“All Harleys leak.”

I hear this often. It’s usually followed by someone spitting, hitching up their drawers, and looking smugly at their own (non-Harley) motorcycle. Unless it’s someone I really like, I usually just say, “Yup, they sure do.” If someone thinks Harley products are poorly made leaking pieces of shit, I’m not going to try to convince ‘em they ain’t. I don’t own a dealership and I don’t need to sell bikes. For those whose opinions I value, though, I generally take a minute to agree with them and then point out why they leak.

There’s some truth in the old adage, “I get worried when it stops leaking — then I’m out of oil!”

Not all leaks are good, but many are not just acceptable, but even intentional. Many people are not aware that there’s a fair amount of drippy-ness planned into a properly functioning Harley-Davidson.

Let’s rewind to earlier days. Until 1936 (for the overhead-valve bikes) and 1937 (for the side-valves), Harley engines did not have provisions to circulate the oil through the engine more than once. This style of lubrication system is known today as “total loss.” Oil made its way through the engine and then was ejected onto the roadway. This was not exclusive to H-D. It was common for stationary and agricultural engines at the time to expel oil directly onto the ground. This system became impractical as engines advanced, but for the time, it was acceptable practice. In fact, modern two-strokes are still total-loss with regard to the lubricating oil.

Notice the breather tube sticking out of the engine case. Oil exits and drips onto the (as of yet uninstalled) spinning chain below. Photo by Lemmy.

Oil systems became the recirculating type in the 1930s. This means that a scavenging pump was added to capture the oil after it completed its circuit through the engine, returning it to the oil tank. Despite the fact the oil recirculated, quite a bit still hit the ground, mostly by design. Engines need to “breathe” in order to equalize crankcase pressure with atmospheric pressure. (If you don’t understand this, hold tight — we’re going to discuss it in just a bit.) Harley engineers knew this, and understood that there must be at least one point at which pressure could exit the system. Rather than try to fight a losing battle and fight the necessary leak, they did just the opposite — they allowed users to increase the severity of the leak. Here’s why.

At the top left of this cam cover, the chain oiler is visible... Photo by Lemmy.

From the first days of the recirculating oil system, a small screw controlled the flow of additional oil routed to the breather tube. (For instance, on Big Twins, the screw was located on the cam cover.) Depending on the year, either shims or a locknut were used to change the height of the screw, controlling the flow of oil directly to the primary chain. This system allowed for a dry clutch, but permitted the primary drive chain to have a steady flow of oil being supplied to it. The tin-type primaries (chain cases) had a hole at the bottom to expel oil that had lubricated the chain and was flung around, to prevent it from building up. This design ran all the way through the Panhead years (late 1940s to mid 1960s).

...and here you can see the oiler and how it works. The tapered section is what controls the flow, and pictured with it are some shims, used for setting the height of the oiler screw. Those were eliminated in 1950, when H-D elected to use a locknut on the Big Twin models, making them adjustable without additional shims. Photo by Lemmy.

Well lubricated roller chains generally offer very long service life, so at the expense of a few pennies’ worth of oil, the drive chains had long, healthy careers. The kiss of death for a primary chain is to run dry. The oldtimers who are still with us will tell you that a bike was properly adjusted if it left a spot the size of a 50-cent piece when parked overnight. (I personally run mine a little juicier than that. Oil's cheap. Primary chains are not.) To take this a step further, Shovelheads actually had two chain oilers — one for the primary chain, as well as an additional one for the final drive chain. The primary oiling changed a bit. The scavenge pump was now connected to the primary. This had the desirable benefit of keeping the primary from dripping onto the ground, but the undesirable benefit of sending oil impregnated with friction material (clutch fibers) back into the engine.

I promised I’d get to crankcase pressure, and now we will. If you can imagine the big pistons moving up and down in a Harley, squeezing air and fuel at around 100 psi, think about the pressure on the other side of the piston rings. It rises and drops dramatically, and blowby - exhaust gases which have blown by the rings - also serve to pressurize the area. The breather system helps to equalize the pressure between the atmosphere and the interior of the engine. It also exploits the regular vacuum that occurs to help return oil to the oil bag. Harleys tend to have greater pressure differentials than other bikes because they have exceptionally large pistons, and there aren’t too many of them. (On smaller displacement multi-cylinder bikes, breathing issues aren’t nearly as prevalent.)

