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-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

Hippysmack 17th April 2021 08:40

Ah Ha...

Thank you for posting that.
So the Buell "PCV" valve is nothing more than an umbrella valve with foam and an oil separator under it.
That's interesting.
It's thought of as a much improved version of the Sportster breather system.
But that foam can stop up and here we go.
The oil separator shelving is old tech to HD.
There doesn't seem to be an oil drain past the umbrella either.
So any oil that gets past the umbrella will blow out the vent if that is the case.

TPEHAK 17th April 2021 17:45

There is no drain after the PCV valve. The oil passed the PCV valve goes into the hose routed into the air filter. After some time it forms puddle of condensed oil inside the airbox under the air filter.

Hippysmack 17th April 2021 17:54

That's not actually a PCV valve in the conventional sense.
It's a variation of the Harley umbrella valve system.
PCV valves don't act as quick.
Since 1979, the breather valves have a drain back into the motor past the umbrella.
Apparently except on Buells. So that was a bit surprising.
The additional baffling below the umbrella will help separate oil which is a plus.
I never have liked the idea of the foam.
2004-up XLs have a matt type strainer below the umbrella and a labyrinth past the umbrella with a drain hole.

The way it looks though, the foam doesn't stop up the center as it has that hole in the middle.
The umbrella is a loose fit when popped in there.
So the it will rise and fall a little allowing air to pass.

TPEHAK 17th April 2021 21:06

I called it foam, but it looks like brown wool on Scotch-brite sponge.

needspeed 17th April 2021 21:15

TPEHAK, did you do the CAD work? Well done.

Hippysmack 17th April 2021 21:33

Great pics! Thank you.
I see now, it looks the same material as in the 04-up breather valve assembly.
(which is also in sealed containment.... sort of)
See the loading of oil on the mesh.

Now I'm wondering if the folks that swap to Buell covers actually see a big difference in the breathers or if it's just a wow factor.

TPEHAK 17th April 2021 21:39


Originally Posted by needspeed (Post 5890677)
TPEHAK, did you do the CAD work? Well done.

Yes, I am planning to reverse engineer the Buell XB12 engine and design and CNC machine my own rocker box, rocker box cover and push rods covers. Currently I have finished the rocker box housings, the rocker box covers, and the PCV valves.

I am tired of leaking gaskets in the rocker box assembly, and I hate to replace them, they start leaking just after a couple hundred miles again anyway, so I decided to fix this once and for all so I will not do this ever again. I have a couple ideas how to fix it.

Hippysmack 17th April 2021 21:50

This is interesting.
I'll be waiting to see what you come up with.
Just keep in mind that sometimes the leaking is because of pressure problems due to the breathers.
Sometimes it can be because of other factors instead.

Steve9 17th April 2021 23:03

So you scan it and digitize it. Create a database and machine from that?

Technology is so good now!

TPEHAK 18th April 2021 04:44

Yes, this is about the process.

carbonizedeyesockets 11th May 2021 08:35

Quite interesting.

Apologies if I missed it earlier in the tread, what are the two large diameter holes on the rocker cover? ( Or are those typical of buell covers? If so what cover them)

Hippysmack 11th May 2021 08:53

Yes, it's Buell rocker boxes.
I don't have part numbers at the moment but we do have a few pics of them installed on an XL1200R.

TPEHAK 11th May 2021 15:06


Originally Posted by carbonizedeyesockets (Post 5894971)
Quite interesting.

Apologies if I missed it earlier in the tread, what are the two large diameter holes on the rocker cover? ( Or are those typical of buell covers? If so what cover them)

Those holes are the holes where the PCV valves are sticked in to release excessive air pressure from the engine housing.

carbonizedeyesockets 11th May 2021 17:48

cool thanks!

Hippysmack 16th July 2021 08:08

I've been charting (from parts books and others) Evo Sportster rocker boxes in the Sportsterpedia.

I've found that the 04-06 lower box was used as a replacement for select 91-03 models.
Selection is based off outside finish.
The 04 lower box (and all rubbermount lower boxes) have the extra screw boss for the plastic breather valve unit.
That also means in reverse that rubbermount owners that don't like the plastic breather valve units should be able to swap their top section for 91-03 middle spacers and top covers.
And use the pop-in rubber umbrella breather valve instead.
It'd be interesting to know if this has already been done (I assume it probably has).

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