Motorcycle Cylinder Honing
text and pictures by Mark Trotta
After engine cylinders are bored to size, the cylinder walls need to be finish-honed for the new piston rings to seal properly and have a long life. Similar to boring machines, machine shops have a special honing machine that does the job fast and easy.
For the DIYer, this is often done by cross hatching with a quality flex hone and an electric drill.
This article covers the two most popular methods for properly honing a motorcycle engine cylinder, which are rigid honing and finish honing.
Bore vs Hone
First, a technicality. Many people use the words "bore" and "hone" interchangeably. In actuality, 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, and 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.
Flex Hone
The best way to finish cylinders that don't need honing or resizing is ball honing, also known as flex honing. This is done with a Flex-hone tool which has dozens of small flexible balls, and is usually operated by a hand-held drill.
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.
Shop: Flex-Hone
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 de-glazing. 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.
Flex Hone Operation
Some people prefer a drill press, but personally, I feel more in control with a hand-held drill. The key is to use the tool at a low speed but 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.
What Is The Best Flex Hone?
Brush Research Manufacturing (BRM) introduced their flex-hone tool in 1972. If you're looking for the best way to hone a motorcycle cylinder, get a BRM flex-hone. They are self-centering, self-aligning, and compensate for cylinder wear.
All Brush Research flex hones are made in the USA.
Shop: Brush Research Flex Hone
A flex hone's diameter should be slightly wider than the bore diameter you're using it in. This slight oversize is usually within 1/8" of your bore size, just enough to produce a soft cutting action.
Order By Bore Diameter
The Flex-Hone tool is produced and used in an oversized condition. Simply order the tool for the bore size you need. For example, a 3" Flex-Hone is ordered for a 3" bore.
Misc Notes
If the exact size is not offered, round up to the closest available size.
If you try to stuff an oversized ball hone into a bore, you can wreck the hone.
Without proper honing, new piston rings cannot do their job correctly, which results in lost compression and high oil consumption.
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Two-Stroke Engine Honing
A bonus when flex-honing two-cycle engine cylinders is, they'll put a slight chamfer on the edge of the ports for you. This eliminates the extra step of radiusing the edge.
Caution: When honing a two-cycle cylinder, the flex hone must be within 1/8" of your bore size, or the stones may catch on the ports and cause damage.
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