Pistons & Cylinders & Rings

Pistons, Cylinders & Rings

Pistons and Cylinders are pretty straightforward. After you remove the head, try to move the piston sideways in the bore. There should be almost no movement. Examine the cylinder. If there is an obvious ridge at the top and you can feel it with your finger, along with any piston movement sideways, you will need to bore it oversize. Factory oversizes are in quarter-millimeter increments, like .25, .50, .75, and 1.0mm. .25mm is about .010", enough to clean up the bore but not significantly increase the engine's size.
To be sure, measure the bore diameter and the piston's diameter using micrometers. Measure the piston about 1/2" up from the bottom of the piston skirt. Then, measure the cylinder bore in several places at right angles to each other. Subtract the piston diameter from the largest bore diameter to get the piston clearance. Piston clearance varies due to several factors:

1. Diameter of the piston.
2. Type of metal the piston is made of.
3. Is it a cast or forged piston.
4. Is the engine air or water-cooled.
5. Is the engine two or four-stroke.

A small (50mm bore) cast piston, 4-stroke, may have a clearance of about .0005" (.0127mm), while a large, forged 2-stroke piston (73mm, 2.874") can need .003" (.075mm) or more. You'll need a shop manual for your bike to find the exact clearance. This is the minimum clearance. For the maximum, consider the following story: I once had the exhaust off a 400cc 2-stroke single and was able to slip a .020" feeler gauge between the piston and cylinder. The bike ran for several more months before the piston skirt broke. The bottom line is to get a shop manual and follow the correct clearance specifications. If it's an aftermarket piston, use the manufacturer's clearance recommendation.

If your old piston is within spec and you wish to reuse it, you'll need to measure the ring side gap. Side clearance (gap) is measured with a feeler gauge, with the ring in its groove on the piston. Try to fit the feeler gauge between the ring and the top or bottom of the groove. They can range from .001" to .003" (.025mm to .0762mm, check the manual). If using a new piston and rings, you don’t need to check this. If the gap is too large, even with new rings, you'll need to replace the piston.

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Many people believe that a new set of rings can fix everything, but that’s rarely the case. On a dirt bike, you frequently need to bore it oversize. Sometimes, on a big street bike, the pistons will be within specification, but most dirt bikes will require boring.

It's best to have the cylinder bored on a boring machine. Honing them as big as .040" over can be done to spec on taper and out-of-roundness, but it takes a lot of time. If I need to go bigger than 1/4mm or .010", I prefer to use a machine. If you decide to hone, use a good hone like a Sunnen or Ammco. Measure the piston first and then bore the cylinder to fit. Do not bore the cylinder unless you have the piston to measure, as this could lead to incorrect clearance.

Remove the circlip and push the wrist pin out with a screwdriver. Lightly tap it if needed, but if the pin is too tight, use a wrist pin puller to remove it.

If the piston has seized and smeared aluminum on the cylinder, you'll need to remove it before measuring. Use Muriatic acid for this. Place a few drops on the smeared aluminum in the cylinder; the acid will eat the aluminum but will not harm the steel cylinder. Avoid getting the acid on yourself. If you do, use water and baking soda to remove it. Also, keep it away from aluminum unless you intend to remove that aluminum. Muriatic Acid can be found at home improvement stores.
Imagine you pushed the engine hot and melted the piston. After fixing and replacing all parts, if it still smokes, the cylinder might be "burnt" or "scorched." The only real solution is a new cylinder or cylinder liner. Check the cylinder head for flatness, as the heat might have warped it. On a 2-stroke, if the top of the piston looks chewed, the bottom end bearings might be failing. Rebuild the bottom end to fix this issue.

Check the cylinder for rust pits. If any exist, you'll need to bore the cylinder to a larger size to remove them. During restoration, rust pits are often found where the rings stopped in the cylinder. Water is produced during combustion, which tends to collect around the rings.
When you get your cylinder back, confirm if it has been cleaned. Take a clean rag with fresh oil and wipe the inside. If dirt appears, wash the cylinder in hot soapy water, then dry and oil it immediately to prevent rust.

