Classic Motorcycle Build

Motorcycle Cylinder Boring

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 restoring an old 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.

read DIY vs Machine Shop

If you are a detail-oriented person and have the right tools, you can bore a motorcycle cylinder yourself to precisely the diameter you want. So rather than relying on a shop to get it as close as the machinist feels like, insure complete accuracy by doing it yourself.

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.

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. Most engines won't need to be line-bored, but you're building an engine for competition, line-honing is mandatory - a perfect cylinder is the basis of a great race engine.

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. Line-honing is mandatory on a race motor, when you want every ounce of horsepower, but certainly optional on a 50 year-old motorcycle.

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 be bought or fabricated.

All aluminum cylinders will require torque plates. 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

The cylinders you see in the pictures are from an early Ironhead Sportster. They are painted the original factory silver.

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.

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. To accurately measure the inside diameter of a cylinder, a dial bore gauge is required. The tool I have is a ToolUSA TM-34260. Bore range is 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.

If there's any deep scratches and grooves you'll want to take them out first, then measure what you have left. Allow the cylinder to cool down between measuring and flipping.

Lisle 15000 Engine Cylinder 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 .010" by boring alone, you always leave a small stock allowance for honing, usually 0.002" to 0.004". If you were boring .020" or more (a bit of a workout by hand) you would still leave the 0.002" to 0.004" for final finishing.

Stop and Remeasure Often

Let the metal cool between boring and measuring. You don't want to take off too much metal, nor do you want to take off too much metal in one spot. Most service manuals call for .002-.003 tolerance - no more. I aim for .003 at a minimum, then concentrate on a good hone.

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.

For my Ironhead Sportster cylinders, I switched the honing stones to 320-grit and then lubricated with a light-weight oil (I use ATF fluid). 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 just wrap some old towels around my vise. Squirt oil frequently into the cylinder as you hone.

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.

Clean and Oil 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 #15500 coarse stones and #15510 medium stones. 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 taking it a reputable machine shop. Same advice goes if your cylinder is way out of round, 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). This missing material may cause the hone to cut oversize below the ports on the sides.

In Conclusion

If you are a detail-oriented person, consider boring and honing motorcycle cylinders yourself. If you're not sure you want to try this, 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.