GM’s massive 900,000-vehicle recall has cost them nearly a billion dollars, but what is the reason behind it? We know that GM has issued a TSB (Tech Service Bulletin) directing dealers and owners to switch away from the 0W-20 oil specified by GM to a higher viscosity 0W-40 oil, but that is the fix, not the answer as to the cause. Lake Speed’s latest video has the answers, and they are fascinating, all revolving around surface Texture

The oil spec change had a lot of people thinking this was a bearing clearance issue, but the reality is that it is all about the crankshaft’s journal surface texture. That’s right, GM cost themselves a billion dollars finding out that you can’t just change the surface texture on machined parts without proper metrology. We even wrote about metrology recently, and how it relates to friction and wear, a bit too apropos. Lake Speed visited Kasse Racing Engines to speak with Mark Malburg, an expert in digital metrology, to get to the bottom of this gripping issue.

What Is Surface Texture?

Surface texture is not the same as surface finish. You can see and feel surface finish; you cannot feel or see the surface texture with the naked eye. How many times have we all felt a scratch in a cylinder wall and said, “Eh, it doesn’t catch a nail, send it.” That is not what we are talking about here. Surface texture is the local deviation from a perfectly flat plane. Measured in the single-digit micron range using a surface profilometer, the surface texture is critical for every moving part in an engine. In fact, it is the cylinder bore that is the single most studied machined surface in an engine.

Is this cylinder bore smooth enough or too smooth? Here’s a hint- you can’t tell.

It may look smooth, but the reality is that machined metal surfaces are quite rocky when you get down to the molecular level. When two machined parts have to fit together and slide, the roughness, even at the molecular level, makes all the difference in the world. Different materials interact with each other differently, and that is the key issue here. Consider a cylinder wall and the piston rings, each surface texture needs to match the mechanics involved. This affects not only wear, but also lubrication, sealing, and friction. Combustion is contained in the narrow spaces between the ring and the wall itself, if you have 1mm rings, those surfaces need to work together. “If I make a surface like a bed of nails and press it against your hand, it matters,” says Malburg. “Scale it down, press it against a piston ring, that’s where it is sealing.”

Similar to a record player, a profilometer uses a two-micron diamond needle to follow the surface, measuring how it moves up and down on the surface to create a profile. For reference, a human hair is about 80 microns. This is good, but Malburg uses a 3D optical measuring system that can create a 3D representation of the surface finish, which is more accurate. In the video, Malburg shows some scaled-up versions of cylinder wall and ring textures in 3D printed form. When scaled up, you can see that several of them look like rough concrete, but that’s only part of the story.

Each of these 3D printed squares represents a 1mm of surface area of a cylinder.

Looking at the examples — a DLC-coated piston ring versus a standard moly ring — the standard ring is substantially rougher, with big valleys and porosity. The DLC ring is relatively smooth in comparison. Next, compare the cylinder wall texture. A standard ring needs a matching cylinder surface to break in. A smooth DLC-coated ring doesn’t trap oil, and is much harder than a standard moly ring, the cylinder wall needs more texture to hold oil and facilitate the wear-in process. The key is creating a system, not just individual parts, which work together. Not only can this increase the life of an engine, but it can unlock hidden horsepower.

Shiny Is Smooth And Slippery? NOPE!

The surface texture can be manipulated to lower both friction and wear, and the idea that a shiny surface is smooth and slippery is a myth that needs to go away. Some cylinder bore materials, like a SUMEbore plasma coating, are honed to a “shiny” finish but still have many holes and valleys that trap lubrication. A smooth ring on a smooth bore does not work; there is nowhere for the oil to cling to, increasing friction and wear. As Speed always says, proper lubrication requires the right oil, in the right place, at the right time, and in the right amount. Having enough retention, such as valleys or pores, is crucial for retaining the oil and preventing problems.

A balance between smooth peaks and rough valleys requires engineering. A rough valley holds more oil, but the peaks need to “plateau” so they can carry the load required. Surface texture is critical to engine performance and durability, but is often overlooked in the pursuit of improvement. Jon Kasse Racing uses a Rottler hone to create engineered surfaces and then dyno test them. The common “plateau hone” is created using a rough diamond hone to create the deep valleys, and then a much finer diamond set smooths out the peaks, creating the desired “smooth peak, rough valley” texture for the cylinder wall. “This is controlled ‘non-cleanup’ for a machinist,” Malburg says. “There is no one-size-fits-all approach; this is where the magic of tuning the surface comes in to fit the application and chemistry.”

In terms of the original question of bearing failure in GM L87 engines, the idea is that we are in the late stages of performance, and efforts to get that last little bit of efficiency in a system tend to overlook the surface texture of the components for the real-world application. “We have testing for materials, we can smash them together, we have testing for lubricants’ chemical makeup, but the level at which surface texture matters is in the real-world applications at real loads and speed, and our tests don’t do that very well,” states Malburg.

With plateau honing, the idea is to rough-hone to cut deep valleys and then come back with a much finer hone to cut off the peaks, making them into plateaus. Nice smooth tops, with deep valleys to hold oil.

This process requires machining, building, testing, and then breaking down the engine to test the actual results of the process. For the average gearhead, this is not feasible, but you can monitor the situation inside your engine with a magnetic drain plug and have your oil filters analyzed. Regular inspection of the filter and plug will make you familiar with your engine’s condition, so you can look for trends over time. Used oil chemical analysis is another way you can monitor the wear. It will take a few times to develop a sense of what is actually happening, so if you don’t already, you should be inspecting your filters with every oil change. Used oil analysis will give you a much better understanding of your engine and its health. As Lake Speed says, make your decisions with facts, and not fear.

If you find this fascinating, like we do, go visit The Motor Oil Geek’s YouTube channel for more information; his videos are second to none in the realm of automotive geekery. The science behind why we do what we do is incredible. Performance engines have gone from the old “loose is fast” idiom, but we are also finding out that fast isn’t always smooth.