View Full Version : Need help & ideas - small ID measurement

03-05-2005, 11:46 AM
Had a boring job last week & didn't have the right measuring tool. Bored a cast iron water pump for a newer style press-in seal. Needed to finish at 1.0015" - 1.0019" hole ID.

Problem was that the bored area was located way down below the top surface. Too far down for my ID mikes to get to. A holemike would be perfect - gonna buy a set as soon as I win the lottery. I got through that job by turning a plug gage to 1.00165", but that's not the answer.

How do YOU measure small ID? Just need to be able to measure to tenths, from 0.5000" up to 2.0000". I really appreciate your help on this.

[This message has been edited by precisionworks (edited 03-05-2005).]

03-05-2005, 11:51 AM
How about turning two pins to 1.0015 and 1.0019 (if I remember the tolerance right)? Once you are able to get the go pin in the hole you can stop opening up the hole.


03-05-2005, 12:16 PM

Don't know if I'll ever do that job again. Looking for a way to measure the whole range from 1/2" up to 2".

03-05-2005, 12:17 PM
I use telescopic gages. Set ranges from 1/2"-6". OK for holes deep enough to get the gages into. BTW, 1) If your bore was to be between 1.0015"-1.0019", how did your plug gage at 1.00655" work? 2) What are you using to measure a diameter down to 100/1,000"?

03-05-2005, 12:53 PM
Probably get sick of hearing this answer, but I did pretty well lurking eBay. It took a few weeks of haunting, but I picked up two Federal dial bore gages for $40 (for both) that range from .6"-1.25" & 1.15"-2.5". Since bore gages make a comparative measurement, you need to set them to a particular dimension (which can be anything in the gage's range) using gage blocks, or you can even use a micrometer if you're careful. Once the gage is set, you can measure tenths easily and the gage will tell you how far away the bore is from the dimension you previously set it to.
Hole micrometers are nice, but they tend to be expensive even on eBay and they are a bit more vulnerable to dirt between the anvils and the measuring surface. The bore gages are actually better for detecting tapers in the bore. A hole mic gives you an absolute dimension and has a relatively wide range of measurement with one setup - a bore gage only has about .030" range of measurement, but interchanging and adjusting the fixed "anvil" allows for a wider range of bores with the same gage - for example, I have one larger bore gage that measures 2" to 8.5"!
.4" is about the smallest bore gage I've seen and that might be the physical limit.
Federal and Standard are two popular and high-quality gages that often sell inexpensively. Mitutoyo makes a good one and there are likely many other good brands.


p.s. I picked up the large bore gage at a pawn shop for $50, complete and in pristine condition. That might be on the lucky side, but they are out there. Of course I wasn't looking for it either - somehow the tools know when to hide and only expose themselves when they can't sense that you need them.

[This message has been edited by vinito (edited 03-05-2005).]

03-05-2005, 12:54 PM
I use telescopic gages. JRouche

03-05-2005, 04:40 PM
Bore gauge, set an outside mike to your dimension and then zero the gauge to the mike.


[This message has been edited by Carl (edited 03-05-2005).]

Jim Luck
03-05-2005, 05:35 PM
An Indi-Cal Caliper Gage set up with a tenths indicator may work, but there's nothing better than the proper equipment. I am used to working to a half a tenth but damn, you really are splitting hairs!!!, I'm jealous --

03-05-2005, 06:59 PM
Splitting tenths really is easier than it sounds. I wear an OptiVisor magnifier. With that, it's not hard to work to a quarter-tenth, a half-tenth, or three quarters-tenth. (Much beyond that would be tough <LOL> )

Just need to check calibration on your mike before you start, and it helps if shop temp (and workpiece temp) are fairly constant. Otherwise you end up chasing the target measurement.

Forrest Addy
03-05-2005, 07:55 PM
I'd check that bore to the limits of my reach with a telescope gage and mike and assert my readings as accurate within 0.0002" had I access to a calibrated length standard.

I've checked holes to the inside corners out of reach of a bore gage using nothing more than inside calipers. This is a skill intensive procedure and it's doubltful if it will pass a third party inspection verification unless they have full faith in the operator providing the data.

A cheap Chinese bore gage set to a stack of gage blocks will give you reliable readings to 0.0001" resolution depending on the dial indicator its furnished with. Since it's a comparative reading and the measurement can be verified between the gage blocks and the part, the reading will pass third party inspection.

Watch your temperatures. 30 degree F difference between the part and the gage blocks will eat up 1/3 of your tolerence.

[This message has been edited by Forrest Addy (edited 03-05-2005).]

03-06-2005, 08:21 PM
PrecisionWorks, Three pin method, If one has 3 pins of equall dia, the circumscribed dia is:
D=(d/cos30*)+d, now using the standard -.0002" pin sets if one averages 3 pins obviously there is some error using that equation but in fact it is usually .000020/.000040" greater and is not a problem to use, I have a spread sheet for pins up to 1.000", I have pins up to 0.750" at home which gets me from .011" to 1.6124" using the pin as is or 3 pin method. Your example 1.0015" to 1.0019", (.4630+.4640+.4670)=0.4647"avg/cos 30*+0.4647"= 1.0012" circumscribed, next set in table = (.464+.465+.466) = .4650" ave = 1.0019" avg, for work as close as your example you would have to measure each pin used in a set and take the actual average as if each of you pins are -.0002" then it changes their circumscibed result, #1 = 1.0008" so on. As a series they increment about .0007" each. It is quite amazing how close you can get to the size you need to measure. Basicaly at the low end .099/.100/.101 ave .100 circum dia = .2155" calculating out to the real inscribed dia probably has about0.000009" Error, at the larger end of the pins say .899+.900+.903 =.900667 avg has an error of about 0.000004"
For general work the table value is close enough, for tight work measure each pin used and re calc D=(d/cos30*)+d
Les H.

03-06-2005, 10:40 PM
Thank you one and all. You've really helped me examine this in ways I'd never thought about.

Vinito, Carl & Forrest, the bore gages are now on my search list. They should do well to fill in the gap in between 1/2" & 2". Seen a bunch of these but never bid on them as I wasn't sure how well they'd work.

Jim, I use an OD IndiCal (not sure if they're still made) & like the way it works. Set it up with gage blocks or adjustable parallels and go. This is also on the search list.

Lew, your method is extraordinarily accurate. It seems like a variation on using toolmakers pins of know OD to precisely locate existing hole centers. Thank you!

Barry Milton