"Topping out", What is it?

2-stroke engines being what they are, they depend on many factors working together to ensure you have good mixture intake into the engine, good flow into the cylinders, and ultimately, good compression and power. One of the most important factors is very good ring seal to the cylinder wall. A 2% leakdown, under load, can result in a 5% power loss (in the neighborhood of 200-300 rpm, easily). Greater leakdown, past the rings, makes the engine difficult to start, can cause running issues, and greatly erodes overall power.

Basically, the rings are the seal area that affects how well your engine will pull fuel-air into the crankcase, how much compression you have, how much exhaust gas contaminates the incoming fuel-air that is stored inside the crankcase, and even how well the piston is cooled. So, those rings play a very important role in the overall health and well being of your engine. A single score mark can lose 100 rpm. Getting the rings hot enough to lose their seat on the cylinder can lose you 400-500 rpm and cause all sorts of tuning issues. Anything that causes the ring to not seal properly will cause starting issues.

Let's look at what happens during start up. If the rings are sealing properly, you flip the prop, a vacuum is formed in the crankcase, the piston comes down and forces the fuel-air mixture into the cylinder, and the next time the prop is flipped it goes BANG!! But, if the rings do not seal properly, you will lose some vacuum, so not as much fuel-air is drawn in. . .you lose some compression, so there may not be enough to fire the engine, and if it does fire, the blow-by will probably contaminate the new fuel-air charge inside the crankcase and cause you to lose power.

Some symptoms of bad sealing rings. . . hard to start when warmed up but let sit for a short while. . . hard to start period. . . loss of rpm in the 200-600 rpm range (if it even runs). . . difficulty starting when cold .. . tends to overheat.

The rings, and their seal to the cylinder wall, are probably the most important parts of the engine. They seal the combustion process, help cool the pistons by moving heat to the cylinders, and from there to the cooling fins, and hold lubricant so that the piston has a layer of oil on which to ride. Any degradation in how the rings perform is going to result in loss of power, heating issues, and hard starting.

One note. . .some engine importers and builders say they "Hone" the cylinders with a Bead hone. .NO NO NO NO NO!!!!!! A bead hone can catch in the edges of the ports and flake off the Nikasil plating (which is only about 2-5 thousandths of an inch thick). This is a VERY bad technique. Secondly, Nikasil is so hard, a normal hone will basically just glide over the surface, barely scratching it, so they are NOT "honing" the cylinders. .they are deglazing them (a true hone for Nikasil has Diamond in it to cut the plating. . normal hones do not have this material). If they really Honed the cylinders your pistons would not fit properly, and the cylinders would be junk. Removing as little as 5 ten-thousandths of an inch (.0005") from the bore renders the cylinder almost unusable because the piston will have too much clearance and sill start flopping around and possibly be damaged, and the rings will have a harder time sealing properly.

Here at MAE we use what is referred to as a "flap hone".. basically a flapping layer of many pieces of abrasive cloth. It will not damage the Nikasil plating, removes all glazing very quickly, and is the PREFERRED method for deglazing Nikasil plated cylinders. If anyone uses a bead hone, small cylinder hone, or other "hone" besides a flap hone, they are not doing the process properly, and risk damaging your engines cylinders in the process.