Why do manufacturers use colour-coded bearings?

Modern blocks, caps, and crankshafts are made to tight tolerances, but normal tolerance stack‑up still exists. To hit the target oil clearance on every engine without rework on the production line, OEMs use selective assembly:

Measure housing bores and journal diameters at the factory.

Assign each to a grade (e.g., A/B/C or 1/2/3).

Fit the prescribed colour‑coded bearing thickness for that grade combination.

Key point: Colour steps are micron‑level adjustments. They are still standard-size bearings—not undersized.

Note: Colours on a used engine reflect as‑built status and no longer guarantee correct clearance after wear or machining.

Section 2 — Aftermarket STD vs. colour-coded OE grades

Aftermarket STD bearings (including Gemini Components STD) are manufactured to the design‑nominal wall thickness for in‑spec housings and journals. OE colour grades nudge that thickness up or down by a few microns to compensate for production variance.

In service work, you should:

Use STD bearings when journals and housings are within spec and oil clearance is correct.

Regrind and fit matching undersize shells (e.g., −0.25 mm / 0.010 in) when journals are worn, tapered, oval, or below size beyond limits.

Do not assume colour = undersize. Colour = micro‑grade STD; undersize applies only after a specified regrind.

Section 3 — Why measuring is the only correct method

Oil clearance establishes the hydrodynamic film that separates metal surfaces. Too tight → seizure risk; too loose → low pressure, knock, rapid wear. Because engines vary in wear and prior machining, the only dependable way to choose bearings is to measure and calculate.

Section 4 — Practical measurement workflow

Tools: Outside micrometers (0.01 mm / 0.0001 in), dial bore gauge (mains & rods), torque wrench, Plastigauge (cross‑check), solvent and lint‑free wipes.

Procedure:

Preparation: Clean; inspect for scoring/fretting/heat; deburr faces.

Journals: Measure at 0°/90° and two widths; record min/max, taper, ovality; compare to limits.

Housings: Torque caps to spec; measure diameters and roundness; confirm alignment.

Trial with STD shells: Install dry; torque; determine assembled bearing ID; clearance = ID − OD; compare to spec.

Decision: In‑spec → STD. Tight → consider light polish within limits. Excessive/out‑of‑spec → regrind to next undersize and fit matching undersize; re‑measure.

Section 5 — Installation essentials

Cleanliness is critical.

Ensure oil holes/grooves align; tangs oriented correctly.

Check bearing crush & seating; shells slightly proud before torque.

After regrind, confirm surface finish and fillet radius suit the bearing.

Use assembly lube; prime before first start.

Rotate the crank by hand after each cap; investigate any tight spots.

Section 6 — Common misconceptions

“Old shells were green; I need green again.” Not necessarily; colours reflect new‑build grading.

“Colour‑coded bearings are better than aftermarket STD.” They’re grading steps around STD; measured clearance decides.

“Plastigauge alone is enough.” Helpful check, not a substitute for micrometers and a bore gauge.

Section 7 — Quick decision flow

Start → Clean & Inspect → Measure Journals & Housings → Trial with STD shells → Calculate Clearance →

• In‑spec & clearance in range → Use STD bearings → Assemble

• Clearance tight (journals large but in spec) → Light polish if allowed → Re‑measure

• Worn/out‑of‑round/excessive clearance → Regrind to next undersize → Fit matching undersize → Re‑measure

Need help choosing bearings?

We stock premium bearings from trusted brands. If you’re unsure whether to remain STD or regrind to −0.25/−0.50 mm, our team can help you review measurements.

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Safety note: Always follow the engine manufacturer’s service data for dimensions, torque values, and clearance limits.