Editing Flathead engine (section)

==The side-valve design==
The valve gear comprises a camshaft sited low in the cylinder block which operates the [[poppet valve|poppet]] [[valves]] via [[tappet]]s and short pushrods (or sometimes with no pushrods at all). The flathead system obviates the need for further [[valvetrain]] components such as lengthy pushrods, rocker arms, overhead valves or [[overhead camshaft]]s.<ref>An exception is the [[Indian Motocycle Manufacturing Company|Indian]] which employs both rocker arms and pushrods to transmit motion from the cam lobes to the valve stems.</ref> The sidevalves are typically adjacent, sited on one side of the cylinder(s), though some flatheads employ the less common [[Crossflow cylinder head|"crossflow"]] [[T-head engine|"T-head"]] variant. In a T-head engine, the exhaust gases leave on the opposite side of the cylinder from the intake valve.

The sidevalve engine's combustion chamber is not above the piston (as in an OHV (overhead valve) engine) but to the side, above the valves. The spark plug may be sited over the piston (as in an OHV engine) or above the valves; but aircraft designs with [[Dual ignition|two plugs per cylinder]] may use either or both positions.<ref>The D-motor flathead aero-engines have both spark pugs above the valves.</ref>

"Pop-up pistons" may be used with compatible heads to increase compression ratio and improve the combustion chamber's shape to prevent [[Engine knocking|knocking]].<ref>{{cite web |url=http://www.hotrod.com/techarticles/engine/hdrp_0511_ford_flathead_engine/viewall.html |title=Ford Flathead V8 – The Flathead Guide of Death |author= Davis, Marlan |date=29 September 2006 |website=Hotrod.com |publisher=Hot Rod Magazine |at=Combustion Chamber |access-date=2014-04-08 |quote=Trying to gain back compression ratio by using popup pistons may improve airflow provided proper attention is paid to the transfer area and overall piston-to-combustion chamber interface. The best balance has been the subject of debate for over 60 years. Currently the most popular approach is running a big popup piston, but with a scallop on the side adjacent to the valves to keep the transfer area clear between the valves and the cylinder bore. Recommended bottom-line street-gas-friendly compression ratios are between 7.5–8:1 on naturally aspirated engines and 6.5–7.0:1 with a blower.}}</ref> "Pop-up" pistons are so called because, at [[top dead centre]], they protrude above the top of the cylinder block.