I'm about to start on a project that will require
me to measure the inside of a bore to an accuracy
of about 1/2 a thousandth. The bore will be some-
where between 2 and 3 inches.
Can someone tell me what the most cost-effective
tooling is to do the measurememt? I've tried the
cheap Enco telescoping guage sets and I can't get
a repeatable measurement from them.
I don't have an unlimited budget so I'd appreciate
a solution that won't make a major dent on my
Do you know why you aren't measuring the bore to the same dimension? The
telescoping gauge set should do the job if you are careful with it. Inside
measurements are rather difficult to master for most people without somebody
to show them when they are doing the job wrong.
Why isn't there an Ozone Hole at the NORTH Pole?
You could buy a dial bore gage but that will probably
bust your budget. I would say that you should practice
with the telescope gages and a good micrometer, without
too much effort you *should* be able to do a half thou
Find somebody near you who has used them a lot, and
practice checking bores with them, and see what kind
of numbers they come up with, vs what you come up with.
You might also consider a pair of spring leg ID calipers,
you might not have good luck with telescope gages but
maybe have a better feel with the spring calipers.
Another approach would be to purchase an ID micrometer,
those work well in the range you are working in. In
between the telescope gages and the dial bore gage in
Someone makes inside diameter "micrometers" that have three hardened, convex
surfaces and can measure down to the levels you need. However, most, if not
all I've ever owned or seen are metric and quite expensive. You can check
eBay for used ones, but you get what you pay for sometimes...
A cheaper fix might be to have someone make you a "go / no go" guage. It's
basically a hardened steel rod ground to .001 unsersize at one end and your
necessary size at the other with a taper of .001 or less over a 3" length,
etc. Honing companies love it when I send these to them as we have a lot of
3", 5" and 10" tubes that are hard chrome coated honed out by them and need
them to within .0005" I think we had ours made for about the price of the
used device I describe above... And at least it won't break if I drop it
and could be somehwat repaired manualy if it got a burr / gouge and if it
wasn't too hard.
Joe Agro, Jr.
I agree with this. The main trick is to develop the right feel.
Start by letting the telescoping gauge expand within the hole tilted
slightly. Tighten the knurled knob at the end just enough to give a
little drag, and rock the gauge through the point of maximum drag. Snug
the knob down a bit more and check whether the gauge feels the same
rocking through that point again. (If it doesn't, you've disturbed the
setting while tightening.)
Once you get it locked with a known drag, then take it to a
micrometer. In the size range you are talking about, I would suggest a
bench mount for the micrometer, so you have your hands free to do other
things than support the micrometer. slowly tighten the micrometer until
you feel the drag as you rock the gauge through the point at which the
gauge rods are parallel to the spindle. Repeat this until you get
Go back to the bore, and try again. Keep doing this until you
are getting repeating readings.
Note that the feel as it passes through center will be different
in the bore and between the micrometer anvils, as the surface finish of
the two is different (unless you are lapping the bore to an amazing
And I have not used the cheap Enco sets. I first learned on a
set sold by Sears (as Craftsman) which is not self-centering, and have
since acquired a smaller set of self-centering ones by Starrett. Given
who made the Craftsman micrometers and combination squares at the time,
I suspect that this, as well, was made by Scherr Tumico, but I don't know
Good advice -- if you can find such a person.
There are two basic styles. The more affordable ones have a
micrometer thimble with a range of 0-0.500", and a set of shanks which
screw on to add precision distances in half-inch increments. The feel
for this should be similar to the telescoping gauges. But it is harder
to use deep into the bore. While there is a handle to hold the gauge
down there, getting a fingertip down to turn the micrometer thimble is a
Much better -- but expensive new, and each covering a limited
range (e.g. 2.4-2.8") are the various flavors of tri-mikes. There
should be a setting ring with each one. The larger ones read in steps
of 0.0002" IIRC. The smaller ones in steps of 0.0001"
Most of mine (including sets) are made by Tesa, and sold under
the Brown & Sharpe label.
The only one I find on a quick search of eBay is too big for
your project, and is a newer one with a digital head as well, so it is
going for a rather steep price -- $350.00. But it will show you what I
am talking about. The auction number is "3872855627", and the URL
(pared to the essentials) is:
The big ring in the second picture is the standard. You measure the ID
of this to calibrate the micrometer.
