Editing Flathead engine (section)

{{Short description|Type of four-stroke engine}}
{{Use dmy dates|date=December 2015}}
[[File:Single-cylinder T-head engine (Autocar Handbook, 13th ed, 1935).jpg|thumb|A crossflow T-head sidevalve engine]]
[[File:Side-valve engine v2.png|thumb|The usual L-head arrangement]]
[[File:Side-valve engine v3.png|thumb|Pop-up pistons may be used to increase compression ratio]]
[[File:Side-valve engine with Ricardo's turbulent head 01.png|thumb|Flathead with Ricardo's turbulent head]]

A '''flathead engine''', also known as a '''sidevalve engine'''<ref name=AmericanRodder1994>''American Rodder'', 6/94, pp.45 & 93.</ref><ref>(As the cylinder cross-section has the shape of an inverted L, other names such as "L-block" or "L-head" are also used)</ref> or '''valve-in-block engine''', is an [[internal combustion engine]] with its [[poppet valve]]s contained within the [[Cam-in-block|engine block]], instead of in the [[cylinder head]], as in an [[overhead valve engine]].

Flatheads were widely used internationally by automobile manufacturers from the late 1890s until the mid-1960s<ref>{{cite web | url=https://www.autoweek.com/car-life/a1713546/what-was-final-year-new-flathead-powered-american-car/ | title=What Was the Final Year for a New Flathead-Powered American Car? | work=Autoweek | date=20 December 2018 }}</ref> but were replaced by more efficient overhead valve and [[overhead camshaft engine]]s. They are currently experiencing a revival in low-revving [[aviation engine|aero-engines]] such as the [[D-Motor]].<ref>{{Cite web |url=http://lightningautogyro.com/gallery.aspx |title=D-Motor image |access-date=29 April 2018 |archive-url=https://web.archive.org/web/20180225114628/http://www.lightningautogyro.com/gallery.aspx |archive-date=25 February 2018 |url-status=dead |df=dmy-all }}</ref>