Blumbly, to measure bore diameter you would need an inside micrometer, not a common micrometer (the latter is a zero to one inch instrument that measures outside diameters only. The ones that measure more than one inch outside diameter are less common, and far more expensive.) An inside micrometer comes as a set of ultra-precision bits in a natty velvet-lined box, requires considerable skill to use, is very delicate to use and maintain, and costs plenty. I doubt that you want to buy one.
To measure bore diameters affordably involves a digital caliper, which copes with both inside and outside measurements from a fraction of an inch to about 6 inches. There is a bit of a hassle when measuring cylinder bore diameters with this kind of caliper because of the unworn "ridge" at the top of the cylinder. Some ingenuity is required to deal with this: you need to measure the diameter about one centimeter down from the top of the bore, at right angles to the crankshaft, because this will be the most-worn spot.
And for anyone who cares, a microtome is an instrument used for cutting very thin slices of tissue to examine under a microscope. Generally, not useful to motor mechanics unless they have a body to dispose of and a very small garbage bin.
