Dail-Bore Guages Compared - Car Craft Magazine

Everybody knows that the right way to build an engine is to use precision measuring tools to establish the proper internal engine clearances. The problem used to be that these tools were expensive (note the dramatic use of past tense). But precision measuring tools are becoming less expensive by the day. Sure, your favorite machinist has spent thousands on high-quality measuring tools—that’s why he is your favorite. But now there are plenty of opportunities to score less expensive tools, such as micrometers and dial-bore gauges, to do your own measuring. But how accurate are these tools? Cheap tools produce cheap results, and engine clearances are too important to take chances, so we thought it would be a good idea to investigate how less expensive tools stack up against the good stuff.

These are two budget dial-bore gauges we’ve been using for the last 10 years. The black plastic case contains a Mitutoyo 511-106, 2- to 6-inch range dial-bore gauge with 0.0001-inch increments. The wooden case contains a budget, 0.0001-inch Peacock dial gauge with a no-name 2- to 6-inch range adapter. The extensions configure each of these tools to measure between 2- and 4-inch diameters, such as on main bearing housings, the big ends of rods, or cylinder bores.
These are two budget dial-bore gauges we’ve been using for the last 10 years. The black pl
This is a close-up of the dial on our Mitutoyo indicator. Note that the gauge indicates its ability to measure down to 0.0001 inch. The small, 1–5 scale indicates the gauge has a 0.050-inch dial range. The large plastic cover protects the gauge from damage, and the locking collar allows you to position the indicator.
This is a close-up of the dial on our Mitutoyo indicator. Note that the gauge indicates it
This is Don Barrington’s Sunnen dial-bore gauge that offers a wide measurement range and accuracy to 0.0001 inch. A gauge-setting fixture is needed in addition to the dial-bore gauge itself. Barrington will use his Sunnen tool to compare our budget tools.
This is Don Barrington’s Sunnen dial-bore gauge that offers a wide measurement range and a

Dial-bore gauges are what are known as comparator gauges. You set the gauge to a known standard or diameter. Most budget guys use a micrometer, but for highly accurate work, you can purchase (for significant cost) large steel rings that are honed to a specific checking diameter and certified accurate to 0.000020 inch—that’s two-millionths of an inch. Professional automotive engine assembly demands accuracy to 0.0001 inch (one-tenth of one-thousandth), but not all tools deliver the same accuracy. It’s also important to mention that all gauges are calibrated and are the most accurate at 68 degrees F. If the tool or the component measured is warmer, let’s say at 90 or 100 degrees F, the results will change dramatically.

In our search for tools, we found a Powerhouse 2- to 6-inch dial-bore gauge set with a plastic case and a 0.0005-inch gauge for $110.81. That means the gauge’s smallest readout is only 0.0005 inch. This might sound acceptable, but if you are going to bother to measure clearances accurately, you want a gauge with 0.0001-inch increments. Powerhouse claims you can substitute a 0.0001-inch gauge for a little more money. For our test, we show how to use a dial-bore gauge and then test two affordable models and compare them using a pro-level $3,000 Sunnen tool from Don Barrington’s Van Nuys, California, machine shop. The results are enlightening.

Conclusion

In this test, we found that the better of our tools was only off by 0.0003 inch when measuring a 3.790-inch bore. This does not necessarily mean that measuring a smaller bore, such as a main bearing clearance, would generate a tighter discrepancy. If you were to perform a similar comparison test using your local shop’s Sunnen tool, you might discover just how accurate your tool is and compensate accordingly. We’d recommend comparing several different diameters to establish a pattern of accuracy. End

Dial-bore gauges are comparators, which means you must use a dedicated calibration ring, a gauge fixture, or a micrometer to set the diameter you wish to measure. We always use the same micrometer for both measuring journal diameter and setting up the dial-bore gauge. For this photo, we’re using a 2- to 3-inch micrometer set to 2.20 inches. Note the short extension plugged into the dial-bore adapter (arrow). We used this same procedure to set up our bore diameter of 3.790, which approximated the bore diameter of the cylinder we were measuring.
Dial-bore gauges are comparators, which means you must use a dedicated calibration ring, a
Barrington used his Sunnen system to record the bore diameter. It was difficult to get this photo, as Barrington was swinging the gauge to get his reading. Holding still for a long photo exposure is difficult, so forgive us that the photo is a little soft. Barrington’s Sunnen gauge generated a bore diameter of 3.7909, indicated on his gauge as 0.0009.
Barrington used his Sunnen system to record the bore diameter. It was difficult to get thi
We then measured the same bore with both our Mitutoyo and Peacock dial-bore gauges, obtaining readings of 3.7912 ( 0.0012) and 3.7915 ( 0.0015) inches, respectively. This produced measurements that were larger by 0.0003 and 0.0006 inches (respectively) compared with the Sunnen readout. While this discrepancy is not excessive, it does show how important good tools are; in this case, the Mitutoyo was twice as accurate as the cheaper Peacock.
We then measured the same bore with both our Mitutoyo and Peacock dial-bore gauges, obtain

Parts List
Description	PN	Source	Price
Fowler 2 in–6 in, 0.005-in graduations	72-646-300	eBay; Beach Audio	$85.77
Powerhouse 2 in–6 in	POW205025	Powerhouse	110.81
Mitutoyo 2 in–6 in, 0.0001-in graduations	511-444	Long Island Indicator Service	629.00
Mitutoyo 0.7 in–6 in, 0.0001-in graduations	511-932	Grainger	1,307.00