Piston

Piston
The piston has a homogenous aluminum alloy with graphite coating on both sides. This coating provides a certain reduction in friction as well as noise absorption. The piston rings are made of different material depending on where they are located in the piston. The upper compression ring is a nitrified steel ring. The lower compression ring is made of cast iron and the oil scraper ring is a design comprising three parts, all in nitrified steel (not applicable to B5xx4Sx). For the B5244S7 engine the oil scraper ring is divided into 2 parts instead of 3.It is slightly more hard wearing than the 3 part ring. The height of the ring is 2.5 mm compared with 2.0 mm for the 3 part ring.

The gudgeon pin is hardened steel and is held in place with 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 hydrocarbons from the engine to the catalytic converter 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.