Harley-Davidson flathead engine

The Valve In Block Design or Flathead Block

Engine block of the Ford flathead V8 engine showing the location of the valve ports (the holes above the large cylinder bores). These blocks were the basis of the original "Hotrod" Engines

A flathead engine (aka sidevalve engine, Ford Sidevalve engine, or flatty)[1] is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine. As the cylinder cross-section has the shape of an upside-down L, this leads to other names: L-block, or L-head.

The sidevalve's poppet valves are usually sited on one side of the cylinder(s)). A recess in the cylinder head creates a corridor connecting the valves and the combustion chamber. The valve gear comprises a camshaft which operates the valves via simple tappets, without any further valvetrain paraphernalia (such as pushrods, rocker arms, overhead valves or overhead camshafts). The sidevalve arrangement was common in early engine designs, but it has since fallen from favor.


The main advantages of a sidevalve engine are simplicity, reliability, cheapness, compactness, responsive low-speed power, ability to use low-octane fuel, and safety from damage at high rpm.

An important benefit of the a sidevalve design is that, if a valve should drop, only limited damage would occur, and the engine might continue to operate with a broken valve. (By comparison, an OHV or OHC engine can suffer piston damage and catastrophic engine failure when a valve drops.) The absence of a complicated valvetrain allows a compact engine that is cheap to manufacture. In particular, the cylinder head may be little more than a simple metal casting, and the simple valve gear engenders reliability.

Because intake and exhaust gases pass through the same corridor between the block and head, and because the light valve gear allows the valves to open and close quickly, flatheads are designed with minimal valve overlap (where both intake and exhaust valves are simultaneously open). This gives better low rpm performance than with OHV or OHC engines which use valve overlap to gain high rpm. Although a sidevalve engine can safely operate at high speed, its volumetric efficiency swiftly deteriorates, so that a high power outputs are not feasible at speed.


The main disadvantages of a sidevalve engine are poor gas flow, poor combustion chamber shape, and low compression ratio, all of which result in a low power output.

The maximum compression ratio is low at about 7:1, reducing efficiency but permitting the vehicle to run on low-octane fuel.

The sidevalve configuration makes intake and exhaust gases follow a circuitous route, with low volumetric efficiency, or "poor breathing". High volumetric efficiency was less important for early cars because their engines rarely sustained extended high speeds; but designers seeking higher outputs had to abandon the sidevalve. A compromise used by Willys (Jeep), Rover and Rolls-Royce in the1950s was the F-head (aka intake-over-exhaust configuration), in which there is one side valve and one overhead valve per cylinder.[2]

An advance in flathead technology resulted from experimentation in the 1920s by Sir Harry Ricardo, who improved their efficiency after studying the gas-flow characteristics of sidevalve engines .[citation needed] The Ricardo head moved the exhaust valve farther from the center of the cylinder than the intake valve (whereas they had previously been equidistant). Ricardo observed how the form of the inlet and exhaust tracts affected gas flow and turbulence both in the inlet stream and within the combustion chamber.[citation needed]

Because the exhaust follows a complicated path to leave the engine, there is a tendency for the engine to overheat. In a T-block (aka T-head), a sidevalve engine has a crossflow configuration, so exhaust gases leave on the opposite side of the cylinder from the intake valve. American LaFrance powered their fire engines with T-head engines from the 1920s to the 1950s. The Cleveland Motorcycle Company produced a T-head four-cylinder in-line motorcycle engine in the 1920s; and early Stutz engines were T heads.

History and applicationsEdit

Although flathead in-line 4 and 6 cylinder engines were frequently used for automobiles, tractors, and other products, the best known flathead automotive engine is the early 20th century Ford V-8, which has both sets of valves (intake and exhaust) located on the inside of the "Vee," and which are all operated by a single camshaft located above the crankshaft. Other common configurations included in-line ("straight") eights and a V 12 Lincoln version of the Ford V 8.

