Crankshaft

The crankshaft and crankshaft bearings together form the mechanical heart of the engine bottom end. In MAN and Mercedes diesel engines, they convert the linear movement of the pistons via the connecting rods into driving torque for the flywheel, clutch or torque converter, while also having to withstand high combustion pressures, torsional vibrations and prolonged loads. In MAN and Mercedes trucks, buses and vans, crankshafts are precisely matched to the engine block, connecting rods, bearing caps, oil pump and vibration damper. Identification based on the chassis number is therefore essential. In practice, these are components that rarely fail without cause: damage to the crankshaft or main bearings is usually the result of poor lubrication, contamination in the lubrication circuit, overheating, incorrect installation tolerances or consequential damage after seizure of connecting rod or piston components. Failing to intervene in time can quickly result in a complete overhaul rather than a targeted repair.

Compatibility of crankshafts, main bearings and thrust bearings

The correct version depends on the engine family, power output, emission variant, crankshaft diameters and any oversize or undersize overhaul dimensions. For MAN, this includes D08, D15, D20, D26, D28, D30 and D38 engines. Mercedes engines include OM 364, OM 366, OM 402, OM 421, OM 422, OM 441, OM 442, OM 457, OM 460, OM 501, OM 502, OM 503, OM 904, OM 906, OM 926, OM 934, OM 936, OM 470, OM 471 and OM 473. Even within a single engine family, there may be differences in stroke length, crankpin diameter, number of counterweights, gear drive, thrust bearing position and balancing.

The crankshaft is mounted in the engine block using crankshaft bearings. Crankshaft bearings, also known as main bearings, are generally multi-layer plain bearings with a steel backing, an aluminium or copper-lead bearing layer and a thin anti-friction layer. These bearings are supplied in standard size but are also available in overhaul sizes. These overhaul sizes are used when fitting a crankshaft with ground journals. In addition, thrust bearings prevent the crankshaft from moving in the longitudinal direction. While the main bearings support the crankshaft radially, the thrust bearings absorb axial forces. Always check the size marking on the crankshaft, engine block and bearing cap. An incorrect bearing shell, swapped bearing cap or insufficient fit causes oil pressure loss, local overheating and accelerated damage to the crankshaft journals.

Wear to the crankshaft and crankshaft bearings often becomes apparent through indirect signs. These include falling oil pressure when the engine is hot, metal particles in the engine oil or engine oil filter, dull knocking noises under load, excessive crankshaft end float, vibrations within a specific engine speed range or repeated bearing damage after a previous repair. In heavy-duty applications involving extensive idling, high thermal loads or delayed oil changes, wear marks and discolouration on the crankshaft due to excessive temperature, grooves on the crankpins, oval bearing fits or damage caused by oil contamination are also more common.

A correct diagnosis does not stop at a visual inspection. First measure the oil pressure using a calibrated pressure gauge, both during cold start and at operating temperature and specified engine speeds. Then drain the engine oil and inspect it for the presence of magnetic plug, copper-coloured, aluminium-coloured or ferromagnetic particles. If readings are abnormal, remove the oil pan and inspect the bearing shells for scratches, scoring, blue discolouration, cavitation and bearing spin.

Measure the diameter, roundness and taper of the main journals and crankpins using an outside micrometer at several measuring points. Check the bearing oil clearance using Plastigage or, more accurately, by measuring the installed bearing diameter and crankpin diameter. Measure crankshaft end float with a dial indicator while the crankshaft is moved in the longitudinal direction in a controlled manner. Compare all values with the specifications for the relevant engine code; a crankshaft that appears visually undamaged may still be out of tolerance.

Never replace bearings without eliminating the cause of the damage. Check the pickup strainer, oil pump, pressure relief valve, oil cooler, oil filter housing and oil passages for blockages, leaks or pressure loss. Thoroughly flush the lubrication system when bearing layers have detached, and always replace the engine oil and oil filter. If crankshaft journals are scored or oval, professional grinding to an approved overhaul size or replacement of the crankshaft is required. Fitting new bearing shells to a damaged journal is not a sustainable repair.

The crankshaft can possibly be repaired. The following repair operations can be carried out for this purpose: grinding (crankshaft grinding), polishing (crankshaft polishing), crack repair (crack repair), welding repair (welding), alignment and balancing, and heat treatment (case hardening, nitriding).  After these repair operations, the crankshaft must be inspected and measured. The measured values must meet the tolerances.

Use the specified assembly lubricant, new stretch bolts where required, and observe the tightening torques and angle tightening specifications for bearing caps. In addition to new parts, Braem can also help select reconditioned crankshafts and inspected engine components based on the chassis number to minimise downtime for trucks, buses and vans.