When measuring, three arms extend outward at 120 degree angles,
so it is self-centering in the bore, and gives excellent repeatability.
And I can tell you that I have gotten a set of four of the
smaller ones (0.400" to 1.200" IIRC) with two standards (the standards
are sized so they are the maximum size of one micrometer and at the
minimum size of the next), plus an extension handle for reaching deeper
into a bore to take your measurement, for less than this one is
currently selling for. (But mine were standard micrometer thimbles, not
Since each covers only a fairly narrow range (the bigger ones a
wider range than the smaller ones, as this one in the auction is 5.000"
to 6.000"), you do need to know your size a bit more closely, unless you
get lucky, and wind up with a set covering the range and with a full set
of standards for zeroing these micrometers.
These are my own favorite way to measure IDs.
There are also, getting into the *very* expensive ones, gauges
which depend on feeding compressed air into a gauge plug which puffs it
out in several directions, and precisely measures the bore diameter by
measuring the change in air pressures. These are used in production
testing, and I believe that they have a very limited range.
I hope that this is some help,
I once used a cheap attachment for a bestest indicator... it was bought
to check an internal ring groove
It mounts to a bestest type indicator and you adjust it to the bore size.
I'm sorry I don't recall the name or brand
I'd want to hear from someone who gages bores every day before commenting on
the practicalities of this, but I sat through a lecture by a real expert on
the subject back when Mitutoyo (then my client) introduced their first
digital bore gage. I had a lot of questions for him, and, if I recall
correctly, these were some of the things I learned:
In an engine cylinder, if you're measuring a newly-machined or Sunnen-honed
cylinder, the three-legged bore gages are great. The cylindricity of those
bores is quite good and what you're measuring is variations in bore diameter
from the top to the bottom of the cylinder.
OTOH, if you're measuring a worn cylinder, they may not be so great. These
cylinders may be elliptical at the bottom, and the three-legged gages won't
tell you the amount of ellipticality [is that a word?]. The important point
is that they will undervalue the maximum diameter, thus misleading you about
how much metal you have to bore or hone to restore cylindricity. Once you
machine the bores, though, and you've restored the cylindricity, the
three-legged gages are, again, great.
To measure an elliptical bore you need something that will gage specific
diameters at specific radial angles around the bore (primarily parallel-to
and perpendicular to the crankshaft centerline), and the practical way to do
that is with a two-legged gage. Of course, using one and getting accurate
results with it is tricky. But according to that lecturer, that's the way to
do it. I have a set of B&S telescoping bore gages, and he told me that I
should practice with those, since it was engine cylinders that I was most
interested in. He said that's the way he does it, and he can get his hands
on any kind of gage he wants.
Again, this guy is a gaging expert, not an engine restorer or builder. His
point may be more academic than practical. But he sure knew his bores.
:) :) :)
How politic, DoN. Honestly I know you are saying,
"fat chance" on that one, but back in the dark ages, folks
measured bores with those things and I bet they got to
within a half thou.
The feel is tougher than the telescope gates, that's true.
Another interesting approach (I think I learned it here)
involves a measuring rod of known length, with points on
the end. It has to be slightly smaller than the bore
of interest, so that when it is fit inside the bore,
the rod will rock from side to side a certain amount.
The amount is then measured by eye, using a good scale.
Turns out that one can measure a bore to within a thou
this way, by reading the scale to (IIRC) 1/64 inch or
so. I never ran the numbers, but it seemed like a nice
Well, not every single day but on a pretty regular basis. Two main types of
bores; engine cylinders and counterbores for valve seat inserts.
before commenting on
Hmmm. On an engine production line it may well be that they take a
statistical view about ovality and use three-legged gauges to just check for
average diameter but I don't think you'll find many engine reconditioners or
race engine builders doing that. The ovality is one of the main things you
want to check for, even on a freshly bored and honed cylinder. Most engine
bores I've measured, and that includes brand new OE manufacturer ones, are
bigger perpendicular to the crank axis than parallel to it.
The only way to get both speed and accuracy is with a dial bore comparator.
