2-Stroke Top End Inspection

Top End Inspection Tips

Inspection Chart
Displacement 80/85cc 125cc 250cc 500cc
Tear Down Hrs 10 15 25 50
 
The top end on your bike should be torn down periodically for proper inspection of the cylinder, cylinder head, piston/rings, power valves, reeds, etc. The above chart is just a general guide. It’s hard to place an exact service interval for the top end due to a few things such as how hard the bike is ridden, hours on the engine, quality of the 2-stroke oil used, and the amount of debris that could have been drawn into the engine. A national MX type racer would perform inspections or replacement of the top end on shorter intervals due to sustained high rpm’s and loads placed on the engine. The average weekend rider could have longer inspection or replacement intervals due to lower rpm range of operation.

Reeds

Reed valves are simple and require no routine maintenance. Reeds only need to be inspected for proper sealing to the reed cage surface. Any air gaps between the reed pedals and the cage sealing surface would indicate lost of spring tension. Reeds will split on the ends or around the edges after many hours of operation and or constant over-revving. Reeds wear from constant opening and closing as the engine is running. Never flip over reeds just to get them to seal to the cage, this can cause them to break. Before removing reeds from the cage mark the top side with a white paint marker if they are going to be re-used. Carbon fiber reeds seem to last the longest and provide the best performance.

Piston/Rings/Cylinders

The piston is placed under extreme load and heat while the engine is running. The condition of the piston can tell you what’s going on inside the engine. For example if the piston has dark brown/black stains below the piston rings then blow by is occurring. Blow by is where the piston rings or cylinder are worn pass service limits and allows combustion gases to seep between the rings and cylinder wall down the side of the piston. Next check for very fine vertical scratches on the piston skirts. These scratches can be from dirt has been drawn into the engine or even composite plating flaking off the cylinder. This kind of wear could result in a new piston kit due to it reducing the piston’s diameter. Check the under side of the piston for burnt black oil this would indicate that the piston crown is to hot and maybe cooling system or jetting related. There are plenty of other signs of problems indicated by the pistons condition these listed are most common. Never reuse circlips and never use a punch and hammer to remove the wrist pin. Circlips fatigue and can break. Hammering out the wrist pin can damage the big end rod bearing. Motion Pro makes a piston pin removal tool.
 
Burned oil spot on the bottom side of piston due to high piston crown temps.
 
Blow by due to worn piston rings and even worn cylinder bore

When dealing with O.E.M parts from Japanese bike manufactures you’ll notice that they use a letter designation to help identify plated cylinders and pistons. You can have of to four letter designation ranging from A,B,C,D. During production there’s no guarantee that all cylinders are the same so the manufactures came up with this letter system to have a wider range of cylinder diameter. Your factory cylinder will have a stamped letter on it and you would need to order the piston with the same letter (that’s if the bore measures correctly). Refer to your service manual for proper cylinder letter specs and piston letter specs. If you purchase a new cylinder it might not be stamped, so you would need to measure the bore to determine its size and compare to the letter chart in the service manual to determine what letter piston you would need.
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Ring end gap can be over looked by most people. To measure ring end gap you will place a lightly lubed piston ring just inside the top of the cylinder and use a piston to push the ring about 1/2 inch down inside the cylinder. Now use a feeler gauge to measure the air gap. Most ring end gaps are going to be .018-.025in but please review your service manual. Check ring end gap each time you have the cylinder re-plated or installing new rings and piston kit.
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Cylinders can be plated or sleeved. Nikasil is the most common material used to plate cylinders. Nikasil is a nickel based material that carries silicon carbide particles, which is a wear resistant material that carries the piston load and resistant to wear from piston rings and attracts oil for lubrication. Check plated cylinders for flaking. Best location to inspect for this is around the port edges. The cylinder would need to be re-plated if the Nikasil is flaking off.  If the cylinder has a brown oil glazing look to it would need to be honed. Ball hones are the only hones to use inside a 2-stroke cylinder. Never use a spring loaded finger style hone, it can damage the port edges. The ball hone will not remove any material. Cylinders can have steel sleeves installed. In some cases a sleeve maybe the only way to repair the cylinder that has been damaged due to engine failure. Sleeves do add weight to an engine and are not as thermal efficient (heat transfer) as plated cylinders.