STD, 0.25, 0.50 — and why you also see 010 on a bearing box

Engine bearings are ordered by size grade, not just by part number, and there are two grading conventions in circulation. Ordering a 0.25 when the crank was ground to 0.010" is a wasted shipment — the numbers look similar and mean different things.

What the grade actually describes

A bearing shell is not machined to fit a specific crankshaft. It is produced in steps. STD (standard) fits a crankshaft at its original journal diameter. When a journal is worn or scored, a machine shop grinds it smaller and you fit a correspondingly thicker shell to close the gap back to the design clearance. That thicker shell is the undersize.

So the number on the box is how much has been taken off the crank, not how much bigger the bearing is. It is a common source of confusion and it is worth being explicit about when you order.

The two conventions

Note how close 0.25 mm and 0.010" are — about four microns apart. They are near enough that people assume they are interchangeable labels for the same step. In practice the shells are produced to different nominal thicknesses, and which one you need depends on which system the machine shop used when it ground the crank. Ask the shop, or measure the journal.

Why both exist in one catalogue

Japanese and European engines are almost always graded in millimetres. American engines — and anything built to an imperial drawing — are graded in thousandths of an inch. A mixed fleet means a mixed parts list. In the Tecnopart catalogue the split is roughly what you would expect: STD is by far the largest group, followed by 0.25 mm, then the imperial 010, then 0.50 mm. Both conventions appear across main and connecting-rod shells.

Ordering without a return

• Give the engine code and the grade together. "6BT5.9 con-rod bearing 0.25" is unambiguous. "Cummins bearings" is not.
• If the crank is already ground, ask the shop what it was ground to, in the units they used. Do not convert and hope.
• If the engine has never been apart, order STD — but if it has any history you cannot verify, measure before you buy.
• Main and connecting-rod grades are independent. A crank can have mains ground and rod journals still at standard, and often does.
• Thrust washers are graded separately again, on end-float rather than journal diameter.

One more thing worth checking

Bearings are supplied as a set for the engine, and set contents differ between engine variants that share a code. If a J08C set and a J08E set look interchangeable on paper, check the shell count and the presence of flanged shells before ordering — the parts that are actually shared are usually fewer than the codes suggest.