Valves and accessories

Intake and exhaust valves and related components regulate gas exchange in the cylinder head and are crucial for compression pressure, combustion, emission values and thermal reliability. This subcategory includes intake valves, exhaust valves, valve guides, valve seats, valve springs, spring retainers, valve collets, valve stem seals, tappets and rocker arms. Exhaust valves in particular operate under high thermal loads due to hot exhaust gases and EGR recirculation. Even a minor deviation in valve clearance, valve seat fitment or spring pressure can result in power loss, oil consumption or severe cylinder head damage.

The correct version must always be determined using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient. Within a single engine family, cylinder head variants may differ in valve diameter, stem diameter, valve length, seat angle, valve guide material, valve spring load and camshaft-operated valve actuation. This applies to MAN engines D08, D15, D20, D26, D28 and D38 and to Mercedes engines 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.

Intake valves are generally designed for optimum airflow and have different steel grades, head diameters and temperature requirements than exhaust valves. Exhaust valves may, for example, require heat-resistant alloys, hardened valve stem tips or a specific surface treatment. Valve seats must be installed in the cylinder head with a controlled interference fit matching the outside diameter. Valve guides must also ensure the specified radial valve stem clearance: too little clearance causes seizure due to thermal expansion, while excessive clearance leads to engine oil consumption, blue smoke and carbon deposits on valves and valve ports.

A burnt exhaust valve often causes compression loss, irregular idling, cylinder misfire, tapping noises in the cylinder head and increased exhaust gas temperatures. In Mercedes OM 457, OM 471 and OM 473 engines and MAN D20, D26 and D38 engines, prolonged high loads, insufficient cooling, incorrect injection corrections or a faulty EGR system can increase the thermal load. A leaking injector, irregular turbo boost control or intake contamination can also cause locally excessive combustion temperatures.

Start by reading fault codes, cylinder balance values and temperature-related faults. Then measure the compression pressure or perform a leak-down test for each cylinder. Air loss through the intake manifold indicates leaking intake valves; air in the exhaust system indicates exhaust valves or valve seats that are not sealing. Use an endoscope to check for valve edge damage, impact marks, deposits and abnormal piston crowns. After disassembly, measure the valve stem with a micrometer and the guide bore diameter with suitable internal measuring equipment. Compare the calculated clearance with the engine-specific workshop tolerance. Also check the free length, squareness and closing pressure of valve springs using a spring tester.

Never replace damaged valves without assessing the associated valve seats and valve guides. A new valve on an indented seat will restore sealing only temporarily. After replacing seats, the contact width, concentricity and seat angle must be checked and machined. Fit new valve stem seals whenever the cylinder head is opened, and replace valve springs, spring retainers and valve collets in the event of corrosion, fatigue, signs of fracture or insufficient spring pressure.

During installation, ensure correct valve clearance, the correct installation position of the valve springs and a clean, undamaged valve stem. Contaminated engine oil, blocked oil passages to rocker arms or worn tappets can cause damage again. In addition to technical support in determining the correct parts, Braem can offer new, reconditioned and used engine parts to specifically minimise downtime for trucks, buses and vans.