On my motorcycle.
I plan on modifying the crankcase venting on my '13 Sportster while I'm off next week. I am keeping the stock ham can air filter box and just want to get those nasty oily watery gasses out of my intake. The bottom rear corner of my air filter has accumulated a fair amount of oily residue over the last 9K miles.
I am venting to atmosphere with a small catch can. I am concerned about the small pinhole of the stock vent bolts. If I vent to the atmosphere, I won't have the benefit of engine vacuum pulling the gasses out and don't want added crankcase pressure causing leaks and robbing power.
So my question is, should I drill out the holes in the breather bolts? And, if so, how much?
Stock bolts are on the right of this picture that I stole from the internet. The bolts on the left are aftermarket breather bolts that use a banjo fitting to vent.
Does your bike also have a one way valve in the system somewhere? And it depends on if there is some kind of a oil separator in the engine somewhere. At least on my 950, the crank gasses have to pass through the center of the balance shaft, which has an oil separator of sorts in it. Then it goes through a one way valve and to atmospheric - I use any oil carryover to lube the chain. The one way valve allows pressure to leave, and actually results in a very slight vacuum in the crankcase, from what I've read. Removing this check valve has been shown to hurt fuel economy on the highway as well, since that engine turns 5k rpm. Probably less critical at low revs.
So TL;DR, I would use the stock bolts with a lot of flowand try to put a check valve in there somewhere. Even if there isn't a check valve, I'd rather have the least possible pressure in the crankcase to reduce windage loss and to keep oil from getting pushed past shaft seals.
In reply to gearheadE30:
Yes, there are umbrella valves in each of the heads prior to the vent bolts. I've heard they begin to fail after 3-4 years.
On the stock bike it comes out of each head and flows directly into the air cleaner through these two holes.
Those holes are so small that I was concerned that without the vacuum in the airbox it wouldn't vent enough.
