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Old 27th July 2021
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Hippysmack Hippysmack is offline
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I need to clear up a few things.
Oil pump pressure has NOTHING to do with oil coming out the breathers or going to the rocker boxes for that matter.
All new umbrellas are soft rubber.
They get hard due to heat in the boxes over time.
They are supposed to close fully on every piston upstroke (suction in crankcase).
Oil pump return pressure is relieved in the tank via the vent back to the camchest.
This is air/oil mist (not like in a handful of oil).
Crankcase, gearcase, oil tank and rocker boxes all share the same air space.
Air/oil mist (not a handful of oil) is used by design to blow around and lubricate stuff.
It's the final separation at the umbrellas where the MoCo wants oil to go back into the motor and excess air out the heads.

The inlet/outlet sizes are by design.
The umbrella is (in theory) getting blown and sucked on at the same speed as the engine.
But the truth is, there is a point at which the engine acts faster then the umbrella does.
So the umbrella kind of floats.
Due to the drainback hole, other forces are at work also from outside upon the umbrella.

Piston size will add/subtract to the volume of air that has to be maintained in the bottom end, yes.
The only difference I've found in the replacement umbrellas is color(same part for all 91-03 Sportsters).

Since there are no replacement intervals for the umbrella in service manuals, people don't think or sometimes believe they are a concern.
So they go out and buy Brand X cause they say their product is THE FIX.
It's actually kind of funny that they say to replace the umbrella along with the Slobbers.
Guaranteed noticeable difference by default of the new umbrellas.
They can't go wrong with that mentality.

Now dangit, I wasn't trying to talk you out of the Slobbers, just didn't want you walking in blind.
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Last edited by Hippysmack; 27th July 2021 at 22:24..
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