Classic Motorcycle Build

Motorcycle Cylinder Boring

(text and pictures by Mark Trotta)

If you want optimum performance and life from your motorcycle engine, cylinder bores need to perfectly round. This means uniformly round from top to bottom. Most manufacturers specify this tolerance to be between .002 to .003 inches.

motorcycle cylinder boring

How Cylinders Wear

A piston has two sides, a thrust side and a relief side. During a four-stroke engine's operation, pistons get pushed down on the stroke cycle. This puts force on the front of the cylinder wall, which is the thrust side. Over time, this creates an oblong, or "out-of-round" shape in the cylinder. To rebuild an engine to factory specs, the cylinder gets bored out, until it's perfectly round again, to the next oversize piston size.

DIY or Machine Shop

If you're rebuilding an old motorcycle engine, boring a cylinder back to round requires removing anywhere from .003" to .010" or more of metal. Professional machine shops have large, expensive, dedicated machines to perform all kinds of cylinder boring. When operated by an experienced user, these machines are quick and accurate.

The same process can be done in your home garage, but will take considerably more time. By using a rigid honing device and a slow-speed drill, motorcycle cylinder re-sizing is possible.

Cylinder Honing vs Cylinder Boring

Many people use the words "bore" and "hone" interchangeably. Boring is a process that is done with expensive machine shop equipment, where as honing can be done in a home garage. The difference is how the metal is removed.

A boring machine has a fixed vertical arm which slowly spirals down into the bore, cutting the cylinder walls like a big drill. Honing, on the other hand, is removing metal by friction, somewhat like sanding down metal with a piece of sandpaper.

For the home machinist, honing is done with a hand-held drill and rigid honing tool. For fastest results, dry, coarse stones (100 grit) are used.

After the honing process is completed, the cylinder walls need to be "finished" to help the new piston rings to seat correctly. This second process to cylinder honing can be done with the same tool using finer grit stones, or a flex hone tool. Either medium (220 grit) or fine (320 grit) stones are used for finish-honing.

Disadvantages Of A Rigid Honing Tool

A hand-held rigid honing tool can only follow the current shape of the cylinder, so if the bore is "crooked" or not perpendicular to the crankshaft, it generally stays that way even though you bored it to a larger size.

Are Torque Plates Necessary?

Torque plates simulate the stresses that are encountered when cylinders are tourqed onto the engine cases. Like line-boring, all race and performance motors require torque plates when cylinder boring. Torque plates can either be bought or fabricated.

All aluminum cylinders will require torque plates during the boring process. Vintage motorcycle cylinders with cast-iron sleeves (liners) inside an aluminum cylinder should use them as well. For a street-driven bike with cast iron cylinders, torque plates aren't necessary.

Sportster Cylinders

All year Ironhead Sportsters (1957-1985) have cast iron cylinders. Later model Evo Sportster cylinders (1986 and up) are of a much lighter construction and will require torque plates.

cast iron motorcycle cylinders

Read: Ironhead Sportster Cylinders

Measure Piston To Cylinder Clearance

Optimum piston-to-bore clearance is usually .004 to .005 inches, but varies slightly depending on the manufacturer. To measure piston to bore clearance, place the piston into the cylinder. A feeler gauge can be used to measure clearance between the piston and cylinder wall. Piston measurement is taken from the bottom of the skirt.

Start with a relatively small (.001") feeler gauge strip, and gradually increase the size until the piston will barely slide in. Double this measurement (per side) and that will give you your running clearance.

Measuring Cylinder Wear

dial bore gauge engine tool

Dial Bore Gauge

Cylinder bore wear needs to be measured in six places: top, middle, bottom (front to back), and top, middle, bottom (side to side). Since the rings don't travel to the top of the bore, do not include the top 1/2" of the cylinder while measuring.

To accurately measure the inside diameter of a cylinder, a dial bore gauge is needed. The tool I have is a ToolUSA TM-34260. Bore range goes from two to six inches. It has a bore depth of six inches, and the dial has .001" increments.

motorcycle cylinder dial bore gauge

Shop: Dial Bore Gauge

Keep in mind that .005" too tight can seize a piston when warmed up, and .005" too loose and you'll get noisy operation (piston slap). Better too loose than too tight - a bit of rattle is much better than a locked-up motor!

