systems require the use of tensioners, which add complexity. In contrast, an OHV engine has the camshaft positioned close to the crankshaft, which may be driven by a much shorter chain or even direct gear connection. However, this is somewhat negated by a more complex valvetrain requiring pushrods. Simpler lubrication system: The lubrication requirements for OHV cylinder heads are much less, due to the lack of a camshaft and related bearings to lubricate. OHV heads only need lubrication for the rocker arms at the pushrod end, trunnion, and rocker tip. This lubrication is typically provided through the hollow pushrods themselves rather than a dedicated lubrication system in the head. The reduced lubrication requirements can also mean that a smaller, lower-capacity oil pump is used. Compared with OHC engines, OHV engines have the following disadvantages: Limited engine speeds: Although OHV engines have simpler drive systems for the camshaft, there are a greater number of moving parts in the valvetrain (i.e. the lifters, pushrods and rockers). Inertia from these valvetrain parts makes OHV engines more susceptible to valve float at high engine speeds (RPM). Constraints on valve quantity and location: OHC engines often have four valves per cylinder, whereas it is rare for an OHV engine to have more than two valves per cylinder. In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods and the need to accommodate them in the head casting. References Cam-in-block valvetrain configurations Engine valvetrain configurations Motorcycle engines Scottish inventions