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bore measurement

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james dethrow 131/10/2020 03:23:16
3 forum posts

hello all new to this forum been playing around with machining for a few years now still green in some areas. im having problems geating consistant bore measurement using telescoping snap gages. is there any other ways of getting accurate say .001 in on small bores? thanks james

John Olsen31/10/2020 05:18:23
1215 forum posts
92 photos
1 articles

There are various more expensive approaches, like inside micrometers, but unless you are doing a lot of small bores you probably don't want to go that way. You can make your own go/nogo gauge by turning the ends of an offcut of metal, one to the exact size and one to a thou under. You can also make a lap, and take the last thou or so out using that. With a lap, you can get a feel for how parallel the bore is too. It also pays in the smaller sizes of bore to make the hole first and then make whatever goes in it afterwards to fit.

The other approach is to use a reamer to take the last cut out of the bore. That means owning a reamer the size you want, which can be expensive, but you can get pretty consistent sizes that way. The size a reamer will leave does depend a little on how much you leave for it to cut out.


Thor 🇳🇴31/10/2020 05:56:52
1433 forum posts
41 photos

Hi James,

A go/nogo gauge is a simple and inexpensive solution. If you want a step up from telescoping gauges, Inside Micrometers like these are not as expensive as 3-point Internal Micrometer sets. I have a cheap Chinese Inside Micrometer and it has worked well so far.


not done it yet31/10/2020 07:15:29
6433 forum posts
20 photos

There are other ways, but better telescopic gauges might help, as well as practice - for both consistency and on known diameter holes.

The link supplied by Thor is only good for consistent diameters - not always good for machined items where the end section may be ‘bell-mouthed’ while machining.

This Old Tony made a video on improving a set of fairly rough (as in ‘cheap&rsquo gauges. Might be worth watching?

John’s suggestions are good alternatives, with likely better outcomes.

David Colwill31/10/2020 08:13:25
768 forum posts
40 photos

The easiest way is to use a three point bore micrometer. The bad news is that they cover a limited range and are hellishly expensive. They do show up used on ebay and you can get lucky.



John Baron31/10/2020 08:20:06
499 forum posts
189 photos

Hi Guys,

I've used inside friction callipers for years, but if I want a precision size I make a plug gauge 1 thou under and lap as needed. Very often a spring cut gets you very close.

Paul Lousick31/10/2020 08:26:59
1868 forum posts
666 photos

The cheap telescopic bore gauges are not that good and the better ones are expensive. You also need practice to use them. Something that I don't have.

For smaller holes, I use a go/no go plug gauge. Quick and easy to make from a piece of bar turned on the lathe. Once made they can be used for other projects.

For larger holes I use a dial bore gauge in conjunction with a micrometer. The cheaper ones on ebay are accurate enough for the jobs I do and includes a dial indicator which you could use for other jobs.


bore gauge.jpg

Tony Pratt 131/10/2020 09:12:38
1752 forum posts
8 photos

With practice a decent quality telescoping gauge will easily get you within .001"


Emgee31/10/2020 09:19:16
2193 forum posts
272 photos
Posted by Tony Pratt 1 on 31/10/2020 09:12:38:

With practice a decent quality telescoping gauge will easily get you within .001"



and these Ball gauges are good to obtain readings from  with a mic or vernier

small hole gauges bb type.jpg

and for bigger holes an internal micrometer

bore micrometer.jpg


Edited By Emgee on 31/10/2020 09:27:37

Edited By Emgee on 31/10/2020 09:29:45

Simon Williams 331/10/2020 09:19:51
617 forum posts
81 photos

OP doesn't identify the make or quality of the telescopic gauges.

With good quality (M&W, Starrett, Mitutoyo) gauges and practice you can get sub 'thou repeatability, then measuring this with an external micrometer introduces its own error. Overall repeatability with practice and developing a feel for the technique, expect to get repeatability of measurement say +/- 1 thou.

Cheap and clonky telescopic gauges won't let you develop the "feel" successfully.

There is a definite knack to setting the gauge in the bore - refer to Adam Booth's technique on Youtube. But you do need to practice.

edit - Emgee and Tony type quicker'n me.

Edited By Simon Williams 3 on 31/10/2020 09:21:06

Martin Kyte31/10/2020 11:30:28
2600 forum posts
45 photos

I use a small inside mike from Arc Euro, not to expensive, for small bores. Bear in mind these will only measure the start of the bore.


As others have said down hole measurements do take practice and feel so take every opportunity to take measurements whenever you have the opportunity and especially when you are in a situation where the bore size is known.

Where ever possible bore nominal size and turn the cylindrical part to fit the bore especially on smaller bores. Reaming will give you consistancy and smaller reamers are less expensive, including machine reamers. Always make a bore gauge (go/nogo gauge) before starting your bore. When all else fails or you need to be spot on, lap to fit.

regards Martin

Howard Lewis31/10/2020 13:10:22
5533 forum posts
13 photos

Where possible, prefer to use three point mic, or ball gauges, rather than telescopic gauges. (Frightened that the setting will change during clamping )

For slightly larger bores prefer to use a Bore Set, if possible, or a Bore mic. if the bore is large enough.

