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They are the head breather bolts. Air does not go into them, but they are vent tubes that connect to a rubber hose, that is connected to the back side of the air filter. The mist that comes from the cylinder heads is burned off in the combustion chamber.
If I could expand on Tynker's excellent answer for a moment, all piston motors have blow-by, meaning some combustion gases go past the rings and into the crankcase instead of out the exhaust port. If the crankcase was sealed up, the crankcase pressure would build up and cause gaskets to blow out. So it has to be vented. On your motor, that venting is through those two bolts.
So what's coming out of them is essentially exhaust that went past the rings instead of out the exhaust. On it's journey through the motor, it picks up oil and water from condensation. So it's pretty nasty stuff. The EPA requires manufacturer to run this back through the engine. That's why those vents come into the air cleaner.
Since the blow by is hot and displaces oxygen and is carrying oil and water with it, running it through the motor is not desirable and it has some bad side effects. It's a very common mod to route those to the atmosphere instead, and it has some benefit - both marginally improving performance and reducing carbon build-up in the motor. Some people route them out through a little breather filter to absorb the moisture component. Others send it into a vented catch can first, that they periodically drain.
They vent the crankcase gases back into the air intake.
If you look into replacing your air cleaner with something different, you will notice that some will allow you to vent to atmosphere and some will recirculate those gases.
Some benefits of venting to atmosphere are slightly lower intake air temperatures and you will not be sucking dirty crankcase gases into the combustion chamber.
I just bought the k&n 3950 for my 2007 roadking custom but I see that the brother bolts don't allow for air to go in or out like the factory ones with the rubber tube's is that safe for the engine not to breathe?
I just bought the k&n 3950 for my 2007 roadking custom but I see that the brother bolts don't allow for air to go in or out like the factory ones with the rubber tube's is that safe for the engine not to breathe?
I installed an aftermarket intake on my 03 883, and one of the plates that sandwich the air filter has passages machined into the face. It also has two ring shaped pieces that the hollow bolts pass through, with channels machined into them. I assume that they're supplied so you can orient them to pass the gasses, or block them if you vent another way (though the instructions said nothing about how to orient them). If you get one like this, be careful to install them correctly so you don't inadvertently block the passagesor as stated in earlier posts, you may end up with a blown gasket.
I just bought the k&n 3950 for my 2007 roadking custom but I see that the brother bolts don't allow for air to go in or out like the factory ones with the rubber tube's is that safe for the engine not to breathe?
Look at the K&N bolts. They'll have hole(s) in the side that allow the gases to pass from inside to outside (OD of the "bolt"). Then there's a passage in the K&N backplate that allows those gases to continue into the Intake Tract. You don't have to orientate the bolts, just tighten them up.
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Last edited by Harley_ish; Jul 21, 2024 at 01:32 PM.
EPA mandated breathers that are directed to the intake of internal combustion engines; in humans it may be compared to flatulence directed to the lungs!
What do the hollow screws in an air cleaner do?
