Agree with all this, and a great thread topic, except that the inside micrometer is not the right tool for this job. There is no way to get an accurate repeatable reading because you can never get it perfectly level ... and when you're dealing with .001", even a very light angle will make a huge difference.
You also need to measure in various places, and 90º to each other to check the bore roundess. All this should be done by your machine shop however, they better have the pistons in hand to check proper clearence ... any machine shop that says they don't need the pistons is not going doing it correctly.
A dial bore guage is the correct tool, and by sweeping it inside the bore you can find the exact measure, and easily find out-of-round. Also, for measuring bearings (which I assume you will also be doing), you will need something accurate to 0.0005". I got one from Fowler canada for a decent price ... you can go much more expensive, but the Fowler gauge seems repeatable, which is a good sign.
For crank journals you will need an outside micrometer and will be zeroing the bore gauge against it.
Originally Posted by matty.dc
Measuring is an absolutely crucial element to building an engine. If it isn't done and a motor is just slapped back together without thought, you will be very sorry(or your wallet will be) when everything breaks at 7000rpm.
The tool I am using here is an inside micrometer. This tool will come in a kit that has many different length inserts to match your desired length. This one only has a 0.500" thimble so the inserts vary by 1/2".
The key to using the micrometer is to not over tighten the tool in place. When you fingers can gently slide around the thimble then the mic is tight enough. The reason you don't want to reef of the mic is because at that point you are deforming both the tool and the material you are trying to measure and will get a false measurement.
On the body of the mic, the numbers represent increments of 0.100". For example, "3" would be 0.300". The lines in between the numbers represent increments of 0.025".
In picture 1 it shows how the mic is exactly in the centre of the bore and level( I leveled it on the minor step at the top of the cylinder).
In picture 2, this shows the actual measurement. To get your starting inch measurement, you would move the thimble all the way to "0" and you would measure the length with a measuring tape. In this case it's 3". In this picture(it's hard to tell) the thimble has moved past "4" and the number 11 on the thimble is just about inline with the line on the body of the mic. This represents a measurement of 3.411".
Other tools used to do this are: (look up pics on google if interested)
Telescopic gauges and an outside micrometer
A Direct read bore gauge