Engine [2015-2016, B5254T12]: Design: Piston

The piston has a uniform aluminum alloy with graphite coating on the sides. This coating gives a certain friction reduction as well as noise damping. The piston rings are of different material depending on where they are mounted on the piston. The upper compression ring is a nitride steel ring. The lower compression ring is made of cast iron and the oil scraper ring is a design consisting of three parts, all made of nitride steel. The piston pin is made of hardened steel and is kept in place with steel lock rings. The form of the piston is oval and tapered with plane piston top. By reducing the topland area (distance between upper piston ring to piston top), the hydrocarbon emissions from the engine to the catalytic converter have been 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.