Motorcycle Cylinder Honing
(text and pictures by Mark Trotta)
Without proper honing, new piston rings cannot do their job correctly, which results in lost compression and high oil consumption.
Bore vs Hone
Many people use the words "bore" and "hone" interchangeably. To get technical, cylinder boring is the process that is done with machine shop equipment. When done in a home garage using hand-held equipment, it's actually honing. It will produce the same results, it just takes a lot longer with hand-held equipment.
Two Types Of Cylinder Honing
There are two distinct types of cylinder honing processes. The first is, as stated above, when you are increasing a bore size to the next oversize with a hand-held rigid hone. This is called rigid-honing.
The second process, commonly called finish-honing, is achieved by using either a stone hone or a ball hone. Rigid-honing removes metal, whereas finish-honing "dresses" metal.
Why Do I need To Finish-Hone Cylinders?
A cylinder needs to be finish-honed for one of two reasons. If you are re-using your old pistons but installing new rings, you may notice the cylinder walls have a shiny finish. This happens by a piston's constant up and down motion, which eventually "polishes" the cylinder walls. Too smooth a finish prevents new piston rings from properly seating with the cylinder bore. So if you're re-using a good used cylinder bore, you want to break the glaze and leave a nice cross-hatch pattern.
The second reason to hone a cylinder is when it has just been refinished to a different bore size. For either of these reasons, you need to provide just a slight amount of roughness for oil to cling onto. The tiny peaks left behind by either honing process provides just enough roughness to allow the rings and cylinders wall to wear together.
Flat Stone Hones
A 3-stone hone like the one pictured below is commonly referred to as a glaze breaker. This is a surface conditioning tool, not a metal removal tool. It will not straighten a bore or to take high spots off - that is the job for a rigid hone.
De-glazing Cylinder Walls
A flat stone hone generally has 3 220-grit stones on a flexible shaft. Operation is done by a hand drill on slow speed of approximately 150 rpm. The advantage over flex hones is it's adjustable, so the same tool can be used on 2" to 7" bores.
The best way to finish cylinders that don't need honing or resizing is ball honing, also known as flex honing. If the bores look good, meaning there's no ridge or scoring, and it measures within specs, a flex hone will add a nice finish without taking any metal off.
Aside from cross-hatching, flex hones can also be used to deglaze a cylinder.
Stone Hone vs Flex Hone
If your cylinder has just been bored by a machine shop, it's probably OK to use a stone hone. But if you're re-using an existing bore, flex honing is the preferred method for deglazing. Here's why: as a cylinder wears, there are slight irregularities up and down the bore that flat stones just can't get. Because it's not flexible, a stone hone tool will not correct any deviations in the cylinder wall. On the other hand, flex-hones are self-centering, self-aligning, and compensate for cylinder wear.
What Size Flex Hone Do I Need?
To get the best results, a flex hone's diameter should be slightly wider than the bore diameter you're using it in. The manufacturer recommends this should be within 1/8" of your bore size.
Example: if a cylinder bore is 3", a flex hone .050" bigger is recommended. Of course, it won't can't always be exact, but try to stay in that range.
If you try to stuff an oversized ball hone into a bore you can wreck the hone.
Flex Hone Operation
Some people prefer a drill press, but I feel I have more control with a hand-held drill. The key is to use the tool at a low speed with quick back and forth strokes.
Tip: For a perfect cross hatching, have your drill spinning at low speed before you slide the hone tool in. This will give it the best 45° cross hatching.
Quick In And Out
- Hold the drill and flex hone above or outside the cylinder
- Start the tool slowly spinning outside the bore
- Plunge straight down and exit the bottom of the bore.
- Pull straight up and exit the top of the bore.
- Do not stop with the tool in the bore.
Standing over a cylinder is awkward for me, but it gives the best position for the up and down action. Alternately, you can mount the cylinder horizontally in a solid vice.
Repeat this process no more than necessary (usually 8-10 times) until you get the pattern you want.
Clean and Oil Cylinder Walls After Honing
Whichever method you use, when you are through honing, clean the cylinders thoroughly with hot soapy water, then dry with compressed air. After they're completely dry, apply a light coating of oil (10W30, ATF, or whatever) to the bores to prevent rusting. If you're not installing them for a few days or a few weeks, put the oiled cylinders in a plastic bag and out of the way until needed.
Two-Stroke Engine Honing
When honing a two-cycle cylinder, the flex hone must be within 1/8" of your bore size, or the stones will catch on the ports and possibly do damage. A bonus when flex honing 2-cycle cylinders is, they put a slight chamfer on the edge of the ports for you. This eliminates the extra step of radiusing the edge.
What Is The Best Flex Hone?
Brush Research Manufacturing introduced the flex-hone tool in 1972. If you're looking for the best way to hone a motorcycle cylinder, get a BRM flex-hone. Order by actual bore size needed, the tool is produced oversized.