What the marking is actually telling you
A crankshaft journal wears. Rather than replace the shaft, a machine shop grinds it back to a true circle at a slightly smaller diameter, and the replacement bearing has to be correspondingly thicker to close the gap. STD means the shaft is at its original diameter and the bearing is nominal. Everything else on the label is the amount that came off the shaft — not the thickness added to the shell.
Across our catalogue about 3,800 line items carry a size marking of some kind, and almost all of them are engine-block parts: main and con-rod bearings, camshaft bushes, thrust washers. Roughly 2,600 are marked STD; the rest are ground sizes.
Two unit systems, one dimension
This is where most wrong orders start. 010 is not a tenth of a millimetre. It is 0.010 inch, and 0.010 inch is 0.254 mm — near enough the same cut as a bearing marked 0.25. They are the same part expressed in two measuring conventions, and a buyer searching for one will not find the other.
| Imperial marking | In inches | In millimetres | Metric equivalent |
|---|---|---|---|
| STD | nominal | nominal | STD |
| 010 | 0.010" | 0.254 mm | 0.25 |
| 020 | 0.020" | 0.508 mm | 0.50 |
| 030 | 0.030" | 0.762 mm | 0.75 |
| 040 | 0.040" | 1.016 mm | 1.00 |
The metric and imperial figures are close but not identical — 0.254 against 0.25 mm. For a bearing shell that difference sits inside the running clearance, which is why the two conventions have been able to coexist for decades. It is still worth confirming which system a supplier's label follows before you order.
The brand pattern — and why you cannot rely on it
There is a rough split, and knowing it helps you guess. Imperial markings cluster on American and older British-lineage engines; metric markings dominate on European, Japanese and Korean ones.
- Mostly imperial in our catalogue: Ford, Fiat, Detroit Diesel, Perkins, Caterpillar, Jeep
- Mostly metric: Mercedes-Benz, Komatsu, Renault, Hyundai, Cummins, Toyota, Kia
But treat that as a hint, never a rule. Several makes appear in both columns: Ford, Detroit Diesel, Caterpillar and Fiat all have parts marked each way, usually split by engine generation or by which plant the part came from. Cummins is largely metric despite being an American engine. Read the label on the part in front of you rather than inferring it from the badge.
Where the size hides in the part number
Most suppliers carry one base number for the family and append the size, which is why two numbers can look almost identical and be different parts:
12341A60D00-0A0— main bearing, STD12341A60D00/050— same bearing, 0.50190918M/010and190918M/020— one main bearing set, two ground sizes
A number quoted without its suffix is incomplete. If you copy a reference off an old invoice and the size has been trimmed from the end, the enquiry cannot be answered without going back to you.
What to send us
Three things settle a bearing enquiry first time: the engine model, the OEM number including any size suffix, and the size you need — quoted exactly as your machine shop wrote it, in whichever system they used. If the shaft has not been ground yet, say so and ask for STD. If you are not sure which convention the marking follows, send us a photo of the old shell; the marking is stamped on the back.
We source these through long-standing factory relationships across Taiwan and China, and we would rather ask one extra question than ship the wrong ten thousandth of an inch.