The Harley-Davidson Knucklehead: The Engine That Built the Big Twin

There are engines that move a motorcycle, and there are engines that move a whole company. The Harley-Davidson Knucklehead engine is the second kind. It was Harley’s first big twin with overhead valves. It set a world speed record in its second year. And nearly ninety years later, it’s still the engine most people picture when they think “old Harley.” When one rolls through our workshop door, everything else stops for a minute.

Here’s the story of the Knuckle, what makes it tick, what goes wrong with it, and what we look for when one comes in for a rebuild.

Where It Came From

In the early 1930s, Harley-Davidson was in a tough spot. The Depression had gutted motorcycle sales, and Indian was still a serious rival. Harley’s big twins used side-valve flathead engines. They were reliable, but the design had little development left in it. The company needed something new, and it bet on overhead valves.

The new engine arrived for the 1936 model year as the 61 cubic-inch E series, roughly 1,000cc. It came in three flavours. The E was the base solo model, making around 37 horsepower. Harley built the ES for sidecar work. The EL, which Harley sold as the “Special Sport Solo,” ran higher compression for about 40 horsepower. It could reach around 95 mph.

A four-speed hand-change gearbox with a foot clutch handled the power. The frame used double down tubes, and a springer fork sat up front. It was Harley’s first production overhead-valve V-twin, and it put the company ahead of Indian at exactly the right moment.

Why “Knucklehead”?

Harley never called it that. The factory simply named it the E or EL. Riders came up with the nickname because of the rocker boxes on top of each cylinder head. They’re polished and bulbous, with big round covers over the rocker shaft ends. Together, they look like the knuckles of a clenched fist.

Once you’ve seen it, you can’t unsee it. The name stuck so well that riders named Harley’s later engines the same way: Panhead, Shovelhead, and so on.

A Rough Start

The first Knuckles weren’t perfect. Early bikes broke valve springs, starved the rocker arms of oil and leaked from the top end. Harley responded quickly. It improved the spring material, redesigned the rockers and sent repair kits to dealers for the bikes already on the road. More than a thousand bikes needed the fix.

That’s worth remembering, because the top end is still where a lot of Knucklehead problems live today. More on that below.

Proving the Point at Daytona

Whatever teething problems it had, nobody could argue with 1937. Factory rider Joe Petrali took a partly streamlined, twin-carburettor 61 cubic-inch OHV Harley to the measured mile at Daytona Beach. He set a world motorcycle speed record of 136.183 mph. The same year, an EL covered 1,825 miles in 24 hours at an average of 76 mph.

One run showed the engine had speed. The other showed it could hold together doing it.

How the Knucklehead Engine Works

Under those polished covers, the Knuckle is a 45-degree V-twin with a single camshaft in the timing case. Pushrods run up from the cam to the rockers in the heads, and the rockers open the overhead valves. Moving the valves into the head gave a much better combustion chamber than the old flatheads. The engine breathed better and made more power from the same capacity.

The other big step forward was the oiling. Earlier Harley big twins used a total-loss system, where oil passed through the engine once and then burned off or dripped out. The Knucklehead brought in a pressurised, recirculating dry-sump system. Oil lives in a separate tank. A pump pushes it through the engine under pressure and returns it to the tank to use again.

Oil pressure rises with engine speed, so the harder you ride, the more oil flows. It sounds basic now, but in 1936 it was a big deal. It’s a large part of why the Knuckle could survive sustained high speeds when older designs couldn’t.

From 61 to 74 Cubic Inches

In 1941, Harley added a larger 74 cubic-inch version of about 1,200cc. It sold as the FL alongside the original 61. More displacement meant more torque, which suited heavier touring bikes and sidecars. The 74 became the engine Harley carried forward.

Then the war got in the way. The government heavily restricted civilian production. In 1943, Harley built just 158 of the 61 cubic-inch EL and ES models, compared with more than 4,000 in 1940. Production recovered after the war, with over 4,300 built in 1947, the final year.

