Editing Flat engine (section)

=== Boxer configuration {{anchor|Boxer engine}}===
[[File:R68-opposed-cylinders.jpg|thumb|right|1954 [[BMW R68]] flat-twin boxer engine]]

Most flat engines use a "boxer" configuration, where each pair of opposing pistons move inwards and outwards at the same time, somewhat like boxing competitors punching their gloves together before a fight.<ref name=Bonk>{{cite web |last=Bonk |first=Aaron |title=How Boxer Engines Work |url=http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |work=Super Street Magazine |publisher=Source Interlink Media |access-date=2013-12-21 |date=2013-02-07 |archive-date=27 February 2014 |archive-url=https://web.archive.org/web/20140227052947/http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |url-status=dead }}</ref> Boxer engines have low vibration, being the only common configuration that has no unbalanced forces, regardless of the number of pairs of cylinders. Boxer engines therefore do not require either a [[balance shaft]]<ref name=Hanlon/> or counterweights on the [[crankshaft]] to balance the weight of the reciprocating parts. However, a [[Couple (mechanics)#Applications|rocking couple]] is present, since each cylinder is slightly offset from the other member of its pair due to the distance between the [[crankpin]]s along the crankshaft.<ref name=Bonk/>