Camshaft

Camshafts and camshaft bearings determine the precise valve timing of the intake and exhaust valves and are therefore crucial for cylinder filling, combustion, emission values and engine performance. In MAN and Mercedes trucks, buses and vans, camshaft configurations vary considerably depending on the engine family, cylinder head and emission standard. The correct camshaft, bearing cap, plain bearing bush, rocker arm contact surface and drive must therefore always be identified using the chassis number. Visual comparison or selection based solely on the model name is never sufficient, even when engine displacement or the number of cylinders is the same.

MAN engines such as D08, D15, D20, D26, D28 and D38 use engine-specific solutions with differences in cam profiles, bearing diameters, axial retention, gear drive and cylinder head design. Mercedes also has significant variations between 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.

Depending on the engine series, the valve train operates with a single overhead camshaft or with two overhead camshafts. Engines with one camshaft generally operate the valves via rocker arms, roller rocker arms or pushrods; versions with two camshafts divide operation between separate intake and exhaust camshafts. As a result, camshafts from a similar engine family are not interchangeable. A different cam height, base circle diameter, cam angle or position of the crankshaft sensor or camshaft sensor trigger can also result in fault codes, poor combustion or mechanical valve damage.

Camshaft bearings are usually plain bearings lubricated by filtered engine oil through drillings in the cylinder head. The bearing caps are line-bored with the cylinder head and must not be interchanged. Always check the markings, installation position and specified tightening torques. An incorrect bearing cap or incorrect bolt tightening sequence causes stress, insufficient bearing clearance and possible camshaft seizure.

Common types of damage include worn cam lobes, pitting on cam running surfaces, blue discolouration caused by insufficient lubrication, grooves in the bearing journals, worn bearing seats and damaged gears or drive gears. In practice, this results in a ticking valve train, irregular idling, loss of power, increased fuel consumption, smoke generation, camshaft-crankshaft synchronisation fault codes or metal particles in the engine oil and oil filter.

Start the diagnosis by checking the oil level, the specified engine oil viscosity grade and the condition of the oil filter and oil cooler. Then measure the oil pressure with a calibrated pressure gauge at both cold and operating temperature. Low pressure may indicate a defective oil pump, contaminated oil passages, leaking bearing locations or excessive plain bearing clearance. Remove the valve cover and check the cam lobes for cam lift using a dial gauge; compare the measured lift between cylinders. Measure bearing journals with a micrometer and determine oil clearance using Plastigage or an internal dial gauge in accordance with factory tolerances.

A damaged camshaft must not simply be replaced. First flush the lubrication circuit and inspect the oil pickup strainer, oil pump, control valves, oil cooler and oil passage drillings in the cylinder head. In case of confirmed wear, always replace the affected camshaft, associated camshaft bearings or bearing caps if available, and also inspect rocker arms, roller followers, hydraulic tappets, valve springs and valve stem guides. A new camshaft fitted with worn contact components will quickly lead to damaged cam profiles again.

If there is any doubt about the reusability of the cylinder head or if bearing seats are out of specification, reconditioning with precise line boring or replacement of the cylinder head is often the safest solution. Braem can assist with chassis-number-based identification and, depending on the application, offers new, used and reconditioned engine parts to minimise downtime for trucks, buses and vans.