What is the standard size of a crankshaft?
There is no universal standard size for crankshafts. Engine designers tailor the main and rod journals, throw length (stroke), and overall geometry to fit a specific engine family, block, and bearing arrangement. In practice, sizes are defined by manufacturer specifications and may be listed as standard or undersize during rebuilding. This article explains how crankshaft size is defined and how to determine the correct size for a given engine.
What defines a crankshaft's size
Crankshaft size is determined by several core measurements that ensure the crank fits the engine block and bearing assemblies. The primary dimensions are:
- Main journal diameter — the diameter of the journals that ride in the main bearings, setting block-to-crank clearance.
- Rod journal diameter — the diameter of the journals on which the connecting rods bear, governing rod bearing clearance.
- Number and arrangement of journals — typically two or more mains and two rod journals; the throw count and spacing influence balance and fit.
- Stroke length (crank throw) — the distance the crank throws move the piston, determined by the offset between journals and the crank's throw geometry.
- Overall length and counterweight design — ensures the crank clears the block, oil passages, and other components, and provides proper balance.
Different engine families have different standard sizes and tolerances. There is no single “standard crankshaft size” that applies to all engines.
Rebuilding: standard vs undersize
When rebuilding an engine, machinists determine whether to use standard dimensions or grind the crank to an undersize to restore roundness and proper clearance. The process follows established specifications for the engine family and bearings.
- Obtain the engine's official specification sheet or service manual for crank and bearing sizes and tolerances.
- Measure the crank journals with precision instruments (micrometer, bore gauge) to assess wear and roundness against standard sizes.
- Decide if grinding to an undersize is required to restore proper clearance and oil film stability (common undersizes are referenced in the specs).
- Select appropriate bearings that match the chosen undersize and the engine's clearance requirements.
- Have a qualified machine shop grind the journals to the chosen undersize and finish the surfaces to spec, then reassemble and verify clearances.
By following these steps, a rebuild can achieve the correct oil clearance, bearing fit, and engine balance essential for reliable operation.
Finding the exact size for your engine
To determine the specific crankshaft size for your vehicle, consult the engine's service manual, OEM parts catalogs, or a qualified engine machinist. Provide the engine family, displacement, and serial or VIN if needed, as many engines share family-level specifications but diverge by revision.
Common sources include the OEM service documentation, aftermarket crankshaft suppliers, and reputable machine shops that can measure and verify journal diameters and tolerances before grinding.
Summary
There is no universal standard size for crankshafts. The correct size depends on the engine design, with main and rod journal diameters, stroke, and overall geometry tailored to each engine family. For rebuilds, adherence to manufacturer specifications and precise measurements are essential, and a machine shop can determine whether standard sizes or undersizes are required to restore proper clearance and balance.