For 1948, Harley replaced the Knucklehead with the Panhead. The new engine kept the same basic bottom end but brought aluminium heads and hydraulic lifters. The Knuckle had a run of twelve model years, from 1936 to 1947.

Why Knuckles Are So Sought After Today

Part of it is history. This is the engine that set the pattern for every Harley big twin since. Part of it is scarcity. Harley built them in small numbers by modern standards, and the war years produced almost none. Plenty of owners crashed, chopped or wore them out long before anyone thought of them as collectible.

Here in Australia, every one of them arrived as an import, so a genuine Knucklehead is a rare sight. And part of it is simply the look and the sound. Those rocker boxes and that lumpy, mechanical beat make the Knuckle a favourite for concours restorations and raw bobbers alike. You can see a beautifully restored 1936 EL in the National Motorcycle Museum’s collection.

What Goes Wrong With Them

Any engine running on and off since the 1930s or ’40s has lived a long life. Most have had a few owners with their own ideas about maintenance. These are the problems we see again and again.

Top-End Oil Leaks

Knuckleheads have a reputation for leaking oil from the rocker area, and they’ve earned it. Worn rocker shafts and bushings, tired gaskets, poorly fitted covers and bad assembly all add up. A Knuckle will never stay as dry as a modern engine. But it shouldn’t drip all over the shed floor either.

Rocker and Valve Train Wear

Rocker lubrication was a weak point from day one. We often find worn rocker arms, shafts and bushings on engines nobody has opened in years. Stretched valve guides and tired valve springs usually come with them.

Bottom-End Wear

The crank, rods and roller bearings take a beating over the decades. Before we quote any rebuild, we check for play in the big end, worn main bearings and a crank that’s out of true.

Oil Pump and Breather Problems

The dry-sump system only works if the pump is healthy and the breather does its job. A worn pump or incorrect breather timing causes poor oil return and wet-sumping. It also pushes oil out where it shouldn’t go.

Electrics and Carburation

Knuckles ran a generator with points and coil ignition. Most also ran a Linkert carburettor. Old wiring, tired generators, worn points and worn carby throttle shafts all cause hard starting and poor running.

Previous Repairs

This is the big one. We see mismatched parts, welded cases and heads from different years. We find badly fitted thread repairs and engines built from whatever was on hand. Part of any Knucklehead job is working out what’s actually there before deciding what it needs.

How We Approach a Knucklehead Rebuild

Every Knuckle is different, so every rebuild starts with a complete strip-down and inspection. We measure everything and check the cases and heads for cracks and old repairs. Then we give the owner a clear picture of what we found and what it’ll take. You’ll know the plan before you spend a cent on parts.

From there, it comes down to what you want the bike to be. That’s where OEM or OEM+ comes in:

• OEM means building it the way the factory did: correct parts, correct finishes, correct specification. It’s the right path for a numbers-matching bike or a display restoration.
• OEM+ keeps the look and sound of a Knucklehead but quietly improves the parts that let it down. Think better gaskets and seals, improved valve springs, upgraded bearings and electrics that start first time. It’s the right path for a bike you’ll ride.

Both are valid. The key is deciding up front, so the parts, machining and finish all suit how you’ll use the bike.

The good news is that parts support for Knuckleheads is better now than it’s been for decades. Suppliers now make quality reproduction parts for most of the engine. A worn-out Knuckle that sat parked twenty years ago can now come back to life properly.

Got a Knucklehead That Needs Work?

Maybe it’s a complete engine rebuild, a top end that won’t stop leaking, or a basket case you’ve just dragged out of a shed. Either way, we’d love to see it. Garage Eleven works on Harley-Davidsons old and new from our workshop in Pakenham, Victoria. You can also freight your engine to us from anywhere in Australia.

Give us a call on 0423 963 402 or book your bike in, and we’ll talk you through where to start.