Due to cooling and efficiency problems, flathead engines fell out of favor in "high power" applications, such as aircraft engines, prior to World War I.[citation needed] However they lived on for some time in the automotive world and were used on the Jeep for instance. Flatheads are no longer in common use for automobiles (except in some rodding and customizing circles),[1][3] although they are still used for some small-engine applications like lawnmowers. Because of their design, the size of valves and the compression ratio are limited, which in turn reduces available power and economy.[citation needed]

A new arrival is the Belgian D-Motor LF26, a compact 2.5 litre flat-four aero-engine that has direct drive to a propeller, without any reduction gearbox[4]. Dispensing with the complexity of an ohv valvetrain, the engine gives peak power of 80bhp at only 2800rpm. The resulting simplicity, lightness, compactness and reliability are ideal in an aero-engine. [5] D-Motor is developing a 120 bhp 6-cylinder version.

Harley-Davidson motorcycle flathead enginesEdit

The flathead engine saw service in Harley-Davidson motorcycles beginning with the "Sport Model" opposed twin produced from 1919 to 1923, and continuing in 1924 with single cylinder export-model 21 cubic inches (340 cc) and 30.5 cubic inches (500 cc) singles and continued in the Servi-Cars until 1973. In the domestic U.S. market, the 45 cubic inches (740 cc) DL model (1929 to 1931) and its technical descendant, the RL model (1932 to 1936), started Harley's side valve tradition in the 45 cubic inch displacement class. The DL and RL models featured a total loss oiling system and were succeeded in 1937 by the WL 45, which had recirculating oil lubrication. The WL went on to serve in WWII as the U.S. and Canadian Army's primary two-wheeled mount and subsequently as a civilian middleweight through 1952. The engine continued virtually unchanged with various G-based designations in the three wheeled "Servi-Car" until production ceased in 1973.

In 1952, the K series flatheads was introduced, selling in parallel with the W series (which was discontinued after 1952), designed to compete with British sporting motorcycles of the time, as the American motorcycle Association allowed the 750 cc sidevalves to compete against 500 cc overhead-valve bikes. The K models featured a unit construction engine and transmission case, right side foot shift and left side foot brake, and evolved from 45 cubic inch (1952 to 1953) to 55 cubic inches by a 0.75 inches (19 mm) increase in stroke length (1954 to 1956) over its five year retail market run. The K series was replaced by the overhead valve Sportster series in the retail market in 1957. However, racing versions of the 750 cc K model, designated KR, continued to be produced in very limited numbers for some time after, winning both roadraces and dirt track events against overhead valve bikes limited to 500 cc through 1969, when the American Motorcycle Association finally decided to change the rules and make the venerable flatheads uncompetitive.[citation needed] The K racers were replaced first by the iron-head XR 750cc overhead valve engine, and two years later by the alloy-head XR, which continues in service in flat track racing to this day.

In 1930, the 74 cubic inches (1,210 cc) VL flathead replaced the JD Big Twin, which had featured intake-over-exhaust (IoE) valve configuration. The VL had a single downtube frame and total loss oiling, culminating in an 80 cubic inches (1,300 cc) version (VLH) in 1935. In 1937, that engine was redesigned to include a recirculating lubrication system, and designated the model U, and it went into the same frame and running gear configuration as the model E Knucklehead, which had originated in 1936. The U continued to be produced in varying configurations as a 74& cubic inch U & UL (1937 to 1948), and 80 cubic inch UH & ULH engine (1937 to 1941). By that time, the first year of the aluminum-head Panhead, it had been thoroughly superseded and outsold in the marketplace by the superior performance of the overhead valve model Big Twins.

Popular CultureEdit

The reason that the overdrive of the Hot Rod Lincoln "just won't stall" is probably that it is a flathead V 12. (Flatheads in general, tend to have good low speed performance because of the lack of valve overlap).

See alsoEdit


  1. 1.0 1.1 American Rodder, 6/94, pp.45 & 93.
  2. Road and Track, some time in the 1960s
  3. "Street Rodder, 1/85, p.72.

External linksEdit

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