Piston Ring Function
B-5 Second Ring
Next comes the second ring, also for an 86mm cylinder bore, with an axial width (height) of 1.2mm and a radial thickness of 2.9mm.
Looking at the appearance of the second ring's running face, we can see that it serves to complement the top ring in its gas sealing function, but it also provides oil control. The photo shows the free gap and running face of the second ring.
When squeezed closed to its 86mm working diameter, the ring's tension will hold it tightly against the cylinder wall, providing a sealing function. At the same time, the ring's 'undercut' bottom side produces a running face profile that acts to scrape oil downward.
A magnified measurement trace shows the ring's running face in profile. This face is not straight-vertical, but is 'tapered'. This ring shows a taper face angle of approximately . This taper-face profile provides good gas sealing action, efficient oil scraping action, and allows for a fast running-in period (break-in phase).