Telescoping (T) gauges are slow and fiddly at the best of times and
generally over read slightly. They work better in hard materials and with
fine surface finishes. In soft materials like aluminium they make a slight
indentation as you rock them over centre. I have to use them for measuring
valve seat counterbores in aluminium heads because there isn't enough depth
to get the dial bore gauges in without fouling the bottom of the bore. I'd
say at best I can work mine to about half a thou. That's within the
tolerance I need for machining the inserts to to get the correct press fit
but if I could use a dial bore gauge I still would.
If the OP needs half a thou accuracy then get a dial bore gauge. If he has
to use T ones then get the best quality he can afford because cheap ones
tend to stick and grab. However top quality T ones can set you back as much
as a good second hand dial bore gauge. To get any repeatability with T
gauges you need to practice on a known bore or a micrometer.
A good way is to get two mikes and set one up in a vice at a given size.
Practice rocking the T gauge over centre across the anvils and measuring the
result with the second mike. You can either aim to get the correct reading
or to get a consistent over reading. The latter is probably easier.
Dave Baker - Puma Race Engines
Using T and dial gauges has been covered in another post. There's a cracking
good cheap way to measure bores if you have a bit of spare time. Get a chunk
of scrap metal and turn up a top hat bung on the lathe. The larger (top)
part should be bigger than the bore by half an inch or more so it can sit on
the bore. The smaller lower diameter should be a couple of thou smaller than
the bore you are trying to gauge. Sit the bung in/on the bore and mount a
DTI perpendicularly against it. Push it back and forth across the bore and
read the movement on the DTI. With an accurate measurement of the plug part
of the bung plus the movement on the DTI you can gauge the bore very
closely. Of course you can only measure the top of the bore this way and you
need a good undercut between the small and large part of the bung so it
doesn't foul the bore on the chamfer between the two.
There's another method which no one ever thinks of but engine builders come
across all the time without necessarily realising what it tells them. If you
insert a piston ring into a bore the gap will be a function of bore size. If
you can measure the gap with feeler gauges to within a thou then you are
measuring the bore diameter to within a thou/pi (about a third of a thou).
You need to ensure the ring is square in the bore and a close fitting plug
to push it level with is handy. It's no substitute for a dial bore
comparator but it works.
Go/no go gauges are probably the most accurate but it takes time to make
them and they still won't tell you about ovality.
Dave Baker - Puma Race Engines
Its not restricted to engine bores. That advice is right on the money for
bores of almost any nature, unless more expensive devices are available.
For the home shop, I can't imagine a more functional, yet inexpensive,
device than telescoping gages for measuring bores. .
I've used telescoping gages for .0002" tolerance with outstanding results.
It has little to do with the gage, and everything to do with developing the
proper feel, and then using it. They're very good in qualified hands.
Proper procedure is important.
Most everyone can learn to do the same thing. It takes dedication and a
well calibrated set of micrometers to cover the desired range of sizes.
I agree. To use them properly takes time.
If you can't make them work in aluminum and hold a couple tenths, it's not
the fault of the telescoping gage. They're certainly capable. I've done
it and had perfect results time and again, and holding .0002" tolerance.
Mind you, my work was always inspected by an impartial third party, so there
was no escaping the truth.
Success in using telescoping gages is all in how they are applied, and
experience. I propose to you that you have too heavy a hand. That's
If you're talking about cylindrical plug type gages, I'm afraid I'd have to
disagree where it comes to accuracy. Unless you have them in tenth
increments, such as the Deltronic pins that are readily available here in
the States, they are a fast gage, but often tell you nothing about the size
of a bore, only that you are too small, on size with the given range, or
beyond. It's often desirable to know the exact size, and that's a function
the typical go/no-go gages don't provide. They are more a gage of
production than anything.
I think my hand is fairly capable but my T gauges aren't. Mine are not the
best set in the world and need to be nipped up a tad tighter than I would
like before you rock them over centre or they slip. A Moore & Wright or
Mitutoyo set would be nice but they cost over £100, or $200 in your funny
(and rapidly diminishing in value) money. My T gauges are Draper Expert ones
which cost about £20 and work well enough to get me within the tolerances I
need for the specific job they were bought for.
