Downunder

Our 4 stroke engines have only one ring, At any given time in any piston engine, there is no absence of a ring end gap or it would create enough friction when the end gaps touched, to remove the oil film and sieze in the cyl. In extreme cases the thermal growth of the ring, with its ends touching, is enough to make its outside diameter larger than the cyl inside diameter. Either scenerio will cause an early failure. With this in mind and the fact that our fuel is the only means for our engines to receive oil, the ring end gap is in fact what is lubricating our bottom end with blow by fuel oil mix. Some will also travel up the valve guides (mainly the exhaust) and find its way through the pushrod tubes to the bottom end but this is secondary to our bottom end and benefits the valve guide. Since we only intend to use our oil once, we can accept the cleaner combustion byproducts from our fuel and use it to lubricate the bottom end of the motor. When this oil builds up to the point that it reaches the crank vent nipple, we just let it blow out. This still allows plenty of compression for our engines to function at a pretty high performance level. With all of that oil on top of our pistons, we get a big boost in the seal for gassious compression and still force a fairly large quantity down through the ring end gap. There is also a negative pressure developed in the crankcase when the piston is on each upstroke helping to pull some of the remaining oil on top of the piston down through the ring end gap on the exhaust stroke.

If your crankcase nipple dosen't emit some oil, then your system isn't working properly. You can test this after a day of flying by removing your glow plug, tipping your airplan/engine to a position that makes your crank vent nipple the low point and turn your prop several times by hand. The positive crankcase pressure developed on the piston downstroke is enough to begin to push the collected oil out through the vent. if you have a drain tube on the vent, the negative crankcase pressure on the piston upstroke will start to draw it back into the crankcase. Some engines, like the Saito 100 have a relitively high crank vent nipple while others like an RCV seem to put them right on the bottom so you may have more or less residule oil in your crankcase. But if you listen to the vent hose or put a finger near the end, you will experience the crankcase pressure cycle.