In short. Engine bearings in a diesel generator are plain shells that run on an oil film a few hundredths of a millimetre thick. They are specified from the engine serial and from the measured condition of the crankshaft journals, because a reground crankshaft needs undersize shells. A failed bearing is evidence about the oil supply, the journal or the load, and fitting new shells without reading that evidence usually produces a second failure.
What a bearing has to do
A crankshaft in a generator engine running at 1,500 rpm (the speed that gives 50 Hz on a four-pole alternator) turns twenty-five times a second, and at each firing stroke the connecting rod pushes down on its journal with a force measured in tonnes. The bearing does not carry that load on metal. It carries it on a wedge of oil drawn between the journal and the shell by the rotation, and the shell’s job is to keep that wedge stable: the correct clearance so that oil pressure builds, a soft overlay so that small particles embed instead of scoring the journal, and a strong backing so that the shell does not fatigue under the cyclic load. Thrust bearings or thrust washers at one main journal control the crankshaft’s axial float against the pull of the alternator coupling and the push of the clutch or flywheel.
From this it follows that a bearing shell is defined by its wall thickness to a few microns, its material layers and its locating features, and that a shell which fits in the housing is not automatically the right shell. The engine cooling and lubrication guide covers the oil circuit the bearings depend on.
Identification: serial number, then crankshaft size
The engine model and serial number identify the bearing family and the standard shell. They do not identify the size that the engine needs now. Crankshaft journals are reground at overhaul when they are worn or scored, and each regrind takes the journal to a defined undersize, typically in steps such as 0.25 mm, 0.50 mm and 0.75 mm according to the manufacturer’s scheme. The matching shells are thicker by the same amount. An engine that has had one overhaul may be on first undersize at the mains and standard at the big ends, or any other combination, and that information lives in the previous overhaul report or on the back of the removed shells, where the size is normally stamped.
If neither the report nor the old shells are available, the journals must be measured with a micrometer at two positions and two axes on each journal before shells are ordered. Ordering standard shells for a reground crankshaft gives excessive clearance, low oil pressure and rapid failure; ordering undersize shells for a standard crankshaft gives no clearance and a seizure on the first start.
| Bearing position | Load character | Size determined by | Typical supply unit |
|---|---|---|---|
| Main bearings | Combustion and inertia loads from all cylinders, transmitted to the block | Main journal diameters after any regrind | Set for all main positions, upper and lower shells |
| Connecting-rod (big-end) bearings | Highest unit loads at firing; most sensitive to oil film breakdown | Crankpin diameters after any regrind | Set per engine or per cylinder, pairs of shells |
| Thrust bearings or washers | Axial location of the crankshaft | Measured end float against the manufacturer’s limit | Standard and oversize thickness, often sold separately from mains |
| Camshaft bushes | Valve-train loads, lower speed | Camshaft journal size and bore in block | Set per engine, usually require line reaming after fitting |
Reading a removed bearing
Removed shells should be kept in order, photographed and read before anything is ordered. The wear pattern is the most reliable evidence available about what the engine has been through, and it is free.
- Fine circumferential scratches with embedded particles indicate dirty oil: a blocked or bypassed filter, a missed oil change or dust ingress past the air cleaner.
- Wiped or smeared overlay on the loaded half with discoloration indicates oil starvation or overheating: low oil level, a failed pump, a blocked gallery, fuel-diluted oil or coolant in the oil.
- Wear concentrated at one edge of a shell indicates a bent connecting rod, a tapered journal or a distorted housing bore.
- Fatigue cracking and flaking of the overlay indicates overload or detonation from an injector fault, or an engine run above its rating for long periods.
- A spun shell with the locating tab sheared indicates loss of crush, usually from a stretched or incorrectly torqued cap bolt or a housing that was already out of round.
Each of these calls for a different corrective action before new shells go in. Oil starvation calls for the pump, relief valve, pick-up and galleries to be checked and cleaned. Dirty oil calls for the air and oil filtration path to be inspected. Edge loading calls for the rod to be checked for bend and twist and the housing bores to be measured.
What a bearing replacement actually involves
Bearing work is never only the shells. The journal must be measured and, if scored or out of round beyond the limit, reground, with the oilways chamfered. Housing bores in the block and rods are checked for ovality and alignment, because a shell takes the shape of the bore it sits in. Main and big-end bolts on most modern diesel generator engines are torque-to-yield fasteners that stretch permanently on tightening and must be renewed, not reused. Clearance is then verified on assembly with a plastic gauge strip or by calculation from measured bore and journal, against the specific engine’s limit; there is no universal clearance value. The oil pump, relief valve and cooler are inspected, and after a bearing failure the whole lubrication circuit is flushed, because fragments of the old overlay are now circulating. Oil pressure is confirmed before the set carries load. The major overhaul page sets out where bearing work sits in the full procedure, and generator engine parts covers the surrounding parts list.
Supply condition and the things not to accept
Bearing shells are precision parts whose quality is invisible from the outside. For normal overhaul work the appropriate baseline is new shells of stated manufacturer origin, in sealed packaging, with the size clearly marked. Used shells have no place in a planned overhaul. Shells of unknown origin are a risk to the whole engine, because a failed bearing usually costs a crankshaft. Our used and refurbished parts policy explains how we describe condition on a quotation.
Related photographs and videos
Common mistakes
- Ordering standard shells for an engine of unknown history; the crankshaft was ground at an earlier overhaul and the new shells run with twice the design clearance and low oil pressure.
- Reusing torque-to-yield main and rod bolts; the caps lose clamp load, the shell loses crush and spins in the housing.
- Fitting new shells without flushing the oil circuit after a failure; debris from the old bearing scores the new ones within hours.
- Diagnosing a bearing failure from one photograph and blaming the parts; the real cause in the oil supply or the connecting rod is left in place and the failure repeats.
- Ignoring crankshaft end float; worn thrust washers allow the shaft to move against the alternator coupling and the rear main seal starts leaking.
What to send us
Send the engine plate photograph and serial number, the hour-meter reading, the reason for the work (planned overhaul, low oil pressure, noise, seizure or contamination), the previous overhaul report if there is one, and photographs of the removed shells laid out in order with the size markings on their backs visible. If journals have been measured, include the figures. Use the spare parts enquiry and we will confirm the size and the parts scope.
Questions engineers ask
How do I know whether my crankshaft has been ground before?
Look at the back of the removed shells, where the size is usually stamped, or check the previous overhaul report. If neither is available, measure each journal with a micrometer and compare with the standard diameter in the workshop manual. Never assume standard on an engine with unknown history.
What causes low oil pressure after a bearing replacement?
Several things besides the bearings: wrong clearance from incorrect shell size, a worn oil pump or relief valve, a blocked pick-up, a faulty pressure sender, oil of the wrong viscosity, or excessive clearance at camshaft bushes or other positions that were not renewed. Check the sender reading with a mechanical gauge first.
Is there a standard bearing clearance for generator engines?
No. Each engine has its own specified clearance range, typically in the region of a few hundredths of a millimetre, and it must be taken from that engine’s workshop data. Generic figures from the internet are not a substitute.
Does a failed bearing mean the bearing was defective?
Rarely. Most bearing failures trace back to oil starvation, contamination, overload, a damaged journal or incorrect assembly. The bearing is the component that shows the damage first, not usually the component that caused it.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.