Editing Overhead camshaft engine (section)

=== World War I ===
[[File:Napier Lion cambox.jpg|thumb|DOHC cylinder head of a 1917–1930 [[Napier Lion]] aircraft engine]]

During World War I, both the [[Allies of World War I|Allied]] and [[Central Powers]]; specifically those of the [[German Empire]]'s ''[[Luftstreitkräfte]]'' air forces, sought to quickly apply the overhead camshaft technology of motor racing engines to military aircraft engines. The SOHC engine from the ''Mercedes 18/100 GP'' car (which won the 1914 French Grand Prix) became the starting point for both Mercedes' and Rolls-Royce's aircraft engines. Mercedes created a series of six-cylinder engines which culminated in the [[Mercedes D.III]]. Rolls-Royce reversed-engineered the Mercedes cylinder head design based on a racing car left in England at the beginning of the war, leading to the [[Rolls-Royce Eagle]] V12 engine. Other SOHC designs included the Spanish [[Hispano-Suiza 8]] V8 engine (with a fully enclosed-drivetrain), the American [[Liberty L-12]] V12 engine, which closely followed the later Mercedes D.IIIa design's partly-exposed SOHC valvetrain design; and the [[Max Friz]]-designed; German [[BMW IIIa]] straight-six engine. The DOHC [[Napier Lion]] W12 engine was built in Great Britain beginning in 1918.

Most of these engines used a shaft to transfer drive from the crankshaft up to the camshaft at the top of the engine. Large aircraft engines— particularly air-cooled engines— experienced considerable thermal expansion, causing the height of the cylinder block to vary during operating conditions. This expansion caused difficulties for pushrod engines, so an overhead camshaft engine using a shaft drive with sliding spline was the easiest way to allow for this expansion. These bevel shafts were usually in an external tube outside the block, and were known as "tower shafts".<ref>{{Cite book|last= Thorpe|first= Leslie Aaron|title= A text book on aviation: the new cadet system of ground school training|volume= 3|pages= 14–15|year= 1936|publisher= Aviation Press|url= {{Google books|aVluAAAAIAAJ|A text book on aviation: the new cadet system of ground school training, Volume 3|page=14|plainurl=yes}}|access-date=27 February 2015|quote= The overhead camshafts are driven by bevel gears and vertical shafts known as tower shafts.}}</ref>

<gallery mode="packed" heights="180px" style="text-align:left">
Hispano Suiza 8 A Brussel.jpg |1914–1918 [[Hispano-Suiza 8|Hispano-Suiza 8A]] SOHC aircraft engine
Hispano-suiza-V8 220PS.jpg |1914–1918 [[Hispano-Suiza 8|Hispano-Suiza 8Be]] SOHC aircraft engine with "tower shafts" at the rear of each cylinder bank
Later Mercedes D III Valvetrain.jpg |Later production (1917-18) [[Mercedes D.III]] upper valvetrain details sketch, its design features copied by the [[BMW III]] and the Allied [[Liberty L-12]] engines
Liberty L-12-2.jpg |Detail closeup of a [[Liberty L-12]]'s upper valvetrain, showing the similarity to the later-production Mercedes design 
</gallery>