The piston's rings need a small gap between the ends when installed. This end gap, specified in the shop manual, can vary widely (.006" to .020" or .15mm to .08mm). Measure the end gap by placing the ring in the cylinder and pushing it up with the piston. This squares the ring in the bore. If the gap is too small, file the ring ends carefully until within spec. If too large, the ring cannot be used.
When installing a new ring, use a ring expander if available. If not, use your fingers, avoiding excessive spreading or bending. Rings are hard and brittle, breaking easily. Always install rings with markings facing up. Note that rings come in various styles. Replace each with the same type and size.
Some believe that the spring pressure of the ring seals combustion pressure, but it's actually the combustion pressure itself that pushes the ring against the cylinder wall. Installing an oversized ring in the wrong cylinder can lead to compression loss. After installing, stagger the ring end gaps to avoid alignment. For 4-strokes, different oil rings exist (one-piece or three-piece), with three-piece rings often performing better but being harder to install.

Before installing the piston, thoroughly oil the piston and cylinder. Ensure plenty of oil on all parts. Pistons have a front and a back, indicated by a mark or arrow pointing to the front or exhaust side. On 4-strokes, "IN" in a valve pocket indicates the intake side.
I prefer using a ring compressor for installation, though a hose clamp can be an alternative. The piston and rings should slide on smoothly with light taps from a piece of wood or a lead hammer. If resistance occurs, reposition the ring compressor.
In 2-strokes with large ports, use a screwdriver to ensure rings don't catch in the port. Position ring ends to the peg in each groove.
To install, fit the piston with rings and one circlip from the top, pushing it down until the wrist pin hole is revealed. Align the cylinder to the rod and bearing, insert the wrist pin, and secure with the circlip. Don't forget the base gasket and sometimes a base o-ring. Always use new wrist pins, wrist pin bearings, and circlips, although reused circlips can suffice.

Circlips come in various types, but 95% of motorcycle pistons use the wire type. Older Harley-Davidsons required special tools for their unique circlips, but newer models and aftermarket pistons now often use the wire type. Always use the recommended type. A failed circlip allowing the wrist pin to hit cylinder walls can cause severe damage.
In the past, high-performance builders used Teflon or aluminum "buttons" instead of circlips, but this method has fallen out of favor.
For engines with multiple cylinders, crank pins are staggered to allow inserting pistons one or two at a time, which can be challenging.

Piston oversizes are usually marked, though sometimes only with ink or worn off. They can be indicated in thousandths of an inch or millimeters, along with a code for oversize and ring type. Always measure the old piston to order the correct size.
Sleeveless plated cylinders are increasingly common, using various plating methods. These cylinders are bored and plated to the right size, using softer metals for rings to reduce cylinder wear. However, wear can still occur. Damaged cylinders may need boring out and inserting a new steel liner. Alternatively, they can sometimes be replated.
On chrome-plated cylinders, avoid chrome rings to prevent peeling. Use cast iron rings instead. Though chrome rings can be used on certain plated cylinders, confirm with the ring's manufacturer.

When working on older bikes, finding the right piston can be challenging. As long as the wrist pin diameter matches and the cylinder can be bored to fit, many pistons can be modified to work. This might involve cutting down the piston skirt, drilling ports, grinding the piston head, or adjusting the skirt's weight.
For example, a Honda 175cc twin cylinder piston could be modified for a 90cc engine by adjusting the bore and filing to fit the combustion chamber. This could significantly increase compression ratio and performance.
Some pistons appear unusual due to design efforts to maximize horsepower. Examples include KTM 500cc pistons with wrist pin posts, Wiseco Yamaha YZ 125cc pistons with side holes for fuel mixture, and differing designs for old and new technology pistons.

In conclusion, there is more to pistons than it first seems. Proper measurement, maintenance, and installation are crucial for optimal performance.

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