Harley Breather Valve Upgrade

The air that evacuates from the breather and into the air box is always an oily mist. A Harley breather valve, in need of service, will allow the evacuating air to be more saturated with oil. An air filter element that is overdue for cleaning; may look like a breather is emitting excessive oil.

Let’s take a look at the 88 Twin Cam top breather assembly and how it functions on this 2005 Harley Davidson Dyna. The purpose of the Harley crankcase breather system is to relieve crankcase pressure which is produced by the down stroke of the pistons. It allows crankcase vapors to be evacuated from each cylinder and be guided into the air filter element.

As each piston pushes downward on it’s power and intake stroke, air in the crankcase vents through the crankshaft bearing and into the cam compartment. From there, the air pushes up the pushrod tubes and into the front and rear rocker housings. The air then pushes under the rocker arm support plate and passes through an opening in the bottom of the plate and enters the breather baffle compartment.

Inside the breather baffle compartment, the flow of air passes upward through the oil filtering material (gauze), where the oil is removed from the air. There are two small holes in the bottom of the rocker arm support plate that work as drain holes for the collected oil. The air that passes through the filter gauze pushes through the rubber umbrella Harley breather valve and into the breather compartment. The flaps of the umbrella valve allow air to be vented in only one direction.

Once the air has entered the breather compartment, the flow of air passes through the holes aligned in the breather baffle, the support plate, and the rocker housing. The air enters a passageway which is cast into the top of each cylinder head.

Finally, depending on the motorcycle’s air filter configuration, the air flows through the head, through the air cleaner backing plate/backing plate hardware/breather tubes, and is directed into the air filter element.

Please make sure to read our Tech Q&A on:

Oil Dripping from Harley Air Cleaner

Check Out Our Related Videos for More Information:

• Dyna Gas Tank Removal
• Air Intake Installation
• Rocker Breather Replacement
• Another Rocker Box Breather Swap
• Top End R&R