Bearing shells are the cheapest component in an engine — and getting them wrong costs the most. A set costs a fraction of the price of a crankshaft, but a bearing one repair size too large or too small means either a seizure within a few dozen operating hours, or knocking and oil pressure dropping to zero. Either way, the engine goes back on the bench.
This post explains how to read bearing size markings, how the different types of bearings differ and which measurements you need before placing an order.
Types of bearings in a Cummins engine
Main bearings
Fitted in the engine block, they support the crankshaft along its entire length. They work under oil pressure and carry the loads of the whole crank mechanism. A set consists of pairs: an upper shell (usually with an oil groove and hole, because this is where the oil is fed in) and a lower shell (often plain, without a groove). They are not interchangeable — fitting them the wrong way round cuts off lubrication.
Connecting rod bearings
They connect the con rod to the crankpin. Smaller than the mains, but working under the highest dynamic loads. They are usually the first to show the effects of neglected oil.
Thrust bearings
They take up the crankshaft’s end play — its movement along its axis. They come either as separate half-rings (thrust washers) or as a main bearing with flanges. A worn thrust bearing gives a characteristic symptom: the crankshaft “walks” lengthwise, which can lead to serious damage at the clutch. In our catalogue they are listed as thrust bearings — e.g. the lower thrust bearing 3945664 or the +0.75 mm thrust main bearing 3283067.
Connecting rod (piston pin) bushings
Formally a separate group, but usually bought for the same rebuild — a thin bushing in the small end of the con rod in which the piston pin runs. You will find them in the connecting rod bushings category.
Repair sizes: what STD, +0.25 and +0.50 mean
This is the most common source of ordering mistakes, so it is worth understanding the logic. A note on terminology first: English-language catalogues usually call these undersize bearings (e.g. “0.25 U/S” or “0.010 undersize”), because they fit an undersize crankshaft; other sources mark the same bearings “+0.25” because the shell is thicker. Both describe the same part.
When crankshaft journals wear or get scored, the crankshaft is not scrapped — it is reground, which reduces the journal diameter to the next repair size. Since the journal is thinner, the bearing must be thicker so that the oil clearance stays correct.
Hence the markings:
| Marking | Meaning | When to use |
|---|---|---|
| STD (standard) | factory size | crankshaft not reground, nominal size |
| +0.25 mm (0.010″) | bearing 0.25 mm thicker | crankshaft reground to the first repair size |
| +0.50 mm (0.020″) | bearing 0.50 mm thicker | crankshaft reground to the second size |
| +0.75 mm (0.030″) | bearing 0.75 mm thicker | crankshaft reground to the third size |
| +1.00 mm (0.040″) | bearing 1.00 mm thicker | usually the last available size |
Remember the rule: the bearing size marking describes how much the crankshaft has been ground down — not how much bigger the bearing is on the inside. A “+0.50” bearing fits a journal that is 0.50 mm smaller than nominal.
Watch the units: some markings are given in inches (0.010″, 0.020″), others in millimetres (0.25, 0.50). These are the same sizes described differently — not two different variants.
One more point: the undersize of the main journals and the rod journals does not have to be the same. A crankshaft is sometimes reground on only one group of journals. Check both before you order a set.
Before you order: measurements you cannot skip
Bearings cannot be selected from the engine number or a photo. You need specific data:
- Measure the crankshaft journals with a micrometer — mains and rods, each in several planes. You are looking not only for the diameter but also for ovality and taper.
- Check the markings on the crankshaft. A reground crankshaft is often marked (stamped on a web or near the flange) with the repair size. It is a good clue, but it does not replace measurement — the engine may have been rebuilt more than once.
- Remove and inspect the old bearing. The size is often stamped or printed on the back. Note: it tells you what was fitted, not what is correct now.
- Check the oil clearance during assembly — with Plastigage or a bore gauge. This is the final check that the selection was right. Take the limits from the service documentation for your engine series.
- Measure the crankshaft end play after torquing the caps — this checks the thrust bearings.
If the journals are oval or scored beyond tolerance, bearings of a new size will not fix anything — the crankshaft has to be reground (or replaced; see crankshafts).
Symptoms of worn bearings
- Knocking from the bottom of the engine, deeper and lower than valve noise, getting louder under load and at low speed.
- Falling oil pressure, especially on a hot engine and at idle. More clearance = oil escapes sideways instead of holding the film.
- Metallic deposits in the oil pan or on the filter — particles of bearing material. This is an alarm: it means the process is under way.
- Oil pressure warning lamp or a low pressure fault code — see our guide to SPN/FMI codes (codes with FMI 1 describe a parameter that really is too low).
There is no point waiting: a worn bearing that seizes takes the crankshaft and often a con rod with it. The difference between a set of bearings and a crankshaft with con rods is usually an order of magnitude.
Why bearings wear
- Dirty oil and neglected filter changes. The number one cause. Particles harder than the bearing overlay act like sandpaper.
- Diluted oil. Fuel in the oil (e.g. from a leaking injector) lowers the viscosity and destroys the oil film — which is why the “rising oil level” symptom described in our post on worn injectors so often ends with the bearings.
- Oil starvation at start-up. Loading a cold engine before the oil has reached all the journals.
- Assembly error. Wrong tightening torque, dirt behind the shell, a shell fitted with the groove on the wrong side, no assembly lubricant.
- Engine overheating. High temperature lowers oil viscosity and the film stops carrying the load — another reason not to ignore overheating.
Assembly: five rules that save the set
- Absolute cleanliness. A single grain of sand behind a shell deforms its surface and creates a seizure point.
- Do not grind, scrape or “fit” the bearing. The running layer is a few dozen micrometres thick — any intervention destroys it.
- Watch the locating tang — it must sit in the notch in the housing.
- Lubricate with assembly oil before putting it together; the first start happens before the oil arrives under pressure.
- Tighten with a torque wrench, following the torque and sequence from the service documentation for the series. Bearings are sensitive to housing distortion.
How to choose a set in our shop
The surest ways to match:
- the part number from the old bearing or from the rebuild documentation — type it into the search; we handle superseded numbers and replacements (e.g. bearing 3016780 also appears as 172020, M-3016780, FP-3016780),
- the engine serial number (ESN) + the journal measurements — the complete information that lets us choose the right repair size; how to find the ESN is explained in a separate guide,
- a photo of the old bearing with the marking visible via WhatsApp: +48 728 626 259.
The full range is in the bearings category. If you are planning a rebuild, it is worth putting together pistons and rings, cylinder liners and a gasket set at the same time — we cover this in our guide to the piston and crank assembly.
Availability and lead time
Some sets are in stock in our warehouse in Poland and ship within 24–48 h. Items ordered in for you typically take 7–10 working days. For a rebuild it pays to order parts in advance: an engine stripped down and waiting for a set of bearings is the most expensive kind of downtime. We confirm the lead time for the specific part number before you buy — contact form.
Summary
The bearing size marking tells you how much the crankshaft was ground down — not how much bigger the bearing itself is. Selection is based on measuring the journals, not on the engine model, and the main and rod journals may have different repair sizes. Measure, write it down, then order — and check the oil clearance during assembly. It is half an hour of work that decides whether the rebuild lasts thousands of operating hours or a few dozen.
Have your measurements and looking for a set? Give us the journal sizes and the engine ESN — phone/WhatsApp +48 728 626 259 or the contact form.