Tools Needed To Bore Cylinders

First, we need to understand the difference between a surface finishing tool, like a ball-type hone or a three-stone hone, and a material removal tool, like a rack-style rigid honing device. You can't bore a cylinder with a surface finishing tool. Technically, you can, but it would take a long, long time.

Hand-held rigid honing tools can be used to bore all types of small engine bores. The tool I use is a Lisle 15000 which services 3" to 4-1/4" cylinders. It is a rack-style and has two stones and two wipers which clip on and off. There is a universal-joint between the handle and the head. After hooking it up to your 1/2" drill, press the trigger and the tool starts rotating, which expands the tool head to the cylinder size.

Lisle 15000 Engine Cylinder Hone

Cylinder Boring Procedure

In a sturdy-mounted bench vise, set the cylinder up horizontally. Stroke quickly but at a slow RPM. The rule of thumb is 100 RPM for every inch of diameter bored. For example: 300 RPM is ideal for a 3" bore, 200 RPM for a 2" bore, etc.

For the quickest results, boring is done dry, meaning, no lubrication is used. This provides the fastest metal removal, but may overheat the metal. Let the metal cool between boring and measuring.

Adding oil will increase the life of the stones and helps keep the heat down, but makes the cutting process slower. Remember that once wet, honing stones cannot be used dry again.

Stop and Re-Measure Often

Keep the hone moving in and out slowly. If there's any deep scratches and grooves, you'll need to take them out gradually. Do not stop in one place for too long--stop and remeasure often. Reverse the cylinder in the vice several times during the process.

Lisle 15000 Engine Cylinder Hone

Remember, you don't want to take off too much metal, nor do you want to take off too much metal in one spot. Stop and measure often. Remember to reverse the cylinder in the vice several times.

Most service manuals call for .002"-.003" tolerance across the bore. I aim for .003" at a minimum, then concentrate on a good hone.

With 100-grit stones you can usually take .010" out of a small engine cylinder in about 30 minutes. Remember that you're not removing the entire .010" by boring alone, you need to leave a small stock allowance for honing, usually 0.002" to 0.003". If you were boring .020" or more (a bit of a workout by hand) you would still leave the 0.002" to 0.003" for final finishing.

dial bore gauge engine tool

Cylinder Honing

Honing cylinders is needed to promote easy break-in of new piston rings. The honing process is much easier and much less time-consuming than the boring process. You're only removing the last .0015" or so with 320 grit stones.

There are several ways to hone cylinders, and there's lots of opinions on what is the best way. Some like to use a flex hone as shown below. I generally follow the manufacturer's recommendations.

Engine Cylinder Flex Hone

Before you start honing, consider where all that oil/lube will collect. I wrap a few old towels around my vise to help catch the mess.

During the honing process, lubricate the bore with a light-weight oil (like ATF fluid) to help keep the temperature down. It also carries away the metal particles.

Squirt oil frequently into the cylinder as you hone. Stop and measure several times. Even though the oil helps keep temperature down, let the metal cool between measuring.

Clean and Oil Cylinder Walls After Boring/Honing

When you are through boring and honing, clean the cylinders thoroughly with hot soapy water, then dry with compressed air. After you are sure 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.

Lisle 15000 Honing Tool

I've found the Lisle 15000 honing tool to be versatile and competitively priced. Included with the tool is a storage case, cleaning brush, and a set of 100 grit (#15500) coarse stones and 320 grit (#15510) medium stones.

Lisle 15000 Engine Cylinder Hone

Shop: Lisle 15000 Engine Cylinder Hone

Additional honing stones for the Lisle 15000 are available in five grits to cover different cylinder's materials. They also offer additional rack sets, to adjust hone range for larger or smaller bore sizes.

Boring a cylinder by hand will certainly take longer than paying a machine shop to do it, but the results can be just as accurate. Remember that even with the best equipment available, an uncaring or incompetent operator can easily screw up, but with the right procedures and correct measurements, excellent results can accomplished with a hand-held rigid honing tool.

A Word About Two-Stroke Engines

If you have a two-stroke engine, boring/honing a cylinder is more difficult because of the missing material on the sides (transfer ports). These port holes may cause the hone to cut oversize below the ports on the sides.

When Not To Bore A Cylinder Yourself

If you have a rare, antique, or hard to replace cylinder, consider sending it a reputable shop that specializes in vintage motorcycle repair.

If your cylinder is way out of round, deeply scratched or scored, or needs .030" or more removed to make it round again, consider having it sleeved and then line-bored.