For a long time I have wanted tools for more exact inner bore measurements. Here is helpful information from James Gray in Arizona [Oct2014]:
Yes, I use the telescopic 2-point T-type bore gauges that you insert, lock, extract and then measure with a micrometer. I also have a direct-reading dial type inside micrometer. More often than not I use the manual gauges because they have long shafts and I am able to measure the entire bore, not just the top or bottom few inches. While I wish I had a direct-reading digital bore gauge with a long shaft I can't justify the cost.
I like the 2-point rather than 3-point types because it is easier to identify exactly which area in the diameter of a bore is out of round. I primarily use these tools for aircraft maintenance work, so they are a bit overkill for assessing clarinets, but they do come in handy for deciding just where to clean up a slightly distorted clarinet bore with a reamer. The distortion is mostly due to movement of the wood over time and in different environments, but sometimes it is caused by sloppy reaming at the factory.
You can get a low-cost set of Telescopic T-bore Gauges (the proper name) at Amazon for under $30 here:Anytime Tools 6 pc Titanium Coated TELESCOPING T BORE HOLE Precision GAGE GAUGE SET
. You really only need the smallest two or three sizes but it costs more to buy them individually than to purchase the whole set. Because they are spring loaded you have to be careful not to get the probes stuck in a tone hole though. It can be a pain getting them unstuck but it WILL happen sooner or later.
I have a good quality precision Swiss digital caliper that is metal and expensive, but they also make some decent Swiss-made plastic ones that are much cheaper: HERE
However, DON'T waste your money on the cheap plastic Chinese-made ones that cost $10, ala Harbor Freight tools. They do not last.
BTW, when I get a new clarinet in for repair, I measure it as-is, and then after oiling. There can be a remarkable difference as the wood swells. The values given for the Kohlert were "dry". It hasn't been disassembled yet for oiling and repair. Here in Arizona you can get some incredibly dry instruments and I do no major work to them until they have been thoroughly oiled and stabilize for a few weeks. That is another story in itself, but sometimes just re-hydrating the instrument cures most ailments unrelated to tenon corks or pads.