There's more than one way to measure the diameter of a bore. If you work in a machine shop chances are you have a diverse selection of measuring tools at your disposal. However, as a hobby machinist you often have to make due with the tools you have, and funds might not allow for more specialized measuring tools like a dial bore gage, telescoping hole gage, or tubular inside micrometer. The purpose of this post is to show a few alternative methods for measuring the diameter of a bore with a reasonable level of accuracy for the hobby machinist.

Measuring a hole using plug gages:
(Accuracy to within .001)

This method is pretty straight forward. Every beginner should have a set of plug gages ranging from .011 – .500. An inexpensive import set (Type: Minus, Class: ZZ) from Enco would cost you around $120 new, and even less if you can find a set in good shape off of ebay. And with a set of .011 to .500 you can measure a hole up to .999 in diameter. How? By using the .500 plug gage and the .499 plug gage. Also, if you add a single 1.000 plug gage to your .011-.500 set, you can now measure from .011 to 1.500! A single 1.000 plug gage costs about $10. Plug gages are very versatile and useful for many other things as well, which is why every beginner should have a set.

Here you can see that I'm measuring the diameter of the cylinder bore on my flame sucker engine using two plug gages, the .500 plug gage and the .221 plug gage – giving a total diameter of .721".

Some might argue that it is better to use a single .721 plug gage to measure this hole. They would be right in a way because using two gages can lead to an erroneous measurement if your hole is slightly out of round. However, if you remember to drill and then bore a hole to its final size you shouldn't have an issue with the hole being out of round.

Another limitation of this method is the level of accuracy. Plug gages step up in size in .001 increments, so you're accuracy is limited to within .001. Also, "minus" class ZZ plug gages are ground to within about .0002 undersized. So the .500 gage might actually measure anywhere from .4998 to .5000, so using two plug gages might introduce an error of up to .0004 if both gages were two tenths undersized. Keep that in mind.

Using a Brown & Sharpe Taper Bore Gage Set:
(Accuracy to within .0001)

B&S used to make a set of tapered bore gages that were similar to an adjustable parallel, but had a rounded edge so that they would fit snugly inside a hole. To my knowledge, nobody is making this type of measuring gage any longer, but you can still find them used on eBay for a reasonable price.

To use these gages you insert them into the hole and slide them past each other along the tapered edge until they grip the sides of the hole. Then you can measure across them using a micrometer. You can see that my micrometer reads .721, precisely the same measurement as the plug gages. However, since I'm using a micrometer that measures to within tenths, the accuracy of this method has the potential to be more accurate than using plug gages – which only measure to within .001 (not to mention the fact that minus class ZZ plug gages are undersized by up to two tenths).

I mention this method because you can occasionally find a set of these taper bore gages on eBay for $20-30, and they don't take up a lot of space in your tool box. Also, don't quote me on what these gages are actually called. I've seen them listed as "Tapered Bore Gages", "Adjustable Taper Hole Gages", and various other names. If anyone has an old B&S catalog, please look them up and let me know what their true name is. Thanks!

Using a Caliper Type Inside Micrometer:
(Accuracy ranges from .001 to .0001 depending on your micrometer)

I also happened to have a caliper type inside mic that I found for $20 on eBay so I thought I'd also show that method of measuring.

Again, this is a specialized micrometer and not as versatile as plug gages which have multiple uses beyond measure holes. This type of micrometer also suffers from a similar limitation when used with holes that aren't perfectly round, which is probably why you'll usually find them with graduations limited to an accuracy of .oo1.

Using a Starrett Taper Gage:
(Accuracy to within .001)

I don't own a set of these taper gages, but they occassionally come up on eBay for a reasonable price. I've only seen ones made by Starrett, but I'm sure other companies make them as well.

One of their limitations include measuring a bore with a chamfer. If your hole has a chamfer or is slightly enlarged at the opening you won't be able to get an accurage measurement. However these would be useful to quickly check the diameter of a hole while you're boring it to size. Once you're within .002 you can switch to a more accurate method of measuring to finish boring the hole.

Summary:

As I mentioned in the first paragraph, there are definitely better and more accurate methods of measuring the diameter of a hole. But as a hobby machinist it sometimes makes more sense to spend your money on versatile tools that are useful for multiple different things, rather than on specialized tools that are made for a taking a specific type of measurement. So if you're new to the hobby, don't run out and buy a $100+ dial bore gage right away, especially when you can spend that same amount of money on a set of .011-.500 plug gages. Unless you need accuracy to within tenths, the plug gages will prove a more worthwhile expenditure, and your plug gages will see much more use in your shop over the years than your dial bore gage ever will.

Do you have another interesting method for measuring hole diameters? Or perhaps you know the true name of the B&S taper bore gages. If so, please post it by visiting the forum topic linked to this post (via the link below). Thanks!