Technical Support

Back to technical library

Plateau Honing 101 - The role of Flex-hones and Ultra Finish Hones

We get a lot of questions here in the Tech Department about Flex-Hones and Ultra Finish Hones. What's the difference? Can I use Ultra Finish Hones to deglaze a cylinder? Are they interchangeable?

Good questions all around. And that's what we're going to talk about this month. But before we do I need to go through a little history.

Back in the old days (maybe 20 years or more) cylinder honing was done almost exclusively with rigid hones and honing stones which led to some problems such as rough cylinder surfaces. It used to be that the rings scrubbed or shaved off the peaks of rough material left by the rigid stones over time. You may have heard of breaking-in a cylinder. That's what this is.

This process works but it has a couple of down sides. First, the rings and cylinders wear much faster and second, all the debris being shaved off of the cylinder surface ends up in your oil which can cause engine component wear.

Understanding the evolution of piston rings is important too. Early thought was wide rings with high tension. The wide rings were tough enough to handle the rough surfaces (to a point) but they also resulted in higher emissions. So as emission requirements evolved, piston rings did too. Today’s rings are narrower with low tension so they result in lower emissions and longer ring life. But they come from the factory pre-lapped so they can’t take the abrading that the older style rings could handle. So as the rings evolved, the honing processes had to as well.

The evolution of the rings gave birth to a finish that’s called a plateau finish.  As the name implies, instead of the peaks and valleys created by the hard abrasive, a plateau is created by knocking down the peaks on the cylinder walls. This is where Flex-Hones® and Ultra Finish Hones come into the picture.

Achieving a plateaued cylinder bore is a multi-step process:

1. Hone to size with coarse grit stones – between 150 and 220 grit – using a portable hone or a honing cabinet with rigid stones and wipers.
2. Remove the peaks using a fine (280 or finer) grit flex-hone depending on the ring face material.
3. Remove the material that has become embedded in the crosshatch with an Ultra Finish Hone.

Keep in mind that this isn’t written in stone (no pun intended). You don’t have to do all of the steps above, but you’ll get a better cylinder surface if you do. You’ll notice that these steps are a bit vague. I haven’t given you any guidelines for how much to remove with each pass of the hone and that’s deliberate. That’s up to you. You’ve developed your own technique based on experience, just like I have so listen to yourself. 

Let's take a closer look at Flex-Hones and Ultra Finish Hones to give you more understanding of their roles in creating a plateau finish.

Flex-Hones®

Flex-Hones®. Ball-Hones. Dingleberry Hones. These are three of the most common names for this honing alternative.

By definition, the flex-hone® is a resilient, flexible, honing tool with a soft cutting action. Flex-Hones are made with a heavy-duty twisted wire shaft that holds flexible nylon strands coated with abrasive globules. These globules conform to the shape of the surface and "float" over the surface you're honing to create a consistent crosshatch finish to enable better oil retention in the cylinder bore. Remember, because the flex-hone follows the shape of the cylinder, you should never use it to do the initial honing. All this will do is perpetuate any flaws that already exist in the bore and that’s what you’re trying to eliminate by honing.

Flex-Hones are available in several abrasives – silicon carbide, aluminum oxide, boron carbide, tungsten carbide, and alumina zirconia with grits from 20 to 800. There is a wide range of sizes from 4mm in diameter all the way up to 24-1/2 inches covering an extensive list of applications.

Here's where two of the biggest differences between Flex-Hones and Ultra Finish Hones can be found. Flex-Hones allow you to deglaze a bore and will remove material from the bore, though not as much as a rigid honing stone.

figure 1

figure 2

figure 3

Take a look at the illustration at left. In fig. 1 you’ll see an overly simpllified rendering of what a bored cylinder looks like after the first honing step. There are sharp peaks that will abrade the rings if you install the pistons and rings now. Back in the day, this was the breaking-in step. But as we’ve already discussed, those days are gone. The newer rings can’t take the abrasion so you’ll need to round over these sharp peaks and that’s where the flex-hones come in.

Fig. 2 illustrates the result of a pass with a flex-hone. You’ll note that the peaks are rounded over and the valleys have been filled slightly with debris from the flex-hone pass. In correcting one problem we’ve kind of created another. It used to be that we’d recommend you clean the cylinder at this point with soapy water and a nylon brush to remove the last of the debris from the crosshatch. It helped.

But a few years back an enterprising person had the idea of adding an abrasive right to the nylon in the brush. It yields and penetrates the crosshatch depth to eliminate debris. So was born the Ultra Finish Hone.

Ultra Finish Hones

The ultra finish hone is made up of a monofilament nylon strand that is impregnated with fine abrasive material that breaks down as the strands brush across the bore surface.

According to one manufacturer, "These tools are specifically designed for mechanical finishing tasks such as: deburring, sharp edge removal, radiusing, edge contouring, de-fuzzing, surface refinement and conditioning, plateau finishing, blending imperfections, reduction of surface stresses and micro crack propagation, cleaning, polishing and surface wiping prior to inspection gauging." Whew, what a mouthful. Ultra Finish hones, aka plateau hones, soft hones, whisker hones or brush hones, will NOT enlarge or deglaze the bore under normal use, unlike Flex-Hones. They are basically deburring tools that remove the folded materials partially filling the crosshatch depth – or as I call them, “the uglies.”

Back to the illustrations. Fig. 3 shows the results of the Ultra Finish Hone. You’ll see that the debris that was left (fig. 2) has now been removed, leaving a plateaued cylinder that’s already broken in.

So we've seen that Flex-Hones and Ultra Finish hones perform similar, but not identical, procedures. They are NOT interchangeable in engine cylinder applications. And there's one other difference. Ultra Finish Hones are available in 120 and 320 grit in either silicon carbide or aluminum oxide abrasives.

There are a couple of cautions when using Ultra Finish hones. Too much plateau honing will make the cylinder too smooth and you won't have adequate oil retention. And on the flip side, too little will leave the surface too rough and will cause ring and cylinder wear. I recommend somewhere between 12 and 20 strokes per cylinder to achieve a properly deburring. Again, I’m leaving the exact application up to you and your feel for what you’re doing.

This gives you a quick tour of Flex-Hones and Ultra Finish Hones, their differences and their similarities. To find out more about Flex-Hones, visit the Brush Research website and for more on Ultra Finish hones, check out Osborn International's site. You can also google plateau hone, flex-hone and ultra finish hone for additional Internet articles and sites.