How do I know my rod bearing size?

Your rod bearing size is determined by your engine’s specifications and the wear on the rod journals. In practice, you’ll identify it from the factory spec, read any size markings on the bearing shells, and confirm with precise measurements of the crank journals. If wear has reduced the journal diameter, you’ll typically select undersize bearings to restore proper clearance.

Finding the official spec for your engine

The starting point is the engine’s documentation and parts catalogs. These sources list the exact bearing size family (standard, undersize, or oversize) that fits your particular model and year. When unavailable, you must rely on measurements and markings to determine the correct size.

• Check the manufacturer’s service manual or OEM parts catalog for the exact rod bearing size or shell code for your engine (make, model, year, and sometimes VIN).


• Inspect the bearing shells you remove for stamped markings such as STD, -0.010", -0.020", +0.010", etc., which indicate standard, undersize, or oversize sizes.


• Note that some engines use metric sizing, in which case measurements and size codes will be in millimeters rather than inches.


• Record all findings before ordering replacement bearings to avoid mismatches during reassembly.

Having the official spec or correctly read markings helps you avoid installing the wrong size, which could affect oil clearance and engine performance.

Confirming with measurements

If you don’t have immediate access to the manual, precise measurements of the crank journals and current bearings provide a practical path to the correct size. This is a two-step process: determine the journal size and then verify the bearing size that will achieve the target clearance.

• Measure the crank rod journals with a calibrated micrometer. Take multiple measurements around the circumference at a consistent location and at several axes to determine the true diameter. Compare this to the engine’s standard journal size in the factory spec.


• If the journal diameter is smaller than standard, you will typically use undersize bearing shells to restore the proper clearance; if it’s larger (uncommon after initial wear), you would use oversize shells. Always confirm the acceptable tolerance from the service manual.


• Inspect and measure the used bearing shells themselves. Note any markings and measure the inner diameter and thickness if possible.


• Optionally, use plastigauge to verify oil clearance after assembling bare components with the new shells. This test helps confirm that the chosen size brings clearance into spec.

Measurements should be taken with clean, dry tools and surfaces, and the results compared against the engine’s official specs. Relying solely on visual inspection can lead to mis-sizing and suboptimal engine performance.

What information you should collect

Gathering the right details ensures you order the correct parts and avoid repeat disassembly. Bring these items to your parts supplier or mechanic.

• Engine make, model, year, and VIN (if applicable).


• Current bearing shell markings (STD, undersize, oversize) and any stamped codes on the shells.


• Measured crank journal diameter (and tolerances) from a micrometer, plus the target clearance from the service manual.


• Recommended bearing size increments for your engine family (e.g., standard, -0.010", -0.020", +0.010", etc.).


• Any machining history (whether the crank has been ground and by how much, if known).

Armed with these details, you’ll have a clear path to the correct rod bearing size and a reliable rebuild plan.

When to seek professional help

If you’re unsure about measuring procedures, interpreting markings, or selecting the correct undersize/oversize proposals, consult a qualified mechanic or engine builder. Incorrect sizing can lead to excessive oil clearance, bearing instability, or accelerated wear, undermining the rebuild’s longevity.

Summary

To know your rod bearing size, start with the engine’s official specification and bearing shell markings, then confirm with precise crank journal measurements. Use undersize bearings when journals are smaller than standard, and oversize bearings if measurements indicate the journal is larger after machining. Verify clearance with plastigauge and, when in doubt, seek professional help to ensure accurate sizing and reliable engine performance.

How to tell what size a bearing is?

To measure a bearing's size, use a digital caliper or micrometer to take three key measurements: the inner diameter (ID), outer diameter (OD), and width (W). First, zero the caliper. Then, use the inner jaws to measure the ID, the outer jaws for the OD, and the side jaws or a rule for the width. 
You can watch this video to learn how to measure the inner, outer, and width of a bearing: 52sFastEddy BearingsYouTube · Feb 8, 2021
Step 1: Prepare your tools

• Ensure you have a digital caliper or micrometer, as these are the most accurate tools for the job. 
• Turn the caliper on and close the jaws completely. Press the zero function to calibrate it before you start. 
• Have a notepad and pen ready to record your measurements to avoid errors. 

Step 2: Measure the inner diameter (ID) 

• Place the inner jaws of the caliper inside the bearing's bore. 
• Slowly open the jaws until they make firm contact with the inside wall of the bearing. 
• Read the measurement on the digital display and write it down. 

Step 3: Measure the outer diameter (OD) 

• Place the outer jaws of the caliper around the outside of the bearing. 
• Gently squeeze the jaws until they are in firm contact with the outer edges of the bearing. 
• Read and record the measurement. 

Step 4: Measure the width (W) 

• Using either the side jaws of the caliper or a ruler, measure the width of the bearing's side from one end to the other. 
• Ensure the measurement is taken straight across the bearing. 
• Record the final measurement. 

How to determine rod bearing size?

Measure the bore of the Rod or Main. Then subtract the diameter of the Rod or Main journal on the crankshaft. A little bit of math will tell you what size bearing you need.

How do I know if my rod bearings are oversized?

An undersized bearing has extra material thickness on the inside of the bearing facing the crankshaft journal. Undersized bearings are used when a crankshaft has been machined or ground undersize. Oversize bearings have more material on the outside facing the bearing housing bore.

What size is a 6203 bearing?

A 6203 bearing has the following dimensions: 17mm inner diameter, 40mm outer diameter, and 12mm width. These dimensions are standard for this type of deep groove radial ball bearing, which comes in various configurations like open, sealed, or shielded.
 

• Inner Diameter (ID): 17 mm
• Outer Diameter (OD): 40 mm
• Width (W): 12 mm