The Dial Bore Gage (Gauge)

Sunnen Dial Bore Gage SetOne of the most important measuring instruments to inspect an engine block is the dial bore gage (gauge). The dial bore gage is specifically designed to take measurements of inside diameters, which makes it the perfect instrument for measuring an engine block’s main housing bores and cylinders. In the picture to the right you can view a Sunnen dial bore gage in its case. As you can tell by the picture, the gage has a variety of extensions that make it possible to measure small and large inside diameters alike.

Dial bore gages are made by many different manufacturers including Fowler, Starrett, Sunnen and other well known companies. The indicator on a dial bore gage often provides readings in increments of .0001” or .0005” and is available with digital displays as well. In the automotive industry, most machine shops tend to use dial bore gages that provide readings in .0001” increments.


Using Dial Bore Gages


Base of Dial Bore GageIf you click on the picture above you will see a larger image that shows a gage point selection table. This table is used to easily pick the correct numbered extension (pictured left) for the housing bore to be measured. Each extension has a lock nut on it so that the extension can be locked in a fixed position for accurate readings. Before the nut is locked, the threaded extension is screwed in to a position that is close to the dimension being measured. This can be set with a rule, micrometer or a setting fixture that acts as a hands-free micrometer. Once the extension is roughed in, the lock nut can be tightened down. At this point the thumb screw on the gage’s face can be loosened and a micrometer or setting fixture can be used to set the indicator to “0.” Once set, tighten the thumb screw and the measuring instrument is ready to use.

Measuring Main Housing Bores

To measure the main housing bores the main caps must be torqued down to specification. The dial bore gage is then inserted into each bore on a slight angle and brought up to a position that reads the tightest point within the bore. Since main housing bores can wear with a spun bearing, it is important to check various positions of each bore. While the up and down reading may be within specification, the side to side clearance may be out of specification. The dial bore gage allows for easy measurements at any position, but the tang groove in each main housing bore may present a challenge for some that are not accustomed to using such a measuring device.

Checking Cylinder Bore Size

To check the cylinder walls of an engine block, the set dial bore gage is inserted into each bore at a slight angle and the tightest reading is recorded as the gage is brought to an upward position. Important areas to check are where the pistons and rings travel, especially near the top of each cylinder. Often a dial bore gage will reveal far more wear at the top of a cylinder where rings travel and less wear near the bottom where the piston skirts travel. Just like with the main housing bores, it is important to check various positions within each cylinder. Inspecting multiple points within each cylinder bore will help to identify runout and can also help to identify cracks within the cylinder wall.

 

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Use of Dial Bore Gages