A bearing clearance test measures the tiny gap between a bearing surface and the shaft (or journal) it supports. In most cases, this gap is measured in thousandths of an inch, and getting it right is essential for engine longevity. The test tells you whether that gap falls within the manufacturer’s specified range, so oil can flow properly and metal never touches metal.
Why Bearing Clearance Matters
Inside an engine, bearings don’t actually touch the crankshaft journals they wrap around. Instead, a thin wedge of pressurized oil separates the two surfaces, allowing the shaft to spin freely. This is called hydrodynamic lubrication: as the shaft rotates, it drags oil into a narrowing gap, building enough pressure to support the load and keep the surfaces apart. The clearance between the bearing and journal is what makes this oil wedge possible.
If the clearance is too tight, there isn’t enough room for oil to flow through. Friction generates heat, the metal expands further, and the bearing can seize. This process can escalate rapidly: localized frictional heating causes thermal expansion, which increases contact pressure, which generates even more heat. In severe cases, this leads to catastrophic thermal seizure where the bearing welds itself to the shaft.
If the clearance is too loose, oil escapes too easily and can’t build enough pressure to support the load. You’ll notice symptoms like a knocking or tapping sound from the engine and low oil pressure readings on the gauge. Excess clearance also introduces vibration and misalignment, accelerating wear on every surrounding component.
Typical Clearance Ranges
A common rule of thumb is 0.001 inches of clearance for every inch of shaft diameter. So a crankshaft journal that measures 2.000 inches across would call for roughly 0.002 inches of oil clearance. That’s about the thickness of a sheet of paper.
Several factors push the ideal clearance tighter or looser. Tighter clearances work well with smaller journals, lower viscosity oils, very smooth surface finishes, and precisely machined cranks and blocks. Looser clearances are appropriate for larger journals, higher viscosity oils, rougher surface finishes, or engine blocks with housing bores that may distort under high RPM. The general principle from bearing manufacturers: run the least amount of clearance you can get away with. Tighter clearances reduce peak loading on bearings, produce less vibration, and help the engine run smoother.
The Plastigage Method
Plastigage is a thin strip of crushable wax material, and it’s the most popular way to check bearing clearance during an engine build or inspection. The process is straightforward and requires no precision measuring tools beyond the Plastigage kit itself.
Start by separating the bearing cap from the journal and making sure both surfaces are completely clean and dry. Oil residue will distort the reading. Cut a piece of Plastigage to the width of the bearing journal and lay it across the surface. Then reinstall the bearing cap and torque the bolts to the manufacturer’s specification. Don’t rotate the crankshaft while the Plastigage is in place.
Remove the cap again, and you’ll see that the Plastigage strip has been flattened. Compare the width of the flattened strip to the graduated scale printed on the Plastigage packaging. A wider, more squished strip means tighter clearance. A narrower strip means more gap. You can interpolate between the markings on the scale for greater precision. Repeat this for every bearing in the engine, both main bearings and rod bearings, since clearances can vary from one journal to the next.
The Micrometer Method
For higher precision, many engine builders use micrometers and dial bore gauges to calculate clearance mathematically. This approach measures each component individually and then subtracts to find the gap.
First, you measure the crankshaft journal diameter with an outside micrometer. Then you measure the inside diameter of the bearing installed in its housing using a dial bore gauge, taken at 90 degrees to the parting line where the two bearing halves meet. Subtract the journal diameter from the bore diameter, and the result is your oil clearance.
This method gives you a more precise number than Plastigage and lets you identify exactly which component is out of spec if the clearance doesn’t fall within range. It’s especially useful when you’re mixing bearing sizes to dial in a specific clearance. Bearing manufacturers sell shells in standard, undersized, and oversized thicknesses, and you can even pair a standard half-shell with a slightly thicker one to split the difference.
Bearing Crush and Its Role
One detail that trips people up is the difference between bearing clearance and bearing crush. They’re related but measure different things. Clearance is the gap between the bearing’s inner surface and the journal. Crush is about how the bearing shell fits into its housing.
A bearing shell’s outside diameter is slightly larger than the housing it sits in. When you torque the cap down, the shell compresses slightly, creating a tight press fit that holds the bearing in place and ensures good heat transfer to the housing. This compression is called crush. If crush is insufficient, the bearing can spin in its housing. If crush is excessive, it can distort the bearing’s round shape and throw off your clearance measurement. This is why bearing clearance is always measured with the cap torqued to spec: you need the crush to be established before the clearance reading means anything.
When This Test Gets Done
Bearing clearance tests happen during engine builds, rebuilds, and any time the bottom end of an engine is disassembled. If you’re replacing bearings, resurfacing a crankshaft, or building an engine from new components, checking clearance at every journal is standard practice. Skipping this step is one of the fastest ways to destroy a fresh engine.
The test also matters outside of automotive engines. Industrial equipment, turbines, compressors, and any rotating machinery with journal bearings relies on proper clearance for the same reasons: oil film integrity, heat management, and vibration control. The principles and measurement techniques are the same, though the specifications and tolerances vary with the application.