How to Tell a Harley's Engine Size

by Chris Gilliland

Harley-Davidson has produced an array of engines to power its classically-styled motorcycles throughout the years, ranging from the original 25 cubic-inch single-cylinder engine built in 1903 to the massive 103 cubic-inch Twin Cam V-twin engine that powers all Harley Big-Twin models built in 2012. Determine your motorcycle's engine displacement -- or size -- by using the engine's bore and stroke specifications, the motorcycle's vehicle and engine identification numbers or simply through visual identification of the engine itself.

Bore and Stroke Specifications

The most accurate method to determine a Harley-Davidson's total displacement in cubic-inches is by using the engine's bore and stroke specifications; however, this can only be done if you can obtain these specifications through a service manual, magazine review or a spec sheet provided by the engine tuner. Bore and stroke refer to the inner diameter of an engine's combustion chamber within the cylinder head and the distance of the piston moves from top dead center to top dead center in a single movement, respectively.

The formula used to determine engine displacement appears as: B × B × p/4 × S × N = Displacement.

In this case, B would equal bore, p/4 is pi -- 3.14 -- divided by four, S equals stroke and N is the number of engine cylinders. Let's use a typical Twin-Cam 88 engine, which was used between 1998 to 2006, as an example. A TC88 had a 3.75-inch bore and a 4-inch stroke. Using the formula above, it would appear as: 3.75-inches x 3.75-inche x 0.785 x 4.00-inches x 2 cylinders = 88.3 cubic-inches. Likewise, the newer TC96 engine has an identical bore, but a longer 4.38-inch stroke. This results in a 96.7 cubic-inch displacement, using the same formula.

This formula is an extremely accurate method that can be useful when determining the displacement of a modified, high-performance engine, but is equally applicable to stock engines, as well.

Visual Identification

Harley-Davidson's engines are well-known by their appearance, many of which are nicknamed after the shape of their cylinder heads or other engine components. These design elements can help you determine the displacement of a particular engine; however, some engine types were available in a range of displacements, making it a bit more difficult to determine the exact size of the engine by visual clues alone. For example, the 45 cubic-inch Flathead engine -- produced between 1929 and 1947 -- had flat-topped cylinder heads and placed its valves along the sides of the cylinders. Likewise, the 1966 to 1984 Shovelhead engine was identified by its shovel-shaped rocker box covers on top of the cylinder heads. Determining the exact displacement for any particular engine often comes down to being able to decode the engine number or by identifying the type of motorcycle it belonged to. Newer Twin-Cam engines, however, often have a decal over the timing cover on the right side of the crankcase denoting its engine type.

Vehicle and Engine Identification Numbers

Nearly all Harley-Davidson motorcycles built have some sort of identification number, imprinted either on the engine, frame or both, that allow technicians to quickly identify the motorcycle's model and engine type. Unfortunately, older Harley-Davidson identification numbers ran through a series of constant changes throughout the years, making it difficult to identify early engine models.

Harley-Davidson used as many as four Big-Twin engines between 1929 and 1968, ranging from the Flathead, Knucklehead, Panhead and Shovelhead engines. Engines built before 1969 typically use a 10- or 11-digit identification number, starting with a two-digit production date, followed by a two- or four-character model type and a four- or five-digit production run number. Using the model type as a guide, you can determine the size of the engine that was equipped with the motorcycle.

Shovelhead engines built between 1970 and 1980 were available with 74 or 80 cubic-inch displacements and used a similar coding system as the previous engine types. However, the model type is indicated by the first two characters, while the next five digits indicate the production run, followed by a two-digit model year identifier. It should be noted that the 80 cubic-inch engines were only available after 1978.

All engines built after 1980, including the 74 and 80 cubic-inch Evolution Big-Twin engines, 883 cc and 1,200 cc Sportster engines, and the Twin Cam 88, 96 and 103 engines, follow the same method as the Shovelhead engine codes.

Current Engine Types

Current model Harley-Davidson motorcycle use three engine types in its motorcycle line: the Twin Cam, Evolution and Revolution engines. Engine displacement is fairly consistent throughout these engines, making it easier to determine engine size by identifying the motorcycle's model and production year. The Twin-Cam 88 engine was equipped on most of Harley-Davidson's motorcycles starting from 1998, excluding only the Evolution-engined Sportster models. The Evolution engine was available with either an 883 or 1,200 cc -- 45 and 74 cubic-inches, respectively -- displacement for the Sportster series. The displacement of the Twin Cam engine was increased to 96 cubic-inches in 2007, followed by a subsequent increase to 103 cubic-inches for most of the 2012 model lineup. The final engine type, the 69 cubic-inch Revolution engine, was introduced in 2001 to power Harley-Davidson's radically different VRSC V-Rod. Harley's sole liquid-cooled engine has remained unchanged through 2012 as is only found in the V-Rod family.

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