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Harley Breather Bypass Mod – Everything You Need To Know

If you take care of your motorcycle in the garage, it will take care of you on the road, and there’s no better way to take care of your Harley Davidson (Milwaukee-8 especially) than by installing a breather bypass mod.

In this article, we’ll explain everything you need to know about the breather bypass – why the engine needs it, what it does, and how much it costs.

What Does the OEM Breather Do on a Harley?

Before we talk about the breather bypass mod, let’s quickly go over the breather system on the Milwaukee-8 engines first.

Like all modern engines, the Milwaukee-8 comes with a closed-loop breather system. The breather system vents positive crankcase pressure and oil mist from the crankcase back into the intake. The sole reason for doing this is to reduce harmful emissions.

Back in the day, engines used to simply dump crankcase pressure and all of the oil mist straight to the ground, but because of the EPA regulations now, it is illegal to vent crankcase pressure out into the atmosphere. Unfortunately, the easiest and cheapest way to comply with the regulations is to simply vent the crankcase pressure into the intake and burn it.

Now, as you can probably tell, this design is not ideal, and forcing the engine to burn oil could cause problems later down the road – more on that later.

What Does a Harley Breather Bypass Do?

Here’s the short answer – the Harley breather bypass mod vents the crankcase pressure and blowby into a catch can or the oil pan, drastically reducing carbon buildup in the combustion chamber.

Now, the main problem with the breather design on the Milwaukee-8 is that in some cases, these engines produce more oil vapor than usual. This breather design can handle small amounts of oil vapor fine and with minimal chances of causing carbon buildup, but if the engine is producing a lot of oil vapor, then things start going sideways.

Excess oil vapor will foul the throttle plate and the air cleaner, but most importantly – it will cause a lot of carbon buildup on the piston. Large amounts of carbon buildup in the combustion chamber can disrupt airflow and even increase the compression ratio, which leads to poor performance and rough idle.

Even Doc Harley admits that venting crankcase gases back into the combustion chamber is not good for the engine in this video:

3 Main Benefits of the Breather Bypass Mod

The OEM breather design kind of sucks. It’s messy (nobody likes oil dripping from the air cleaner) and it’s certainly not good for your engine.

Luckily, all of that can be easily taken care of with the breather bypass mod. Here’s a list of a few huge benefits of running the breather bypass mod on a Milwaukee-8.

1. Increased Engine Life

This is a big one. Nobody wants to waste money (especially these days) and running a breather bypass will significantly increase the life of your Milwaukee-8.

You’ll have much less carbon buildup on the pistons, rings, and valves. This means minimal power loss through the life of the engine as well.

2. Slightly Better Performance

Not a lot of people know this, but the engine is tuned from the factory to accommodate the hot and oxygen-depleted blowby vapors while running.

By installing the breather bypass mod, your engine will have access to more fresh oxygen and thus slightly more power. Furthermore, the engine will run smoothly for longer since the carbon buildup won’t reduce airflow.

3. No More Oil Inside the Air Cleaner

One of the most annoying things about the stock breather setup is how messy it gets over time. It’s not uncommon to see oil dripping from the air cleaner.

Luckily, the breather bypass mod will take care of it and keep your engine clean on the outside, as well as the inside.

How Much Does a Harley Breather Bypass Cost?

One of the coolest things about the breather bypass is that it’s extremely cheap to do and you can even do it yourself for under 20$.

Now, the biggest drawback to the breather bypass is that it’s pretty difficult to pull off and you do need decent mechanical skills to do it yourself. It requires drilling, tapping, etc. However, if you’re not comfortable with that, there are kits available online that make the job much easier, but they are more expensive.

DK Customs make one of the most popular breather bypass kits for Harleys, thus we recommend you start there. At the time of writing this, a breather bypass kit for the Milwaukee-8 will cost you around $70 on average.

If you want to give it a go yourself, here’s a good tutorial for 2000 – 2016 models courtesy of HDforums. For 2016 and up Milwaukee-8 engines, check out this video by BLOCKHEAD. It covers pretty much everything you need to know.

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

So far, we’ve seen how cheap and beneficial the mod is, but why don’t Harley Davidson motorcycles come that way from the factory in the first place?

Well, Harley Davidson is not the only manufacturer that’s using the same breather system. In fact, nearly every single engine made in the last 25 years uses a similar breather system.

But why don’t engines come with oil catch cans or something similar from the factory? Why do they vent oil vapor to the intake?

First, it would be yet another maintenance item, and most riders already don’t have enough time or skills to replace their brake fluid or coolant. There are bound to be some riders who will forget to drain the catch can and let it overfill and then sue HD. Obviously, the company doesn’t want that.

Sadly, this type of crankcase ventilation system is the cheapest and easiest way to comply with EPA regulations. That’s why nearly every manufacturer uses it.

Final Thoughts

The breather bypass is undoubtedly the best mod you can do for your Harley Davidson.

Not only is it cheap to do, but it will also make your engine last significantly longer and help maintain its power figures for longer.

We hope you’ve found this article helpful and that it made you consider doing the breather bypass on your Harley. Ride safe!

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