Agree with Ray.
The reason that a fellow would bore the cylinders is usually because they are imperfect for installing new pistons and/or rings. The cylinders do not wear evenly side to side or top to bottom and can be eggshaped as well as funnelshaped or both, so you want to bore and hone the cylinders to make them perfectly round again, like they were from the factory. At the same time, you would hone the cylinders with a certain grit number of stone, depending on the material the piston rings will be made from, in order to allow the rings and the cylinder surface to "wearin" together and form a smooth surface with each other to seal in the combustion gases and make power.
Ray already knows this stuff, but we'll have a little math lesson for ferabel while we're at it.
.40" or .400" or 0.400" would be read as four hundred thousandths of an inch, or nearly half an inch.
0.040" would be read as forty thousandths of an inch and would be an acceptable overbore dimension. The reason for adding a zero in front of the number is to tell the person looking at it that the dimension is less than an inch, so that there are no mistakes when using the number to calculate other dimensions from it. For instance, 4.040" would be the cylinder bore diameter of a 350 Chevy cylinder block after boring it 0.040".
Also, you have to think about this....When cutting the cylinder bore, you would only take out 0.020" (twenty thousandths of an inch) on the radius of the cylinder, because as the boring bar turns to take material out, it is cutting then entire diameter of the cylinder larger, so 0.020" on a side (the radius) equals 0.040" finished bore (the diameter) increase.
0.004" would be four thousandths of an inch....
0.0004" would be four ten thousandths of an inch, also expressed as "four tenths".
0.00004" would be four one hundred thousandths of an inch, also expressed as "four hundredths". Automotive machine work does not get into hundred thousandths, I'm just showing this so that you can begin to get wrapped around the numbers.