For anything else I can get a dial comparator into I use those instead. I
have three sets which cover the range from 0.5" to 6" and cost me only £100
for the lot second hand from a metrology lab. The nicest is the 2" to 6"
Mitutoyo which reads to 0.0001". The other two are fairly old ones with
0.001" dials. For under 0.5" I have an expanding small hole gauge. A split
ball end with a tapered expander inside which opens the two sides as you
turn the adjusting screw. Adjust until it just rubs in the bore and then
mike it up across the ball ends.
As I said to the OP, if he wants precision with T gauges then buy the best
he can afford but they'll cost as much as a good second hand dial comparator
and still be slow and fiddly to use. Frankly they aren't state of the art
technology anymore and there are better ways of measuring things.
Not that I doubt you but using T gauges to 0.0002" is stretching my
credulity a bit. That's close to the limits you can repeatably use a
micrometer to for an o/d measurement and well beyond what most people would
claim they can do for i/ds with T gauges. However perhaps you are a
measuring god with abilities beyond those of us mere mortals.
Dave Baker - Puma Race Engines
There are cheap dial bore gages available, and sometimes by changing
the cheap indicator in them, you can get reasonably accurate results
with them. However:
Brown and Sharpe sells the inside micrometers, they measure on three
points. No good, they will NOT show whether the bore is round like an
O or shaped like a goose egg. Federal made some dial bores with a
quite wide range, but three points, also not what you want and
fantasitically expensive. The inside micrometers, the "adjustable end
measureing rods" made by Starrett, B&S and others can do it, but
require practice and a good feel with them. Standard probably makes
the best dial bores available, but unless you have need for them,
damned expensive, in that size range $500 plus. The Sunnen honing
gage is probably the best available, but let's get down to earth
again, more than $3000 for the one I had.
Plugs won't show out of round condition, three point won't show out of
round, you need two point, and there are some out of round conditions
that even two point won't show. (Your chances of seeing that are
somewhere between slim and none.) For a quick and dirty, "one off",
if you can find an old used good quality dial bore, Standard, older
Federal, even Boise, it will probably do the job nicely if the
indicator is good. A cheaper dial bore can be made to work, but they
are all dependent on what you use to set them. IF you can find a
setting ring that's within a couple thou of the size, that would be
the best, just adjust your zero to compensate. Dial bores are
comparators, they compare what you make to a known standard.
Buy a cheapie, see if you can get any repeatability against known
settings, it'll probably work well enough. CHeck it against your
standard before and after each measurement and you can be reasonably
sure that the result will be good.
THese are rips of the old Standard gage design, but not only do not
address the problems inherent in the Standard, they're not as well
made. I had a set of them, from .750" to 6", figure I was lucky to
get the $20 I sold it for. (I also told the sucker, ummm, customer
not to bring them back if they started to stick, I won't fix them.)
The footpiece on the Mitutoyhoho is aluminum, galls if not kept lubed
well, and temperature sensitive beyond any use in a non controlled
environment. Standard is bronze, better, but the cause of most of the
problems we had. Tool steel would be better, but probably more prone
to corrosion and even worse problems.
The micrometers with three contact points will show triangular bores
but not elliptical bores. The micrometers with two contact points will
show elliptical bores and not triangular bores. so you need to know
what kind of bore you are producing.
You may be able to get a good reading with your telescoping gauges. I
hate 'em but sometimes use 'em for speed. It's longer to set up a bore
gauge than it takes to measure 1 bore with a telescoping gauge. Here's
how I do it: Like everyone says, put it into the bore at an angle and
then tighten the thing. Not too tight. Just enough to keep it wherever
it ends up after rocking it through the bore. After rocking through
the bore by holding the very end of the handle grasp it in the middle
of the telescoping part and rock it back through the bore. You will
have to rock the gauge side to side to get it to go back through the
bore. Don't force it. It should go through one spot with just a tiny
bit of drag. If it goes through easy then it wasn't centered the first
time. You will have to try a few times to get the feel of it. Then,
mike the gauge and rock the gauge through the mike with the same
amount of drag you got in the bore. Finally, after measuring the gauge
by rocking it through the mike measure it by screwing the mike down
onto it several times until you find the largest reading. Compare this
reading to the one you got rocking it through the mike. It should be
just a couple tenths larger. As a test, turn two slugs to the
different diameters. The one measured by rocking the gauge should just
fit. The one that is turned to the diameter measured by just bringing
the mike to touch the gauge should not. Most important is to use
always a light touch. Nothing should be forced.