Classic Motorcycle Build

Harley 45 Flathead

Produced from 1929 to 1973, the Harley 45 flathead motor was offered in motorcycles until 1952 and Servi-Cars until 1973. These rugged, reliable V-twin engines were easy to maintain and would continue to run even in the worst of tune. They were simple and efficient, but not particularly powerful in stock trim.

Harley 45 flathead

The first Harley 45 flathead was the DL model (1929-1931), followed by the RL model (1932-1936). Both the DL and RL engines used a total-loss oiling system, and were succeeded in 1937 by the WL45, which had recirculating oil lubrication. Original compression ratio was 4.5:1, which increased to 4.75:1 in 1941. Remember that at this time in America, there was no high-octane gasoline to allow high-compression engines to operate. A bore and stroke of 2.75" x 3.18" made 742cc, advertised as 750cc, which is 45 cubic-inches. Horsepower was rated at 21.

read Harley 45 Drivetrain

Side-Valve Simplicity

Advantages of a flathead (side-valve) engine over an OHV (overhead valve) engine were cost of manufacture and simplicity. With valves positioned in the engine block beside the piston, instead of in the cylinder head as in an OHV engine, push rods and rocker arms were not needed. Cylinder heads need only be a simple casting with threaded holes for the spark plugs.

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Factory bore, stroke, pistons, and rods were the same for all years of Harley 45 motors. Be aware that on 1936 and earlier engines, the oil holes and key-ways on the flywheels are different than on later 45 engines.

Harley 45 flathead

Harley-Davidson WLA

Nearly all Harley-Davidson motorcycle production was for military use during the years of 1940-1945. More than 88,000 WLA models were built; "W" indicating 45ci flathead, "L" signifying high compression, and "A" for Army use, both American and Canadian. When WW2 was over, there was a great surplus of these bikes, with many being sold very cheaply. These surplus WLA models became some of the first modern-day choppers.

WLA Harley 45 flathead

Harley WL Solo

The WL series was built from 1937 until 1952 and had a 45ci high-compression motor. Harley-Davidson called this model a Sport-Solo. The 45's highest output model, however, was the Harley WR factory racer.

Harley WR Racer

In 1946, the 45ci WR Flathead racer was introduced. It conformed to stricter Class C AMA rules, which were intended to reduce costs for competitors. In Class C rulings, flatheads are allowed to displace 750cc, while OHV motors were limited to 500cc.

With increased compression and hotter cams, the WR-750 shared a large number of parts with the mass-produced military WL series. The WR racer saw considerable success, competing against rival Indians on dirt track speedways throughout the 1930s.

Harley 45 Flathead Models

In the 1975 film, The Rocky Horror Picture Show, Eddie (played by singer Meatloaf) rides out of the deep freeze of Dr. Frank-N-Furter's lair on a Harley-Davidson WLA.

Rocky Horror Harley WLA

Harley 45 Generator

There are two types of generators found on Harley-Davidson 45 flatheads. Early-style motors (1937 to 1958) have a three-brush generator, which is mounted to the engine with two 1/4" bolts, and utilizes a strap which goes around the generator body. Later 45 motors (1959-1965) have a two-brush generator which does not require the strap, and has two 5/16" bolts holding it in place. Servi-car motors from 1966 and later came with an car-style alternator which was externally driven by a belt.

Harley two-brush generator

Generators and voltage regulators are always suspect on old Harleys. Consider upgrading to a trouble-free aftermarket unit like Cycle-Electric.

Harley 45 Piston Pins

There are two styles of piston pins found on the Harley 45 flathead. The early-style pin used an L-clip and was seen until about 1952. The late-style was the more common E-clip seen from 1952 until 1973. Pictured below is the early (pre-1952) style.

Harley-Davidson Flathead 45 pistons

Harley 45 Engine Serial Numbers

Stamped in the left engine case, the serial (VIN) number starts with the last two digits of the year, followed by the letter designation, then a 4 or 5 digit assembly sequence code. Made from 1941 through 1945, the WLA engine had serial numbers beginning with 42WLAxxxxx. There were seven different configurations of 42WLA's during those years.

Most Harley Servi-car motors are stamped 'G' or 'GE' if it's an electric start 1964 and later. Very early models may be stamped 'GA' or 'GD' or 'GDT'. The letters (up to four) are the model code. The first two numbers are the year and the remaining numbers are the production number.

45 Engine Case Numbers

This example may help decipher the engine case numbers on your Harley 45 motor. In number 361-1722, the number 3 will probably indicate a round top case. Second digit, decade 6, will probably have a round back case. Third digit, year 1, will probably be stamped in a closed-top letter style with a short horizontal stroke across the top.

Harley 45 flathead engine assembly

read Harley 45 Bottom End Rebuild

read Harley 45 Cam Timing

read Assemble Harley 45 Cylinders

Harley 45 flathead motors never needed lead. The reason tetraethyl lead was added to gasoline in the first place was to increase octane for vehicles with high compression. This is not an issue with old Harley flatheads.

The 45 Harley flathead motor looks similar to a Big-Twin flathead. The easiest way to tell them apart is that the big flatheads have their drive chain on the left, and the 45 has them on the right.

The 45 flathead was left pretty much unchanged throughout it's 41-year production run. Until the Sportster surpassed it several years ago, it was Harley-Davidson's longest running engine in production.

read Harley 45 Gas/Oil Tanks