Connecting rod

Connecting rods and connecting rod bearings convert the linear motion of the pistons into rotary motion of the crankshaft. They operate under high alternating tensile and compressive forces and at high engine speeds. Proper lubrication is crucial for all engines. Bearing fit and dimensions are decisive in this regard. The bearing fit and dimensions of connecting rod bearings determine how tightly the bearings sit in the connecting rod and with what clearance they rotate around the crankpin, ensuring a correct oil film and reliable engine operation. A defect in these components can quickly lead to damage to crankpins, pistons, cylinder liners and the complete engine block. For MAN and Mercedes trucks, buses and vans, connecting rods, bearing caps and bearing shells must therefore always be determined based on the chassis number and exact engine code. Visual comparison or selection based solely on the model name is never sufficient.

Depending on the engine family, output, emission standard and production version, connecting rods differ in centre-to-centre length, small-end bore, big-end diameter, material, weight class and bearing cap design. This is relevant for, among others, MAN D08, D15, D20, D26, D28, D30 and D38 engines and Mercedes 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 934, OM 936, OM 470, OM 471 and OM 473 engines.

By design, there are one-piece and two-piece connecting rods. In two-piece versions, the big-end bearing cap is fitted separately; cap and connecting rod form a matched set and must never be interchanged. They are secured using connecting rod bolts. With a fracture-split connecting rod, the rod and cap are first manufactured as a single unit. The connecting rod is then controlled-fractured along a predetermined fracture line. Due to the irregular fracture surface, the two parts can only fit together precisely afterwards, ensuring highly accurate alignment without dowel sleeves.

The big-end encloses the crankpin with two connecting rod bearing shells; the small-end carries the piston pin, generally via a pressed-in or bushed plain bearing bush. Connecting rod cross-sections may be designed as I-profile, H-profile or solid. There are also standard connecting rods, reinforced connecting rods and forged connecting rods. Especially in heavy-duty applications, at high cylinder pressures and during engine overhauls, the correct weight class must be maintained to prevent imbalance and uneven bearing loads.

Conventional connecting rod bearings generally consist of steel backings with multi-layer bearing coatings. Sputter bearings feature a physically vapour-deposited, highly wear-resistant top layer and are used where higher surface pressures and fatigue loads occur. Bearing damage can result from insufficient oil pressure, contaminated engine oil, blocked oil passages in the crankshaft, overheating, incorrect bearing clearance or a distorted connecting rod.

Typical fault symptoms include a deep knocking noise that increases under load, metal particles in the oil filter, falling oil pressure when the engine oil is hot, and blue discolouration or seizure marks on the crankpin. If suspected, remove the oil sump, check the bearing shells for copper exposure, scratches, pitting and spinning, and inspect the crankpins for ovality and taper using an outside micrometer. Then measure the bearing diameter and oil clearance using a bore gauge or plastigage according to the engine specifications. Also check the axial and radial clearance of the connecting rod, the small-end bore and straightness on a connecting rod measuring bench.

The following step-by-step procedure can be used:

1. Visual inspection : 

Check connecting rod bearings for scratches, grooves and copper exposure. Check connecting rod bearings for discolouration due to overheating. Check connecting rod bearings for wear of the top layer on tri-metal or sputter bearings. Check connecting rods for cracks and fractures. Check connecting rods for deformation. Check connecting rods for damaged bearing seats or  threaded bolts.

2. Measuring dimensions and fit:

At the big-end side, i.e. the crankshaft side: Check bearing clearance using a feeler gauge or plastigage. Check the diameter of the crankpin.

At the small-end side, i.e. the piston side: Check the fit of the connecting rod in relation to the piston pin

Connecting rod itself: Check alignment using a dial indicator.

3. Replace the old connecting rod bolts with new ones. Connecting rod bolts are stretch bolts that plastically elongate when tightened. This achieves an exact and constant clamping force. Reuse does not guarantee the correct clamping force.

4. Ensure that everything is clean and free of grease before reassembling the engine. Apply a thin layer of oil to the connecting rod bearings before installation.

If the crankpins are within tolerance, there is no heat discolouration and the connecting rod bearing seat is undamaged, targeted bearing replacement may be justified. If there are signs of seizure, an enlarged fit or visible deformation, the connecting rods must also be replaced. Always replace connecting rod bolts. Use the correct tightening method, including angle tightening where required. This prevents repeated failure shortly after reassembly.