Two-Stroke Top End Rebuilding

Some unfortunate guys do more damage replacing the piston than the actual wear on the piston! Remove the circlips with a small needle-nose pliers and throw them away. It is a common mistake to reuse circlips, but the cheap spring-steel wire clips will fatigue and break if you install them for a second time.

After removing the circlips, you have to remove the piston pin. Never use a hammer and punch to remove the pin. That will damage the connecting rod and needle bearings. Instead, use one of the pin-extractor tools available from your local franchised motorcycle shop. You can also grasp the piston with one hand and use a 3/8-inch socket extension to push the pin out with your other hand.

Too many people replace their pistons too often. The exact service interval for your bike depends on how hard the bike was run, for how many hours, the quality of the lubrication, and the amount of dirt or other debris in the intake air. Bikes that are run hard with dirty air filters may wear out pistons in only 6 hours, while bikes that are ridden easy with clean filters and adequate fuel octane may last 60 hours.

Measuring the Piston

The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies such as Enco or Harbor Freight. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximum wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the skirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb's main jet needs to be richer.

Letter Designations on Cylinders and Pistons

The Japanese manufacturers use a letter designation system for plated cylinders. They intend for you to order replacement pistons based on the letter designation printed or stamped on the cylinder. This is the reason why they need this type of system. In mass production you can't guaranty that all parts will be exactly the same size. The size variance is based on an acceptable level of quality. Tool bits become dull, temperatures of machine tools change through production runs, and machine operators have inconsistent performance. The Japanese manufacturers have between two to four different sized pistons and cylinders. Normally labeled A, B, C, and D. If they only had one size, the piston to cylinder wall clearance would vary between .001 to .006 inches. In the standard Japanese alpha labeling system, A denotes the smallest bore or piston size and every letter after that is slightly larger, usually in increments of .0015 inches. The danger is that if you try to put a D piston in an A cylinder the piston to cylinder wall clearance will be so tight that a seizure might occur.

Pro-X Oversize Piston Kits

Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers. These pistons are the same quality as the OEM pistons, and they are available in sizes larger than the alpha pistons available from franchised dealers. Also the Pro-X pistons are usually priced lower than the OEM pistons. If the cylinder bore is slightly worn (up to .005 inches) with only a small area of bare aluminum exposed, you can install a Pro-X oversize piston. The Pro-X pistons are graded oversize in smaller increments than Wiseco pistons, but a wider range than the OEM pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches increments. Before attempting to order a Pro-X piston, you must measure the cylinders bore at the smallest point and allow .002 inches clearance between the piston and cylinder.

Measuring the Ring Gap

The best way to know if the rings are worn is to measure the ring end gap. Put the ring in the cylinder and use the piston to push it down about 1/2 inch from the top evenly spaced. Now use a feeler gauge to measure the width of the ring gap. Normally, the maximum gap is 0.018–0.025 inch.