Am lucky to have acquired a Compaq bore set for 0.8 to 2" bores, with a few missing extensions / contacts. Spares are not available and the complete unit is excruciatingly expensive USD 5+K, with a six month wait for a new one. But it is industrial Swiss quality.

So finished up making the fittings with Tap and Die for the 0.5 mm pitch threads.

Ultimate test has to be the fit of the mating part, (Turned after having measured the bore )


Dave S31/10/2020 13:44:07
257 forum posts
56 photos

If they are a cheap set then dismantle them and deburr the bits. A small dab of moly grease on reassembly helps as well.
“this old tony” on YouTube has a video on improving cheap bore gages.

Then using them is a question of practice mostly.
I do it like this:

select a gage where the range is appropriate - if there is an overlap I usually use the larger size gage as the telescoping pieces are less sticking out that way.
with the gage in springy mode plonk into the bore and cant over so the handle touches the side of the bore.
wiggle the gage a little so it’s as extended as it can be in the bore at an angle.

Then tighten the end piece so the gage will move, but is stiff. You may need to figure this out out of the bore until you have done it a few times.

move the handle from the side of the bore through the centre to the other side. Try to sweep in a single movement and along the diameter. As you pass through the centre the gage should tighten and then loosen. This is the bore squashing in the telescopes and then as you pass the centre they become shorter than the diagonal.

fully tighten the lock.

Only sweep once, then carefully remove the gage from the bore without bumping it.

Now you can measure over the ends gently. Be aware that a micrometer is also a g clamp - if you monkey it you will change the measurement.

You should be able to repeat measurement to under a thou without much practice, but it does take some practice.

If you have some ball races then you can practice with those, and know your measurements are correct, as the bores are quite tightly specified.

hope that helps


John Baron31/10/2020 17:09:42
499 forum posts
189 photos

Hi Guys,

This is how I measure bores ! Use callipers to get the bore diameter and then measure with a micrometer.


In my last post I said friction callipers, I meant to say spring callipers. Sorry.

In this picture I'm measuring the bore in a Norman tool holder block. Its 35 mm diameter.


This shows the technique used.

Emgee31/10/2020 18:45:32
2193 forum posts
272 photos

These dial spring caliper gauges are particularly good to measure bore diameter, roundness and taper. If set zero at a known diameter you can read directly, alternatively you need to use the method shown by John Baron but taking the measurement at the point on the gauge when inserted in the bore.

calipers 0.5 to 1.75.jpg


old mart31/10/2020 19:13:00
3397 forum posts
210 photos

I use a set of three Moore & Wright teliscopic gauges down to 3/8" and. then some of their ball gauges to 3/16". They are good enough for 0.0005" with practice.

jimmy b31/10/2020 19:23:58
741 forum posts
42 photos

Hi James,

A lot depends on the end use of the item!

i use one/some of the following, depending on the job

Calipers (digital/vernier)

Telescoping gauge

bore/inside mic

comparator (John Bull etc)


Gauge pins

Slip gauges and accessory set.

Very much "horses for courses"!


Nigel McBurney 131/10/2020 20:26:15
945 forum posts
3 photos

Before all the fancy bore gauges etc were available in the early part of the 20th century, and before,,turners used calipers and a rule ,their calipers were the firm jointed type,they would not use spring type calipers with screw adjustment,they would get a close fit in a bore by adjusting the calipers by hand and then adjust to get a very fit by just tapping the arms of the caliper usually on the lathe toolpost and apparently they were very accurate,if large numbers of one size were required then hardened go/no go plug gauges would be used,a lot of shops only had a few micrometrs usually in the care of the foreman. During my apprenticeship our boss used to tell us how jobs were done when he started work. For small bores the M & W bore gauges are very accurate, for larger work I have M&W plus some Mitutoyo telescope gauges plus an internal mike with with a range of 2 to 6 inches, I find the M & W tele gauges are better than the oriental ones,they have a better feel. when I can get at the open end of a bore I tend to use a Swiss Rochalon vernier where the outside of the jaws is machined to a radius, and then use these jaws as a bore gauge I find knife type jaws are not accurate enough, by using a mike to measure across the jaws and using the same mike to measure the mating piston or shaft only one measuring instrument is used,for better accuracy ,if the verniers reading is used then it introduces a possible secondary error.with all these methods it does require a lot of practice .

james dethrow 101/11/2020 15:17:13
3 forum posts

hello all thanks for all the great help. my biggest problem is age my hands are a bit on the shakey side. i have mueller and kafer guges some alina most of the guages and hand tools came from my uncle hes a retired tool and die maker from germany. ps im from ansbach germany just stuck here in the usa. chers james

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