Without a profilometer to measure surface finish, this variable remains hidden because your eye can’t see in micro inches. What is a profilometer, you ask? It is a precision measuring device capable of tracing surface characteristics to a single micro inch (1 millionth of an inch).
Just for reference, 100 micro inches equals .1 of thousandth (.0001).
Keep that conversion in mind as we continue.
When looking at the graphical trace created by a profilometer, it becomes obvious that the majority of the valley in a good plateau finish is roughly 100 micro inches deep.
Without enough valley in the cylinder wall finish to retain oil, “ring seal soup” will either turn out bad or spoil quickly. As I have travelled around to engine shops this past couple of years with my profilometer in my backpack, I’ve seen a wide variety of surface finish measurements.
Between these field tests and actual dyno testing, the hidden variable is no longer hidden. You just need to correct tool to see it.
Let me give you an example. For several years I wondered how a couple of shops had great success using a certain oil for break-in that would never work in the engines I helped build and test. In fact, I had actual engine tests with measured wear data that proved a drastic difference between the two oils in question.
So why could these shops use one oil (let’s call it oil A) but not the other(let’s call it oil B)? Why couldn’t I use oil A but could use oil B? The differences were actually two-fold, but the common theme was surface roughness.
The shop successfully using oil A had a completely different surface finish than my SBC dyno mule engine. There was a good reason for these wildly different surface finishes. While my SBC was a grey cast iron block, the other engine was a Nikasil plated bore in an aluminum block.
These different bore materials required a different bore finish, which required a different oil chemistry to get the soup right.
Because Nikasil contains silicon nodules trapped in the metal matrix, it requires a smoother bore finish compared to grey cast iron. The smoother surface finish of Nikasil didn’t need the powerful boundary layer chemistry that the grey cast iron bores love.
At this point you may be saying that the difference in bore material is a known and primary variable, so why bring up the secondary variable of surface finish? Well, that is because sometimes Nikasil engines do respond well to the oil with the powerful boundary layer chemistry. Why is that? It’s because the inconsistency of the surface finish. When the surface finish is rougher, the rings are protected by the boundary layer lubrication chemistry, so the hidden variable that actually effects the “soup” is the surface finish - even over the cylinder bore metallurgy.
In conclusion, Scooter was right. You need to measure everything that can change the outcome of your “soup”.