Classic Motorcycle Build

Harley 45 Flathead (History and Specs)

Article by Mark Trotta

The Harley 45 flathead motor powered two-wheeled motorcycles until 1952, and three-wheeled 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 motorcycle history and specs

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 WL series 45, which had recirculating oil lubrication.

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.

Harley 45 Engine Specs

A bore and stroke of 2.75" x 3.18" displaced 742cc, advertised as 750cc, which is also 45 cubic-inches. Throughout it's long production run, engine bore and stroke remained the same. This makes parts interchangeability between different years excellent.

Harley 45 motorcycle left side
Picture Courtesy American Classic Motorcycle Museum

Pistons, connecting rods, cylinders, and many other parts were the same for all years of Harley 45 motors. One minor difference is that on 1936 and earlier engines, the oil holes and key-ways on the flywheels are different than on later engines.

In stock trim, Harley 45 output was about 21 horsepower. Standard 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.

Harley 45 flathead

The Harley 45 flathead engine was left pretty much unchanged throughout it's 41-year production run. Until the Sportster surpassed it in 1998, it was Harley-Davidson's longest running engine in production.

Harley 45 Flathead Models

Harley-Davidson WLA

During the years 1940-1945, nearly all Harley-Davidson motorcycle production was for military use. The vast majority was the WLA military bike. Accessories included a luggage rack, ammo box, and leather rifle scabbard. Painted in olive drab or black, bikes were equipped with blackout lights to reduce nighttime visibility.

WLA Harley 45 flathead

Read: Harley WLA History

When World War II was over, there was a large surplus of WLA bikes, most of them being sold very cheaply. In America, these surplus military models became some of the first modern-day choppers.

Harley WL Solo

The WL series were manufactured from 1937 until 1952 and featured a "high-compression" motor. The compression was increased through use of different cylinder heads. Harley-Davidson called this civilian model the Sport-Solo. The 45's highest output model, however, was the WR factory racer.

Harley 45 motorcycle history and specs

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 were 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.

During the Korean War, manufacture of Harley WLA started up again.

Small vs Big Harley Flathead

The Harley 45 flathead motor looks similar to a Big-Twin Harley flathead. The easiest way to tell them apart is that big-twin flatheads (UL and VL) have their drive chains on the left, and the 45 has them on the right. The two engines do not share many interchangeable parts.

Harley 45 flatheads 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 vintage motorcycles.

Harley-Davidson Servi-car Gas/Oil tanks

Read: Harley 45 Gas/Oil Tanks

Spark Control Grip

Retarding the distributor timing helps cold engine starting, and on early Harley flathead models this was done manually. The left hand grip was referred to as the "spark control" grip.

Before kick-starting the engine, the rider twisted the cable-activated left grip outward, moving the ignition timer outward. This would retard the ignition timing. Once the engine started and was warmed up enough, he would twist the grip back inward to it's fully advanced normal running position. Sometimes when riding, a rider might retard the distributor slightly to avoid engine knock under heavy load or acceleration.

Auto Advance Timer

In 1964, flathead 45 Servi-cars were upgraded with "auto advance" ignition timers. Auto-advance doesn't make an engine run any better, it just makes it easier to operate.

Harley 45 auto advance circuit breaker

Read: Harley 45 Timer And Ignition

Mechanical and auto advance ignition timers are interchangeable with each other. Custom bike builders favor the latter because it's visually cleaner; removing the spark control grip and cable unclutters the handlebar area.

Harley 45 Magneto

High performance flatheads like the WR factory racer were equipped with magneto ignition. Unlike an ignition timer, they don't require a coil or battery to produce spark. Instead, spark is created by internal magnets. Magnetos work well at higher speeds but produce weaker spark at low RPMs.

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.

Harley two-brush generator

Your classic bike's generator really doesn't charge the battery until engine rpm is considerably higher than idle. Trips that are less than 15-20 miles are typically not enough to recharge the battery's losses from starting.

Read: Harley Generator - Repair or Replace?

Harley 45 Alternator

Servi-car 45 motors from 1966 and later came with a car-style alternator which was externally driven by a belt.

Harley 45 motorcycle gas and oil tanks

Read: Custom-Wiring A Motorcycle

Harley 45 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.

Harley WLA motors had an additional number in the VIN (example: 42WLAxxxxx). NOTE: all WLA motors began with "42", regardless of the year they were produced. This was unique in H-D's history, and was presumably done to simplify paperwork.

Harley 45 WL flathead motorcycle history and specs

Although pre-1970 Harley-Davidson motorcycle frames were never numbered, engines were usually sequentially-numbered. So, even though they all have the same "42 WLA" designation, it's possible to identify the year of manufacture from casting numbers and date codes on the engine cases.

Most Harley Servi-car motors are stamped 'G' or if it's an electric start (1964-1973) will be stamped 'GE'. 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.

Harley 45 Belly Numbers

This example will 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. The second and third digit is the year (1961), and will probably be stamped in a closed-top letter style with a short horizontal stroke across the top. The four numbers after the dash are a sequential assembly-line number.

If you're building a Harley 45 motor from the cases up, belly numbers that match usually mean that line-boring may not be required, although its always a good idea to check.

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Read: Harley 45 Drivetrain

Harley 45 flathead engine assembly

Read: Harley 45 Bottom End Rebuild

Harley 45 flathead engine assembly

Read: Harley 45 Cam Timing

Harley-Davidson flathead 45 cylinder heads

Read: Harley 45 Cylinders and Heads

Harley flathead 45 oil pump install

Read: Harley 45 Scavenger Oil Pump Repair