As we discussed earlier, Knuckles and Pans vented to that chain area. Like the total-loss oiling system, this was in keeping with the technology of the times. Automobiles used a similar system, called the road draft tube. Effectively, it was an open pipe that ran from the engine down to the road surface, performing much the same function without the pleasant byproduct of automatically performing a necessary lubrication task. Harley was actually ahead of the design curve with their oilers.

Near that red arrow is the case breather. Normally, that would run up to the air cleaner as in modern Harley products, but this one has been set up to simply vent to atmosphere.

In the Shovelhead era, environmental reality set in. Releasing burnt hydrocarbons into the air contributes to smog and air pollution. The solution? The breather lines were divorced from the chain oilers. Rather than vent them to the open atmosphere, breathers were routed to the air cleaner. From there, the oily mist they generated could be sucked back into the engine’s intake tract with the incoming air-and-fuel charge. This also had the benefit of filtering the air the crankcase breather took in as well, keeping oil cleaner.

Here you can see a pair of head-breathers. These were introduced in the early 1990s, depending on model. Because this Big Twin Evo is not running a factory air cleaner, the horseshoe-shaped piece collects breather mist and vents it remotely. Photo by Lemmy.

Unburnt hydrocarbons pollute, too. The chain oilers disappeared in the the Evolution era. The primary chain cases were sealed, eliminating the source of another oil leak. Rather than drip oil, the clutches were changed to a wet design to help to cool the stator, which had been relocated to the primary. Rear drive chains gave way to belts, so a rear chain oiler also became unnecessary, but the breather-routed-to-the-air-cleaner lived on.

This same system is in use today, which is why you’ll see oily air cleaners on an engine that’s been run hot and hard — the engine is “breathing” into the air cleaner. When the housing loads up with too much oil, droplets form which will fall downward when the bike is still or spray backwards if it’s in motion. If you see a Harley piddling oil from the air cleaner — even a modern one — that’s what they’re engineered to do.

Many modern bikes from other manufacturers use an airbox buried deep within the bike, and the filter sits horizontally, not vertically as on a Harley. Even if the bike does breathe heavily, the airbox contains most of the mess. Harley air cleaners, hanging off the side of the bike, are considerably more visible than their multi-cylinder counterparts, so any airbox oil that's found its way out is much more noticeable.

The breather system vents inside the air cleaner housing, into the back of the filter, which has been removed for this photo. Obviously, if very much oil gets into the filter, it leaks out, eventually exiting the housing, causing a drip. RevZilla photo.

One other leak that’s also common on most Harleys is the dreaded wet-sump, when oil collects in the crankcase. Harleys are dry-sump bikes, meaning the oil is stored remotely and pumped in. All older Harleys and some new ones locate the oil in a tank that is higher than the bottom of the engine cases. Liquid oil seeks to fall, and the only thing holding it in the oil tank is a small check ball. Dirt and grit moving through the system, wear and tear, weak spring pressure, and imperfections in the seat can allow the check ball to let oil slip by. (For those of you who have had a carb needle not mate properly to its seat, this is a very similar situation.)

If the bike sits without running for any appreciable time period, the crankcases fill with oil that slips by. Upon startup, the scavenging pump gets overwhelmed. Thus, the oil blows out the crankcase breather or air cleaner in copious amounts. It’s totally normal — Bill Harley and Artie Davidson designed bikes that were going to be ridden, not parked for months at a clip.

It seems ludicrous in these days of $10-a-quart oil to waste it, but given the cost of parts and specialized labor, a small quantity of oil dribbling out of a bike made a lot of sense 70 years ago. These machines were designed to be used and used hard in a country that did not yet have an interstate highway system. Whether it's crankcase pressure equalization, primary or final drive chain maintenance, or the scavenge pump tirelessly working to clear the bottom end of a bike that hasn’t been run in a bit, one thing is certain: Harleys leak.

As they're meant to.