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Shop Tools: Bore Gauge

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What is a bore gauge 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 a dial bore gauge, 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.

A Dial bore gauge is also known as comparator gauges. You set the bore 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 dial bore 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 bore 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 bore gauge is and compensate accordingly. We’d recommend comparing several different diameters to establish a pattern of accuracy. End

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 bore gauge 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

Sources

Barrington Engines; 818/442-9409

Long Island Indicator Service; 631/269-9357; LongIslandIndicator.com

Dial Bore Gauge: Powerhouse Products; 800/872-7223; PowerhouseProducts.com

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1 comments
waynep712222
waynep712222

with the sunnen dial bore gauge setting master.. please check the ends of the standards for wear marks.. the standards are the precision sections that extend the range of the micrometer setting base for different bore sizes.. there is a 2", 3", 4" and 5" version in the case.. see picture 3 abovethe carbide balls on the dial bore gauge wear divots into the ends of the standards and this throws off the actual measurement you have dialed in..

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