Now you might be thinking that smoother surface finishes on the cylinder wall must be the answer, but a certain amount of roughness is needed.
Just like the deck on the block, the cylinder walls need roughness to hold the oil. Because the oil is fluid and not a stationary gasket, the roughness on the cylinder wall retains the oil so it can function as that gasket to assist with ring seal.
I experienced this first hand back when the NASCAR and NHRA ProStock teams began to experiment with mirror smooth bore finishes. The idea was to reduce friction, but these super smooth bores didn’t retain enough oil. As a result, blow-by increased and crankcase vacuum decreased. In an effort to fix the problem, higher viscosity oils were employed, but the increased fluid drag off-set any reduced friction due to the smoother cylinder wall finish.
In the end, getting the correct plateau finish was the best answer. Smooth on top, but with deep valleys to hold the oil.
Now you might we thinking to yourself, I’ve never had any problems getting my engines to seal up, so why even worry with plateau finishing.
Well, the traditional honing and finishing techniques tend to leave a rougher surface, which is good for retaining oil. However, the peaks from those rougher surfaces can wear the face of the ring faster, especially with the softer molybdenum face coatings.
While Moly is great at holding oil - it has more porosity than PVD coatings - it is softer, so it will wear faster than a PVD coating. When combined with a rougher cylinder wall finish, a Moly faced ring will break-in quicker, but it will also wear out faster, especially with low lubricity fuels like Methanol and E85.