What Are Weather Bolts on Harley-Davidson Motorcycles?
Harleys are one of the easiest motorcycles to buy today, thanks to their huge ecosystem. One of these options is breather bolts. These are small, CNC machined aluminum or stainless steel bolts, usually sold in pairs. They replaced the factory breather hardware on the engine’s air assembly. As for why you might want to do that, it might help to know what the stock system does in the first place. Every engine produces exhaust gases as a by-product of the combustion process. This leak is past the piston rings – and the process is no different on Harleys. And when gases escape into the atmosphere, they cause emissions.
That’s why the factory reset on Harleys captures all that and returns you to the air cleaner. Along the way, the blow-by passes through one-way umbrella valves and filter screens in the rocker boxes, which are designed to separate the oil mist from the crankcase air. From there, it goes back into the intake to be reheated. That sounds responsible enough on paper. But the system still has its limitations. Flammable gas isn’t just any old gas. It is a mixture of unburned fuel, exhaust gases, and oil mist. Over time, the oil vapor that makes it through the stock split can leave a film on the intake valves, which can bake in carbon deposits. This build-up can eventually cause engine knocking or prevent valve shutoff. The recirculated gases also displace fresh air and fuel from the intake manifold, resulting in less power per stroke.
Breather bolts do away with all of this. Rather than returning crankcase air to the intake, they vent it directly into the atmosphere, filtering the debris through small, built-in mesh screens. The result is a cooler, denser air charge and a combustion chamber that stays cleaner over time.
It’s easy to install, but there’s a big catch
Making breather bolts an attractive option is the fact that they are one of the easiest changes you can make. Most kits on the market include everything you need to replace the hardware: two bolts, mesh screens, and snap rings. Usually you don’t even need to reconfigure. Don’t miss this one extra step: installing an aftermarket support bracket. That said, this is only necessary if your new air cleaner doesn’t include a solid support plate of its own (such as when installing a low-speed stack).
But there are downsides. The breather bolts vent the crankcase into the open, and that oil mist has to go somewhere. Riders who ride these long highways at high RPM sometimes report a fine, oily film on the adjacent surfaces. Clearing all that from time to time can change quickly. It can be a distraction for many. To avoid this nuisance, passengers often use grab cans.
Then there’s the legal reality, and it’s the exact reason Harleys don’t come with this setup. Under the US Clean Air Act, it is a federal offense to interfere with or exceed the factory emissions controls of a registered motor vehicle. This includes the crankcase ventilation system. The government takes this seriously; in 2016, the DOJ and EPA slapped Harley-Davidson with a $12 million civil penalty over aftermarket “super tuners” that beat emissions systems. Although the breath bolts are not tuners, changing the factory exhaust system means that these parts are not legal for street use. As a result, they are often sold strictly for off-road or show use only, and an aftermarket mod like this may affect your Harley-Davidson warranty.
