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Harley Breather Bypass – Everything You Need To Know

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If you’re like me, you’re always looking for ways to improve your Harley.

I’m not talking about slapping on a new air cleaner or exhaust – I’m talking real, useful improvements to your bike. And if you want to keep your Milwaukee-8 running smoothly for a long time, then the Harley Breather Bypass is a must.

But what is the Breather Bypass mod? What does the breather even do and why on earth would you want to bypass it? In this article, I’ll explain everything you need to know about the Breather Bypass mod, what it does, and whether or not you should do it to your Harley.

Also Read: The Best Stage 1 Air Cleaner For Harley Davidson M8?

What Does a Breather Do on a Harley Davidson M8?

Before we talk about the mod and its benefits, let’s first quickly talk about the breather system and its purpose.

The Harley Davidson Milwaukee-8 engine comes with a closed-loop breather system. This means that the engine vents positive air pressure, blowby, and oil mist out of the crankcase through the crankcase breathers (located on the right side of the engine) and back into the air intake, right before the throttle body.

The idea behind this is to reduce harmful emissions by letting the engine burn the blowby, instead of just dumping it on the ground, like most engines used to back in the day.

Due to EPA regulations, it is illegal to vent crankcase gases out into the atmosphere, and the easiest (and most profitable financially) way for engine manufacturers to comply with that is by using this closed-loop PCV system. There are better ways to do it, but we will talk about those later on in the article.

What Does a Harley Breather Bypass Do?

The Harley breather bypass simply vents blowby to a catch can or the atmosphere, thus dramatically reducing carbon buildup inside the combustion chamber.

The main problem with the breather design on the Milwaukee-8 is that some of those engines like to vent excessive amounts of oil vapor through the breathers, and all of that oil is sucked through the throttle plate and burned inside the combustion chamber.

This breather system is fine if there is little oil vapor, but if there’s too much, things start going sideways.

All of this oil fouls the air cleaner, the throttle plate, and most importantly – it causes a lot of carbon buildup on the pistons. That carbon buildup can disrupt airflow and actually increase the compression ratio if ignored for a long time, which will lead to poor performance and rough idle.

Even Doc Harley had to admit that venting crankcase gases back into the combustion chamber is not good for the engine:

Benefits of a Breather Bypass

Okay, since now we know that venting blowby and oil mist into the combustion chamber is not good for the engine long term, let’s talk about the benefits of a breather bypass.

1. Increased Engine Life

Bypassing the breather will reduce carbon buildup on the pistons, rings, and valves, which will lead to less wear and tear on the engine, and thus – a longer engine life.

2. Better Performance

A Harley with a breather bypass will also perform better than one without. Normally, your Milwaukee-8 is tuned from the factory to accommodate the hot, oxygen-depleted blowby vapors, but once you remove them from the intake, your bike will have access to more fresh oxygen.

Furthermore, your bike’s engine will run smoothly for longer since there won’t be large amounts of carbon buildup that reduce airflow.

3. Reduced Oil Consumption

One of the main benefits of the Harley breather bypass is that it will reduce your engine oil consumption. If you’ve been burning oil through the breathers, then you’ll notice that your oil level stays nice and full after switching to a catch can setup.

4. No More Oil Inside The Air Cleaner

Another Harley breather bypass benefit is that you won’t have to deal with oil inside the air cleaner anymore. This will save you cleaning time and make your air cleaner last longer.

Why Don’t Harley Davidson Engines Come With a Breather Bypass From the Factory?

Harley Davidson is not the only manufacturer that’s using this closed-loop breather system. Actually, almost every single engine made in the last 25 years uses this system.

But why? Why don’t engines come with oil catch cans from the factory?

First, that would be yet another maintenance item for the riders, and we already have enough of those.

A lot of people don’t have the time or the skills needed to replace their brake fluid or coolant every two years, so how could they drain the catch can multiple times a year?

Secondly, there are bound to be some riders that forget to drain their oil catch cans and destroy their engines by letting them overfill.

Can you guess what they would do afterward? They would simply sue Harley Davidson, and I’m pretty sure the company does not want that.

Sadly, this crankcase ventilation design is the only real solution that we have right now, and I don’t think Harley Davidson should be blamed for going with it.

How to Do a Breather Bypass on a Harley M8

Now that you’re up to speed with pretty much all there is to know about the Harley breather bypass, it’s time to actually do it.

The thing with the breather bypass mod on Harley Davidson M8 engines is that there really is no very easy way to do it.

Apart from DK Customs, almost nobody sells ready-made air cleaners that are compatible with a breather bypass.

In most cases, you will have to drill a hole into your air cleaner housing in order to pass the breather hoses through and to the outside of the engine. Furthermore, some air cleaners are designed in such a way, that you would have to drill two holes into them, tap them, and screw in two hose fittings.

There really is no set way to do the breather bypass mod as there are dozens of different ways to do it and they all depend on your specific air cleaner and its design.

We do highly recommend going with the DK Customs breather kit. Here’s a video on how to install it:


The Harley breather bypass is one of the best mods you can do for your engine’s longevity. Not only does it have multiple benefits, but it’s also very inexpensive to do.

We hope you found this article helpful and that you will consider doing the Harley breather bypass mod on your engine.

As always, ride safe!

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