The set you are looking at has a vernier scale to allow measurement expressed to the nearest 0.0001" (mentioned above by Moke spider)
You will need practice to actually repeatably measure to that nearest 0.0001" (you could say it was 2.7803" according to the mic but whether it actually was 2.7803"?)
This is at least the level of accuracy required for pistons, cranks etc
Of note - metric kit gives better resolution as 0.0001" = 0.0025mm but a metric mic can have a resolution of 0.001mm but whether anyone can accurately measure to the nearest 0.001mm with a mic? 0.001mm = 0.00004" or just about twice the resolution of a 0.0001" reading mic
Telescopic bore gauges can be relatively cheap and they are only a means to get something the same size as the hole that can then be measured with the micrometer BUT again getting the TBG to the same size as the bore needs practice and then measuring the TBG with the micrometer needs practice - better ones will have harder tips for longevity:
I think there might be some confusion about the bore gauging as people might be talking about these:
What you actually need are some test pieces of known sizes to practice on.....
Feeler gauges - buy a reputable brand but some are harder than others (for longevity) but hence more brittle so can snap if bent too much so take care
Cleaning threads - agreed thread chasers or I heard a way was to grind a slot in the thread of a standard new bolt/screw and use that as a chaser?
Thanks for the videos. I did work out that Bore Gauges and Telescopic Gauges were different, but it took me a while.
With regards to your comment:
"Of note - metric kit gives better resolution as 0.0001" = 0.0025mm but a metric mic can have a resolution of 0.001mm but whether anyone can accurately measure to the nearest 0.001mm with a mic? 0.001mm = 0.00004" or just about twice the resolution of a 0.0001" reading mic"
Should I have bought the Metric set? The same set in Metric has a resolution of 0.01mm.
Or are you saying I should have bought a Metric set with a resolution?
The metric set with 0.01mm resolution is not good enough you need 0.001mm but as DeadSquare says measuring imperial stuff in imperial is easiest to be consistent with numbers.