I made this about 40 years go from a Model Engineer article before digital calipers became so affordable. Overall it works very well, it has a range of 3/8 to 2 inches. The slightly awkward part is transferring the reference to a micrometer it being a bit of a juggle to hold both items at once, the answer of course is to use a micrometer stand (my next project). Over the years I have gained some bore mikes to make this a bit redundant. Personally I find telescopic bore gauges just as good to use but this seemed like a good idea at the time like so many things.