Engine (B5244S4; 2004-2010): Design: Piston

The piston has a homogenous aluminum alloy with graphite coating on the sides. This coating gives reduced friction and noise. The piston rings are of different materials depending on where they are located on the piston. The upper compression ring is a nitrated steel ring. The lower compression ring is manufactured from cast iron and the oil scraper ring is a three piece design, all made of nitrated steel (does not apply to B5xx4Sx). For B5244S7 engines the oil scraper ring is two piece instead of three piece. It is more wear resistant than the three piece ring. The ring height is 2.5 mm compared to 2.0 mm for the three piece ring.

The piston pin is hardened steel and is held together using steel snap rings. The piston is oval and conical and flat at the top. By reducing the area from the upper piston ring to the top of the piston, the release of hydro-carbons from the engine to the three-way catalytic converter (TWC) is reduced.

The pistons are oil cooled so that they can have a lower compression height. Oil is led through a valve in a longitudinal channel in the lower section of the cylinder block on the exhaust side. There is a nozzle screwed to the channel at each cylinder. This leads the oil towards the underside of the piston.