Short bloc

The engine block is the load-bearing foundation of the internal combustion engine and determines the geometric accuracy of the cylinders, main bearings, thrust bearings, coolant passages and oil galleries. In MAN and Mercedes trucks, buses and vans, the engine block must withstand high combustion pressures, thermal loads and torsional forces from the crankshaft and drivetrain over extended periods. Depending on the engine family, this may involve cast-iron engine blocks with wet or dry cylinder liners, or engine blocks with cylinders bored directly into the block. All crucial components that allow the engine to run, breathe and cool are mounted on the engine block. The cylinder head is mounted at the top. The crankshaft, pistons, connecting rods and oil sump are mounted inside and underneath. The timing system, water pump, oil filter housing, oil pump, flywheel housing and engine mounts are fitted to the sides, front and rear. A suitable engine block must always be identified using the chassis number.

Engine blocks differ in, among other things, the number of cylinders, cylinder diameter, stroke, bore spacing, deck height, cylinder liner seats, position of oil galleries and main bearing cap design. For MAN D08, D20, D26 and D38 engines, blocks are not automatically interchangeable, even where engine displacement or cylinder count is the same. The same applies to Mercedes engine families such as OM 904, OM 906, OM 926, OM 457, OM 460, OM 471 and OM 473. Variations in EGR configuration, compression ratio, crankshaft bearing diameter, sensor connections and mounting points for auxiliary units, such as the alternator, starter motor and air conditioning compressor, determine the correct version.

When selecting a replacement engine block, therefore check not only the engine or casting number, but also the dimensions of the cylinder bores, the main bearing tunnel, the fit of the cylinder liners and the connections for coolant, engine oil and crankcase ventilation. On engine blocks with replaceable wet cylinder liners, liner protrusion above the deck surface is particularly critical. Insufficient or uneven protrusion prevents proper sealing of the cylinder head gasket.

Cracks in the engine block are often caused by frozen coolant, local overheating, cavitation around the cylinder liners or a faulty cylinder head gasket that blows combustion gases into the cooling system for an extended period. Other common defects include corrosion in coolant jacket cavities, worn main bearing saddles, damaged threaded holes for cylinder head bolts and erosion of the liner seats. Practical symptoms include coolant loss without a visible external leak, engine oil contaminated with coolant, rising cooling system pressure, repeated cylinder head gasket failures or oil pressure that is too low despite a correctly functioning oil pump and the correct engine oil viscosity.

Completely dismantle the block and clean the oil and coolant passages before taking measurements. Check the deck surface for warping using a calibrated straightedge and feeler gauge; then measure the cylinder bores at several heights and in mutually perpendicular directions using an internal micrometer. This determines ovality, taper and wear in relation to the repair size. Measure the main bearing tunnel with a line boring gauge and check the bearing fit with the specified dial gauges or plastigage during trial assembly. Magnetic crack testing is suitable for ferromagnetic cast-iron blocks; additionally, pressure testing the coolant jacket can reveal hidden leaks.

An engine block can only be repaired when the deck surface, cylinder bores, liner seats and main bearing tunnel remain within the permitted overhaul tolerances. Replacement is necessary in the event of severe cavitation, a cracked main bearing bulkhead structure, deep corrosion around the cylinder liner seals or structural cracks. During overhaul, always fit matching cylinder liners, pistons, main bearing shells, connecting rod bearings, core plugs and seals in accordance with the specified size classes. Thoroughly flush the oil cooler and oil galleries as well: remaining metal particles can immediately damage the crankshaft or turbocharger again.

For fast and technically sound repairs, new, reconditioned and inspected used engine blocks are available. Technical support with chassis number identification helps prevent incorrect orders, installation problems and unnecessary downtime for trucks, buses and vans.