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.
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
|Fowler 2 in–6 in, 0.005-in graduations
||eBay; Beach Audio
|Powerhouse 2 in–6 in bore gauge
|Mitutoyo 2 in–6 in, 0.0001-in graduations
||Long Island Indicator Service
|Mitutoyo 0.7 in–6 in, 0.0001-in graduations
Barrington Engines; 818/442-9409
Long Island Indicator Service; 631/269-9357; LongIslandIndicator.com
Dial Bore Gauge: Powerhouse Products; 800/872-7223; PowerhouseProducts.com
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 it
This is Don Barrington’s Sunnen dial-bore gauge that offers a wide measurement range and a
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 thi
We then measured the same bore with both our Mitutoyo and Peacock dial-bore gauges, obtain