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Dial Bore Gauges

A dial bore gauge is a special purpose dial indicator for checking bores, and has such they are pretty expensive. The most accurate dial bore gauges have an accuracy of 0.0001 in. , or 1/10,000 in. ! Less expensive dial bore gauges will check to within 0.0005 in.
The dial bore gauge consists of a dial indicator at one end of a long stalk and a tripod arrangement positioned 90°to the stalk at the other end. The tripod is made up of a single interchangeable post that goes against one side of the bore being checked and two hardened buttons at the other end that automatically center the plunger in the bore. The gauge plunger is between the two buttons, directly opposite the post. To allow for different bore sizes, the post is interchangeable. This allows you to measure the big end of a connecting rod or the diameter of a cylinder bore.
Reading Technique---To achieve a reading, the gauge is first zeroed. This is done by measuring across the gauge with an outside mike set to the specified bore size and rotating the dial face until 0 aligns with the needle. The gauge is then inserted into the bore to the desired depth and rocked back and forth until the lowest reading is achieved. When the gauge is square to the bore and the indicator needle reverses direction, the lowest reading is read. This may be on the plus or minus side of the zero, indicating an oversize or undersize bore. Because it’s so fast and accurate, the dial bore gauge is most helpful while honing bores for fitting pistons.
Although this tool is the easiest and most accurate one to check bores, the cost of a dial bore gauge restricts its use to the professional engine builder or engine machinist. But if you’re a very well healed do-it-yourself and you feel you must have one. Great! Get a dial bore gauge , if you’ve ever done any engine building without a dial bore gauge, you’ll wonder how yuu ever got along without one.