Two-Stroke Top End Rebuilding

By Eric Gorr

Displacement: 80cc 125cc 250cc 500cc

Note that air-cooled bikes should be inspected more frequently. Also, you may want to inspect more often if you are riding in fine sand or lots of mud. When you tear down the engine, inspect each system and look for the following trouble signs.

Reed Valve

Check the reed petals for open gaps between the sealing surfaces. In time, the reed petals lose their spring tension, and the back-flow can cause a flat-spot in the throttle response. Stock nylon reeds tend to split at the edges on bikes that are constantly over-revved. Expert riders find that carbon fiber reeds last much longer.

Cylinder Head

Check the head at the edge of the chamber for erosion marks—a sign that the head gasket was leaking. If the head or top edge of the cylinder is eroded, it must be turned on a lathe to be resurfaced.

Cylinder

All cylinder bases use aligning (dowel. pins around two of the cylinder base studs. These pins are made of steel, and after heavy power washing, they get corroded. That makes it difficult to remove the cylinder from the crankcases. Never use a pry bar! That will damage the cylinder. Instead use a plastic mallet to hit upward on the sides of the cylinder at a 45-degree angle. Alternate from left to right sides so the cylinder lifts up evenly. After you remove the cylinder, stuff a shop towel into the open crankcases to prevent debris from entering the engine.

The Different Types of Steel-Lined and Plated Cylinders

There are two types of cylinder bores used on dirt bikes, steel or cast iron sleeves or ones with plating on the aluminum. Most dirt bikes made after 1989 have plated cylinders. You can check a cylinder with a magnet. If it sticks to the bore then it is a sleeve. If it doesn't stick then it is plated. There are three types of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel silicon carbide. There are several variations of the nickel silicon carbide process but the most common trade name is Nikasil. The nickel-based processes have many advantages over hard-chrome, Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier material for silicon carbide particles, a wear resistant material that carries the load of the piston. This material is electro-plated right on to the aluminum cylinder for the optimum thermal efficiency. Nickel can be honed with diamond stones which leave distinctive peaks and valley scratches in the cylinder wall which retain oil and provide a certain bearing ratio between the running surfaces of the bore. It's possible to rebuild a plated cylinder by fitting it with a sleeve. However you can expect to pay more for bore maintenance over the life of the bike, and lose thermal efficiency and horsepower. Plated cylinders are harder and last longer than sleeved cylinders. Kawasaki cylinders with the original Electrofusion coating or hard-chromed cylinders can be repaired with nickel plating or sleeving. Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder. The reason is that the pretreatment for the plating would disintegrate the aluminum. There are four companies that replate cylinders in the USA. The average price to replate a cylinder is about $200.