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How to Know When to Replace a Motorcycle Piston?

How to Know When to Replace a Motorcycle Piston?

The importance of replacing the piston at regular intervals in high-performance powersports engines cannot be overstated.  The piston is one of many items in your motorcycle that will wear over time.  It may last longer than tires or a chain, but it should still be treated as normal maintenance when the time comes. Read through our tips to help you know when it's time for a refresh.

Quick Article Summary

  • Piston replacement intervals are typically outlined in your machine’s factory service manual.
  • Monitoring the engine’s health through periodic checks such as compression and leak down tests
  • Piston wear typically occurs in four key areas: the piston crown, piston skirt, wrist pin bore, and ring grooves.
  • We sell the highest quality pistons from industry leading manufacturer Wiseco.  Click here to see our current prices!

Piston Maintenance

The piston in an internal combustion engine is arguably one of the most important components found in the engine. When it comes to high-performance engines used in powersports, the piston is also a component that is regularly replaced and serviced. Knowing when your piston should be replaced and how it wears is key to maintaining a reliable engine. To help you make that decision, we've laid out replacement intervals, piston wear, why it’s important to replace the piston, and piston replacement options.

Piston replacement intervals are typically outlined in your machine’s factory service manual. Using dirt bikes as an example, many manufacturers outline a piston and ring replacement schedule of every six races or 15-30 hours for a four-stroke, depending on the machine. If you’re new to the sport or have never looked at your factory service manual, these service intervals may seem shockingly short. The service intervals are based on the service schedules required to maintain a high-level racer’s machine. Unfortunately, for the average rider, the outlined service intervals commonly end up being conservative.

In reality, piston replacement intervals should be established based on how the individual owner rides and maintains their machine. It’s true that forged pistons exhibit greater strength and wear resistance, but the variables of rider and maintenance still apply. Engine displacement, engine make, air filter maintenance, environmental conditions, riding style, and the type of riding the machine is used for will all influence how long the engine should be operated before servicing it. Monitoring the engine’s health through periodic checks such as compression and leak down tests is the best way most riders can appropriately time major service tasks, such as piston and ring replacement. Due to the number of variables that affect engine wear, it is simply not possible to specify a replacement schedule that fits everyone’s needs other than a very conservative schedule.

Piston wear will typically occur in four key areas:  the piston crown, piston skirt, wrist pin bore, and ring grooves.

Piston Crown Wear

Piston crown wear occurs as a result of improper tuning, or a damaged valvetrain on four-stroke engines.  When an engine is run at full throttle, along with a lean mixture, will see high combustion temps and can cause detonation. These detonation results will show on the piston crown as an eroded or pitted surface.

Piston Skirt Wear

Piston skirt wear occurs from the inherent geometry of the crank mechanism as the engine fires. Peak combustion pressure occurs slightly after top dead center, causing the piston to thrust into the cylinder wall.

On two-stroke engines, skirt wear can sometimes be heard audibly while the engine is running.  A rhythmic metallic sound along with a loose or worn piston when the engine idles. What you're hearing is the piston rocking back and forth in its bore as it reciprocates.

Skirt wear can be observed by measuring the diameter of the skirt and referencing it against the diameter outlined in your service manual. Wear can also be obverved visual as a polished area on the thrusting faces of the piston.

Your pistons may feature one of a few different types of skirt coating. Wiseco pistons utilize different types of skirt coatings depending on the piston, including ArmorGlide and ArmorFit coatings. These coatings are screen printed on and will last for the life of the piston.  We sell a variety of Wiseco pistons with different types of skirt coating.

Wrist Pin Bore Wear

A wrist pin is a small cylinder that keeps the piston attached to the connecting rod.  Wrist pin bore wear occurs naturally from the loading of the wrist pin joint through inertia and combustion. The wrist pin bore will typically become misshaped as it wears.  Usually, a portion of the bore will appear polished or worn. Also, the wrist pin bore can be measured from top to bottom and side to side. These measurements can be compared to the diameters specified in the service manual.

Replacement piston options are plentiful and can be overwhelming, and the most common upgrade riders are faced with is whether or not to move to a forged piston. Forged pistons are a nice upgrade because they offer additional strength and wear resistance over cast pistons.

Forged pistons have greater strength than cast pistons by using different aluminum alloys and manufacturing processes. The forging process results in finished components that have a tighter molecular structure, and optimized for strength. On the other hand, cast pistons are not cast under high pressures and their molecular structures are not as tight.  In severe cases this can lead to air pockets or voids.

Ring and Groove Wear

Located between the cylinder and the piston, piston rings are necessary components that allow the engine to operate efficiently.  Combustion chamber ignition causes the piston rings to move in and out of their grooves.  Once the air/fuel mixture is ignited, pressure within the cylinder increases and forces the piston ring against the cylinder wall, which causes the ring to slide in its groove.

Ring and groove wear occurs due to the reciprocating motion of the rings and can be made worse by carbon deposits accumulating in the ring groove. The current state of your rings and grooves can be obtained by thoroughly cleaning the ring and groove, then measuring them. Most service manuals outline specifications for ring width, groove width, and ring to ring groove clearance.

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