Engine Building Fundamentals: Bore Gauge
A Bore Gauge or Dial Bore Gauge, as it may also be referred to, is used to accurately measure inside diameters.
And as its name implies, it's often used to measure the bore diameter of an engine block.
This isn't its only use though, and essentially it can be used to measure the inside diameter of many critical components.
The bore gauge is used, for example, to check and confirm bearing clearances for both the main bearings and big end bearings of the engine by measuring the inside diameter of the con rod big end journal with the bearing shells installed, or alternatively, the main bearing tunnel in the block, with the main bearing shells installed.
The bore gauge has a dial indicator on it, however the dial indicator doesn't specifically measure the component we're interest in.
Instead, the dial indicator is used to compare two measurements, and as such, first of all we need to set all zero, the Dial Bore Gauge at our required specification using a micrometer.
Once we've done this, we can the use the bore gauge to confirm a specific measurement.
If for example, we zeroed the Dial Bore Gauge with the micrometer set to exactly two inches, and the tunnel we're measuring showed a measurement of two and a half thou on the dial gauge, this would mean the tunnel diameter is 2.5 thou larger than our measurement, or in other words, we have 2.5 thou of clearance.