system I actually tend to work and think in imperial units when assembling engines.
01:45 Many of the components that we will deal with in the aftermarket such as cams for example, originate from US manufacturers, and hence many of the specifications are in imperial units.
01:57 So even if you tend to work in metric units, it's essential to have an understanding of imperial units and the conversion factors so that you can swap easily between the two.
02:10 For the remainder of this course, I will be using both imperial and metric units so that you can get familiar with both.
02:18 It's easy to swap between the two, if you can just remember that there are 25.4 millimetres in one inch.
02:26 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 Likewise if we have a metric measurement, then we can swap to imperial by dividing by 25.4 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 use terms such as thou, or hundredths.
02:59 For example when we're talking about a bearing clearance, it may be expressed as 2.5 thou.
03:06 What this actually means is that the oil clearance is 0.0025 of an inch.
03:15 To understand how this works we need to look at where the decimal point is.
03:20 The first number to the left of the decimal point represents one inch.
03:25 As we move to the right of the decimal point, we're expressing some fraction of an inch.
03:31 For example the first digit to the right of the decimal point represents tenths of an inch.
03:38 The second represents a hundredth of an inch, and finally the third number represents thousandths of an inch.
03:46 So let's use that understanding to look at the value, 0.0025 inches.
03:52 Here the digit two is in the third decimal place, which we know is thousandths.
03:59 Since we'll commonly use thousandths or thou in reference to these measurements, we represent this value as 2.5 thou.
04:08 Likewise if we were working in the metric system, it's common to speak in hundredths of a millimetre.
04:15 Using our same imperial oil clearance of 2.5 thou, we can convert this to metric by multiplying by 25.4 This gives us a metric value of 0.064 millimetres, which we would express as 6.4 hundredths, since the digit six is in the second decimal place, which represents hundredths of a millimetre.
04:40 You'll see more examples of these measurements as we move through the course.
04:45 But for now I want you to understand that you may use either measurement system and that it's beneficial to be able to understand and work in either.
04:55 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.