Bergs. Best saved until you are back from the pub.
MalcD.. The breather outlet is the hole above the gearbox sprocket. Early rigid and plunger engines have a neat pipe running down to the sump plate, from which condensed oil mist or neat oil, (depending on how lucky you are) will drip. Later plunger bikes have a short tube, directing the mist onto the top of the chain, as a crude chain oiler.
S/A bikes just have a hole, so here its an oily mess, but a well preserved mudguard.
How does it work? Well, along the lines of GB's carb gnomes, here goes. In the crankcase the breather gnomes are relaxing, each one with an empty bucket. When the engine starts, pistons descend, the oil vapour in the crankcase is first compressed, and condenses. The gnomes catch this in their buckets, and in a neat line make their way to the breather bush. However as the pistons rise, the pressure in the crankcase is reduced, sucking the line of gnomes back, and they spill their oil buckets, just leaving a thin film in the bottom of each bucket. The majority of the oil then returns to the sump, and is scavenged to the oil tank, by the so called Scavenger Gnomes of the Deep, who have altogether bigger buckets.
Now the breather gnomes are determined fellows, and make the trek to the breather bush again, but now the buckets contain very little oil, just oil vapour and a lot of air. As the breather bush rotates, each gnome hangs on tightly with one hand and tips whatever is in the bucket into the bush, just as the exit holes align. There you are, oil vapour comes out of the breather in time with the rise and fall of the pistons. Well it does most of the time, when the engine speed is at normal town traffic speeds. At high revs those gnomes just can't bucket fast enough, and a more sophisticated approach is needed as on modern machinery. Plenty about alternatives elsewhere within the Forum.
So, depending on how many gnomes you have, and how big the buckets are, you may get little or no oil or plenty from the breather. All the more remarkable as they do this in the dark.
The whole system depends on a good cork washer seal between the breather bush and the camshaft drive gear. The machining of breather bushes and timing inner castings changed over the years, make sure holes in the bush align fully with the breather drilling in the inner timing cover. Many mix'n match parts don't, so use the best combo if you have a choice. The inner cover external drilling should be blanked off in production. The only link from the crankcase to atmosphere is via the breather exit.
Swarfy.
