On
7 August 2026 at 12:17 John MC Said:
Those telescopic bore gauges. They seem to be popular but I’ve never had much luck with them. Fortunatley, there are much better ways to measure a bore.
I was gifted a set of them many years ago, I was delighted and grateful until I tried to use them to determine the diameter of a bore accurately. When asked, by the gifter, how I was getting on with them, I answered honestly! He agreed with my thoughts and suggested passing them on to anyone who wanted them…
I have, due to circumstance, tried to use them occaisionaly, still hate the b****y things.
Can you describe how exactly you’re using them?
There are a few techniques, and some work better than others.
Here’s a video that I think demonstrates the most practical and repeatable technique:
Yes, he has a far bigger workshop than most of us, and bigger machine toolsbthdn we’re likely ever to have, and worst of all, he’s a frightfully vulgar American! 🧐
But seriously, his videos are pretty good, and this one in particular is excellent.
I’ve started building up a collection of analog ‘tri-mike’ type, bore micrometers, so I don’t use them as often, but when I have to use them on larger bores (have you seen the cost of the larger ‘tti-mikes’?🫣), I can generally get a repeatable reading within a couple of hundredths of a mm.
I’m afraid that this is one of those instances where it doesn’t pay to go down-market on these things.
They key to them producing repeatable results is the ability to snug up the ‘lock’ just enough so they are set to the bore diameter as you rotate them out of the bore, but will remain at that settings afterwards, and that the tightening up of the ‘lock’ to then measure in the micrometer doesn’t alter the setting. Cheaper telescopic gauges tend to feel gritty, and aren’t smooth moving, and the tightening of the ‘lock’ can affect the final result.
Mitutoyo are always a reliable manufacturer and their set, whilst not cheap, is not unaffordable (£180 from Digital Micrometers Limited, an official Mitutoyo dealer). You can find a second-hand set, for about £100.
M&W? Made in China, and whilst they may be fine, for £170, I’m not willing to take a punt.
Starrett? Who knows where they’re made these days.
This Old Tony has a video where he finds a Browne and Sharp set is less than brilliant. He then goes on to fettle them to a good usable state, so this video is worth watching for anyone who has a set that isn’t as smooth as they might wish:
He’s also quite amusing in a surprisingly gentle, rather ‘unamerican’, self-deprecating way! 😄