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 at all points 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, high-quality motorcycle cylinder boring is possible.

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). Always err on the side of loose, a bit of rattle is much better than a locked-up motor!

Cylinder Honing vs Cylinder Boring

Many people use the words "bore" and "hone" interchangeably. The main difference is the grit of the cutting stones. For cylinder boring, coarse stones (100 grit) are used.

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

Is Line-Boring Necessary?

To run at peak efficiency, cylinders should be a perfect 90 degrees to their base. They probably already are, but good engine builders don't like leaving anything to chance.

Line or deck boring is the most accurate method to verify that the cylinder bore will be perpendicular to the crankshaft/block. When using a block-mounted boring bar, the machine centers off the original hole and cuts a true 90-degree bore.

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.

Most engines won't need to be line-bored, but if you're building an engine for competition, line-honing is mandatory. When you want every ounce of horsepower, a perfect cylinder is mandatory. Some builders will tell you that a perfect cylinder is the basis of a great race engine.

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.

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

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

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.

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.

Stop and Remeasure 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 remeasure often, reversing 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. There's lots of ways to hone, and lots of opinions on what is the best way. I usually follow the manufacturer's recommendation.

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

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. Stop and measure several times. Even though the oil helps keep temperature down, let the metal cool between measuring.

Squirt oil frequently into the cylinder as you hone. The oil increases the life of the stones, and also helps keep the heat down. Remember that once wet, honing stones cannot be used dry again.

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

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. 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.

Lisle 15000 Engine Cylinder Hone

Shop: Lisle 15000 Engine Cylinder Hone

When Not To Bore A Cylinder Yourself

If you have a rare, antique, or hard to replace cylinder, consider sending it a reputable machine shop that specializes in vintage motorcycle repair. Same advice goes if your cylinder is way out of round, deeply scratched or scored, or needs .030" or more removed to make it round again. If your cylinder is that bad, consider having it sleeved and then line-bored.

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.


If you're not sure you want to bore and hone a motorcycle cylinder yourself, practice on a scrapped lawn mower engine first.

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.

Read: DIY vs Machine Shop