Main Engine Bearings & How to Use a Micrometer [with VIDEO!]

Operation Stude has given me the opportunity to completely dismantle an engine for the first time, and while there’s been plenty to learn it’s also given me the hands-on realization that an engine really isn’t all that complicated! Since we turned in the old crank for a “new” one at the machine shop and were replacing the bearings, Ethan said it would be a good idea to “mic” (aka use a micrometer to measure) the clearances.  (The cost wasn’t much different to replace crank vs. machine the old one, and I had messed up the front threads using the pulley puller…oops! It’s all part of the learning process…sometimes you WILL mess things up!) Beginners Note: A micrometer is a just a tool that measures things with precision – in this case up to 0.000″. Since engines have clearances within those ranges, it’s important to be able to determine the size of openings and diameters of things…and a micrometer enables you to do just that! This video outlines the process of changing the main engine bearings and miking them…and it gives you a good look at the purdy new gold paint! Follow along below with a few extra notes for those learning:

What’s a main engine bearing?

The crank is the rotating mechanism that your pistons are attached to and is responsible for converting the up and down motion of pistons to rotational force that will ultimately power your car. The main engine bearings are the parts around the rotational area where the crank meets the block, and therefore they’re responsible for keeping the crank in place and rotating without friction…which is why the clearances are so important. If you’ve ever wondered why changing your oil is such a big deal…here’s one example. Oil needs to be around the crank in order to keep it rotating happily, without friction…and the clearances give space for that necessary oil. Too little space and there won’t be enough room for oil – too much space and the crank will be wobbling about. It’s a little like Goldilocks and the Three Bears…everything needs to be just right. For this application we were looking for a target of a 0.002″ clearance space.

What is a micrometer?

As Wikipedia puts it, a micrometer uses a calibrated screw to make precise measurements. There are inside and outside micrometers. “Inside” measure inside a space (i.e. engine cylinder) whereas “outside” micrometers measure the outside of a space (i.e the round part of an engine space). Because they’re expensive, many people only keep a set of outside micrometers and use those in combination with telescoping snap gauges (like this $14.99 set I used from Harbor Freight) to measure inside distances. micrometer_operation_stude

First lets go over how to use a micrometer…

  1. Get an even reading across the “equator” of the space you’re measuring, the gauge should be firmly around it – but not clamped down on it.
  2. To read, start with the size of the  micrometer (in the video’s example I was using a 2-3 inch micrometer). To it add the number on the sleeve (the stationary part with numbers). In between the whole numbers on the sleeve are three other marks, separating it into quarters. Think of this as dollars (whole numbers) and cents (inside marks representing 0.25, 0.50, & 0.75). So if the thimble was marking the sleeve two marks past the number 3, that would be 2″ + 3.50 thousandanths = 2.350″.
  3. Next, look at the rotating portion and add that to the cents. So if the rotating “thimble” was at the 3.50 mark on the sleeve and crossed it at the 9 placement…it would equal 2.359″. It may sound a little tricky, but once you get your hands on it, it’s pretty easy – here’s another helpful micrometer reading tutorial if you need practice.

If you’re measuring an inside space rather than an outside, you’ll either need an inside micrometer – or you’ll measure it with a telescoping snap gauge like I did- placing that inside your outside micrometer for the reading. “Miking” my engine bearing and cylinder spaces may have taken a little extra time, but it’s well worth the peace of mind knowing that it will fit together and work as it should. If someone is building an engine from scratch, this miking thing gets much more technical…but for now this is enough of a lesson for me!

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3 Responses

  1. Arielle West

    This is good to know! I’ve been watching Horsepower TV for a few years now and as I was reading it, I was asking myself “Did Kristen do this, or that, and were the journals at all scraped, oil starvation, etc?” My mum being an engineer, I know exactly what a micrometre is, and what it does, and have actually used one, and hers a few times. I also looked and realised that it had single main bolts. I just watched the video and yep, the engine was starved for oil.

    • Kristin Cline

      Thanks for the video link, I finally got a moment to watch it and it’s really informative!


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