The nightmare of every engine repairer: a new cylinder liner is fitted to an engine block. The cylinder head is put in place, the cylinder head bolds are fastened – and suddenly, a faint “ping” can be heard. The reason for this peculiar cracking sound becomes obvious when the block is dismantled afterwards: the flange of the cylinder liner has snapped off along its entire circumference – and is lying loosely on top. When it is removed, it can be seen that the break happened below the cylinder block face at an angle of about 30°. The fracture surface shows a course break structure. However: material or casting defects cannot be detected even with a magnifying glass or under a microscope. This pattern of damage is typical for a forced fracture.

Cylinder liner flange and flange seat – like lid and pan
The cylinder block has circular, plane recesses, the so-called flange seats. These fix the cylinder liners in axial direction in the cylinder block. The flange of a cylinder liner must fit exactly into such a recess – so that the liner rests on its entire circumference on the flange seat. The cylinder head gasket is then placed on the cylinder block. The sealing of the combustion chamber (for older models a metal border, for newer metal gaskets a profiled surface) has to fit correctly onto the top of the liner flange.

When the bolts are tightened, the cylinder head is pressed firmly against the cylinder block. The cylinder head bolts and the tightening instructions are designed to achieve a firm connection between the cylinder head and the cylinder block – even for ignition pressures of more than 200 bar at times. This means that enormous forces are introduced from the bolts via the gaskets to the liner flange. This makes it even more important that the forces via the head gasket are transmitted vertically to the liner flange. (Sketch 1 shows correct fitting)

Sketch 1: Only correct fitting ensures the right transfer of forces.		Sketch 2: Dirt particles under the liner flange – a cause for dangerous flexing moments.

Danger of damage: Inclined force lines
Cylinder liners are made from grey cast iron. This material has many positive properties – however, flexing is not tolerated by this brittle material. When the force lines are only slightly angled through the cylinder liner flange, flexing results in the upper part of the liner that can lead to fracture of the flange.

Common reasons for bending moments
Dirt particles: to avoid contaminations between the flange and seat in the block (i. e. dirt, chips, gasket residue, sealing material, etc.) cleanliness during fitting is important. Also sealing paste should only be used when it is stipulated by the engine manufacturer – according to the motto „the less the better“.

Our tip: when pre-machined cylinder liners are pressed in, it is a good idea to stop a few millimetres before the block and use compressed air to remove material that may have been scraped off from the gap between flange and flange seat.

Uneven surfaces and warpage at the flange seat of the cylinder block:
the underlying reasons for this are increasingly lighter engine designs – and the warpage at the component will be worse, the thinner the wall thickness of the block. At the same time, power output, combustion pressures and torques are rising all the time in modern engines. In engines that have been running for 500,000 kilometres (or the equivalent in running time) warpage can be that excessive, that the seat in the block should be reworked. The plane-parallel redressing should be carried out on a boring mill or with a hand operated, mobile flange facing unit. Important: the surface must not be slanted (see sketch 3 + 4) – and after machining, the sharp edge of the seat surface should be chamfered (about 1x45°, see sketch 5). If there is no chamfer, fracture damage can result (sketch 6).

Incidentally: Also the specified liner projection has to be assured before fitting. To adjust the size, a suitable metal ring can be placed under the flange. For many cylinder liners, MAHLE offers also suitable liners with oversized flanges. When the cylinder block is resurfaced, the flange seats have to be redressed by the same amount.

Sketch 5: Chamfer at the sharp-edged flange seat		Sketch 6: Fracture damages due to missing chamfer.

Wrong cylinder head gaskets:
also this can introduce angled force action into the flange (sketch 7), either because the diameter of the combustion chamber border is too small or the gasket has the wrong thickness. Our tip: only use the type of gasket that was originally intended for the engine. Cheap reproduction gaskets can differ in dimensions and material from the original gasket – and this can result in expensive consequential damage.

Sketch 7: Force transfer at an angle due to wrong cylinder head gasket.		Sketch 8: Cylinder head machining: if the grove is not allowed for, the forces are transferred via the fire prevention edge – and the liner fractures.

Incorrect machining:
some cylinder heads, like those for certain Volvo types, have a full perimeter groove into which the fire prevention edge of the cylinder liner projects – and cylinder head and cylinder liner must not come into contact. If the cylinder head is resurfaced due to damage or warpage, the perimeter groove must also be redressed by the same amount. Otherwise, there is a risk that forces are not introduced via the gasket but at an angle via the fire prevention edge of the cylinder liner (sketch 8).

Ignorance becomes expensive
Good hearing is needed when tightening the cylinder head bolts. If the dreaded cracking sound is heard straightaway it is still a blessing in disguise. The cylinder can be dismantled immediately and the cause can be analysed and redressed – and then a new liner can be fitted.

Otherwise, it will get expensive: the broken liner moves gradually in direction crank shaft after the engine is started up again – and as soon as the location of the break is level with the first piston ring in TDC, the piston ring springs open above the break location. At the next downward movement of the piston it pulls the cylinder liner downwards. The rotating crankshaft shatters the liner and piston and connecting rod are also wrecked – and the engine repairer is greeted by a connecting rod that protrudes sideways from the engine block.

A cylinder liner, fractured at the flange due to fitting error.

The coarse break structure – a sure sign of force action.