Harley-Davidson Timed Breathing Explained

HARLEY engines (excluding early engines with total loss oil systems) manufactured up until 1999 used some form of timed breathing to scavenge oil from the crankcase and deliver it to the vicinity of the return oil pump so that it could be sent back to the oil tank.

Timed breathing was introduced on the first Knuckleheads in 1936 and remained fundamentally unchanged on OHV Big Twins through to the end of production of the Evolution engines in 1999. Sidevalve engines and Sportsters used a similar system in as much as a breather opened and closed in relation to the position of the pistons in the cylinders.

With timed breathing, as the pistons travel down the bore, a rotary breather valve starts to open and the pressure created under the pistons pushes the oil-air through the open breather valve window into the cam chest and pickup area for the scavenge pump. When the pistons commence their trip back up the cylinders, the breather valve closes and a vacuum is created which sucks oil from the crankcase into the breather cavity. As the engine continues to rotate, the suck and blow cycle continues.

The vacuum created in the breather valve by the up-stroke of the pistons has also been used for other scavenging operations. Knuckleheads use this vacuum to return top-end oil and early Electra Glides relied on vacuum to retrieve engine oil from the primary chaincase where it was used to lubricate the primary chain.

Harley-Davidson Timed Breathing Explained

At the left side of this crankcase you can see the window leading into the timing chest.

Harley-Davidson Timed Breathing Explained

When the opening in the rotary breather valve lines up with the window, a passage is formed and oil-air is forced out of the crankcase cavity by the pressure under the pistons travelling down their bores. In this photo the breather valve is just starting to open. The rotary breather valve makes one full revolution for every revolution of the engine.

When building engines for racing and high performance applications, the breather window in the crankcase can be opened up somewhat with a resulting increase in horsepower and scavenging efficiency. This is a very exacting job and the precise opening and closing times of the breather valve needs to be calculated in crankshaft degrees.

Traditionally, the breather valves were machined from steel with a ground finish on the outside diameter. These gave very little trouble, and unless foreign material from an engine failure tried to get through, they would last forever.

In late 1977 Harley replaced these steel breather valves with a moulded plastic or nylon version. These were not as durable and premature wear in the breather valve bore was common. A steel breather valve in these engines is a very worthwhile update.

Harley-Davidson Timed Breathing Explained

A period repair. This breather valve bore had been damaged and repaired with a bronze sleeve. This same repair can now be accomplished with an oversized breather valve produced by S&S. The S&S breather valves are available in standard and oversize diameter for all models 1936 through to 1999. Although this crankcase has been glass-bead blasted and is very dusty, the various passages that line up with their respective holes in the rotary breather valve can be easily seen.

Harley-Davidson Timed Breathing Explained

Sidevalve breather valve

Harley-Davidson Timed Breathing Explained

Sportster breather valve

Harley-Davidson Timed Breathing Explained

Knucklehead breather valve

Harley-Davidson Timed Breathing Explained

Evolution breather valve

Harley-Davidson Timed Breathing Explained
Picture Page 67-69 lower left hand corner S&S catalogue

For 1993 through to 1999 Big Twin engines, S&S has devised an alternative to the gear-driven breather valve in the form of a reed valve. This clever gadget fits in the existing breather valve bore and does not rotate although it still uses the same window. The end result is much the same but is achieved in a slightly different manner.

As with the timed breather, the pistons travel downward in the cylinders and the pressure in the crankcases increases. This pressure causes the reeds to open, allowing the air and oil mist in the crankcase to escape into the cam chest. When the pistons change direction and begin travelling back up the cylinders, a vacuum is created in the crankcase which causes the reeds to close, preventing air from entering the crankcase. The effect is that a slight vacuum is maintained in the crankcase and oil scavenging is greatly improved. These reed valves are also available in oversize for damaged crankcases.

A full range of S&S breather valves and related parts are available from Redgrave Motorcycles at all times. Call for more details. Redgrave Motorcycles: 02-9484-9955.

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One Comment

  1. if you happen to put the breather gear in and not line up the timing marks on the gear with the cam would this stop oil returning to tank when bike running

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