Cylinder head

The cylinder head provides the seal between the engine block, cylinder head gasket and combustion chambers. It includes, among other things, intake and exhaust ports, valve seats, valve guides, coolant passages, lubrication oil passages and – depending on the engine family – camshaft bearings, injector seats and EGR connections. The correct cylinder head and cylinder head bolts are always determined based on the chassis number, engine code and part number. Visual comparison or selection based solely on the model name is never sufficient, as revisions differ in port shape, injector fitment, compression height, sensor connections and bolt lengths.

For MAN, cylinder heads are used on D08, D15, D20, D26, D28 and D38 engines. Mercedes engine families 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. These engines are fitted in a wide range of trucks, buses and vans and may differ in emissions standard, injection system, turbocharger configuration and cylinder head design.

On two-valve cylinder heads, each cylinder generally has one intake valve and one exhaust valve. They have a simpler port design and fewer valve components, but more limited gas flow. Four-valve cylinder heads use two intake and two exhaust valves per cylinder; this improves cylinder filling, combustion, power output and emissions control, but places higher demands on valve guides, camshafts, valve bridges and adjustment. There are also engines with one cylinder head spanning multiple cylinders and engines with separate cylinder heads per cylinder. Individual heads, often found on older V-engines such as certain OM 400 series engines, reduce the scope of replacement in the event of local damage. A one-piece cylinder head generally requires more extensive reconditioning or complete replacement in the event of warping or cracking.

Typical fault symptoms include coolant loss without a visible external leak, excessive pressure in the expansion tank, white exhaust smoke, emulsion formation in the engine oil, loss of power, irregular combustion and localised overheating. Causes include a defective cylinder head gasket, corrosion in coolant passages, hairline cracks between valve seats and injector bores, recessed valve seats, worn valve guides or a warped sealing surface following thermal overload.

Begin diagnosis with a pressure test of the cooling system and check for combustion gases in the coolant using a CO₂ test. Then measure compression pressure or carry out a cylinder leak-down test; air leakage into the cooling system, crankcase or adjacent cylinders indicates a sealing problem. After dismantling, check the flatness of the cylinder head surface using a certified straightedge and feeler gauge. Compare the measured deviation with the factory tolerance. Have heads suspected of cracking pressure-tested and, where the material permits, inspected by magnetic particle or dye penetrant testing. On aluminium cylinder heads, checking for corrosion around coolant passages is particularly important.

Always replace cylinder head bolts when the manufacturer specifies torque-to-yield bolts. Reuse can result in insufficient clamping force, local cylinder head gasket leaks or deformation of the sealing surface. Carefully clean the threaded holes in the engine block, check them for stretching and remove coolant or engine oil from blind holes to prevent hydraulic lock during tightening. Install the bolts according to the specified sequence, tightening torques and angle turns, using the specified lubricant on the threads and contact surfaces.

Before installation, also check cylinder liner protrusion, the condition of the engine block surface, the correct gasket thickness and the fitment of injectors or injector sleeves. A new cylinder head fitted to an insufficiently flat block or with the wrong gasket grade will not provide a lasting solution. Braem can assist with chassis-number-based identification and in choosing between new or reconditioned cylinder heads, helping to minimise downtime and repeated dismantling.