Understanding Crankshaft Diameter Measurement

Measuring the diameter of the crankshaft is a crucial step in engine maintenance, repair, or rebuilding. The crankshaft has main journals and connecting rod journals whose diameters must be within specific tolerances. Using the correct measuring tool ensures accuracy and helps identify wear or damage.

Choosing the Right Tool for Crankshaft Measurement

The question asks about measuring the diameter of the crankshaft. The crankshaft is a solid shaft with circular journals on its exterior surface. To measure the diameter of these external circular features accurately, a tool designed for measuring outside dimensions is required.

• Bore dial gauge: This tool is used to measure the inside diameter of bores, such as cylinder bores or bearing bores. It is not suitable for measuring the outside diameter of a crankshaft journal.
• Feeler gauge: A feeler gauge is used to measure small gaps or clearances, such as valve clearances or bearing clearances (when plastigage is used). It does not measure diameter.
• Outer-measuring micrometer: Also known as an outside micrometer, this precision tool is specifically designed to measure the external dimensions of objects, including diameters of shafts like the crankshaft journals. It works by using a calibrated screw mechanism to determine the distance between two measuring faces (an anvil and a spindle).
• Inner-measuring micrometer: Also known as an inside micrometer, this tool is used to measure internal dimensions, such as the diameter of holes or bores. It is the opposite of an outer-measuring micrometer and cannot measure the external diameter of a crankshaft.

Based on the function of these tools, the outer-measuring micrometer is the appropriate instrument for accurately measuring the diameter of the crankshaft journals.

Why Outer Micrometer is Used for Crankshafts

The crankshaft's main journals and crank journals (where connecting rods attach) are external cylindrical surfaces. To get an accurate reading of their diameter, you need a tool that can span across the outside of the circle. An outer micrometer, often simply called a micrometer, does exactly this. Different sizes of micrometers are used depending on the diameter range of the journals being measured.

Measurement Process Steps (General)

While the question only asks about the tool, understanding the process helps:

1. Clean the crankshaft journal thoroughly.
2. Select an outer micrometer with a range suitable for the journal diameter.
3. Calibrate or check the micrometer's zero point.
4. Place the micrometer squarely over the journal, ensuring the anvil and spindle contact the journal surface.
5. Apply consistent measuring pressure (often using the ratchet stop or friction thimble).
6. Read the measurement on the micrometer scale.
7. Repeat the measurement at several points around the journal and along its length to check for ovality or taper.

This precise measurement using an outer-measuring micrometer is essential for determining if the crankshaft journals are within specifications or if machining (like grinding) is required.

Revision Table: Measurement Tools

Additional Information: Crankshaft Journals and Wear

The main journals support the crankshaft within the engine block's main bearings, and the crank journals connect to the pistons via the connecting rods. These journals are subject to significant friction and wear during engine operation. Regular measurement using a precise tool like an outer-measuring micrometer is vital to assess their condition. Wear can lead to reduced oil pressure, knocking noises, and eventual engine failure. If the journals are worn or out of round, they may need to be ground down to a smaller, standard undersize or the crankshaft may need replacement. The correct measurement ensures the proper clearance with the bearings.