are the hottest part of the engine and require adequate cooling, typically air cooling, oil cooling or liquid cooling. Some motorcycles such as Harley-Davidsons, Moto Guzzis and BMWs become identifiable by their cylinder-head types, namely airhead, panhead, oilhead, and even knucklehead. The Ducati desmos head enables higher rpm to be achieved without detrimental valve float also known as valve bounce. Valve control in four-strokes In a side-valve engine, the valves are operated from the "underhead" cam without special valve gear. Overhead valve (OHV) engines have valves operated by pushrods. Single- and double-overhead camshaft (SOHC and DOHC, respectively) engines have overhead camshafts typically operated by chain, belt, gear train or bevel gear drive. The earliest motorcycle engines had exhaust valves operated in the same way as a side-valve engine, but they had overhead intake valves held closed by a weak spring. The intake valve was opened by the suction caused by the intake stroke. As the need for mechanical control of the intake valve arose, the design was modified by adding another cam to the camshaft and using a pushrod and rocker arm to operate the intake valve. Honda equipped the CBR400F with REV, described as "revolution responding type valve pausing mechanism", in 1983. This system enabled switching over the number of valve operations per cylinder between low and medium speed revolution range and high speed revolution range. In 2002, Honda introduced HYPER VTEC in the VFR800 Interceptor. In 2006, Kawasaki introduced VVT in the Concours 14. Unit construction Engines and gearboxes were originally separate items, the gearbox being driven by a "primary chain". Later designs combined the two elements into a single "unit construction". Some manufacturers, such as Norton and Enfield continued with "pre-unit" construction until recently, and Harley-Davidson still makes pre-unit engines today. In the early 1960s, BSA introduced new short-stroke unit construction 500 and 650 cc parallel twins to replace their existing pre-unit long-stroke engines. Triumph modified their large pre-unit 500 and 650 cc parallel twins into unit-construction engines. The smaller Triumph 350 and 500 cc twins had been unit construction from their introduction in 1957. The decision by Lucas to discontinue making dynamos and magnetos, and to produce only alternators and coil ignition systems, was a significant factor in forcing the redesign. Traditionally, engines and gearboxes had separate lubrication systems with different grades of oil; but modern practice is to use the same oil for both. Cylinders