Gents,
I am returning to this topic in reponse to Pim's question asking, "...what this 'breather' is everyone is talking about?"
In this thread, we've talked a lot about the oil drip from the breather, but I don't think we answered the question as to why the breather is really there and what the timing implies. Pim's question made me think a little harder about this. With quite a bit more more than a little repect for dpaddock, I'd like to suggest a different reason than blow-by or pressurization by the pistons. While it is true that pressure in the crankcase due to blowby gets out through the breather, the breather is not timed for this, as there are two ignitions for each breath from the breather. With regard to pressure from the pistons when they come down, that pressure is relieved when they go back up.
Even though I wanted to, I did not try to calculate the pistons' position when the breather port is open. That would be interesting, but take too much time, also, my crankcase is now out for machine work. If someone knows, please share.
Anyway, my opinion is that the breather is serving two purposes, the most important being to achieve near-atmospheric pressure in the crankcase so that oil can be pumped in and sucked out. In a worn engine with enough blowby to presurize the crankcase, oil could probabaly, be sucked out, but it might not get back in. The second purpose is the exhausting of blowby, for the reason mentioned. While there may be something to the timing of the breather, I don't see it and I think it may not matter. Instead, I think it is just a convenience of the architecture of the crankcase.
The vent on the oil tank serves a similar purpose as the breather (except for the blowby issue) in that it allows oil to be sucked out and pumped back.
As always, I am quite interested in contrary and abundant more expert opinion.
Richard