The piston protrusion defines the distance between the piston and the cylinder head. It always has to be checked when installing new pistons in the engine, if parts of the crank mechanism have been modified or if the engine block face has been reworked.
Measurement of piston protrusion
A dial gauge with a measurement bridge is used for measurement of the piston protrusion. Before the measurement, it is vital to ensure that the engine can be rotated completely. The engine block face must also be clean. This is the only way to avoid measurement errors.
If the piston is in the upper position, top dead centre can be determined using the dial gauge. This can be recognised by the change in direction of the dial gauge needle. Once top dead centre has been determined, the pretensioned dial gauge is placed on the surface of the engine block and the needle is set to 0. If the dial gauge is then placed on the piston again, the piston protrusion can be read on the scale. Always perform the measurement on both sides of the piston to prevent measurement errors - the piston can also tilt.
Measure each individual piston, then compare the values with the manufacturer’s specifications, which must not be exceeded. If the piston protrusion is too large, the piston bumps against the cylinder head once it has reached operating temperature. This causes damage during the combustion process.