Ok, I hope these are good enough.
Ok, I hope these are good enough.
Visibly, the cylinder looks ok but shows signs of the rings being worn out for some time. Note the patches of blow-by above the exhaust port. Note the many other areas where the rings are not making even contact indicated by dull and shiny spots on the cylinder wall.
Lack of scuffs or seize marks on the intake side of piston shows that lubrication was still present between the piston and cylinder wall even though the piston clearance had disappeared and the piston was operating with a press-fit for clearance. The temperature on the exhaust side of the piston skirt and cylinder wall was enough hotter that the lubrication film could not hold up operating with a press-fit for clearance. When the lubrication film breaks down (reaches the critical temperature where the oil does not provide any lubrication) it only takes less than a second for the friction to generate enough heat to cause the surface of the piston to melt making the seize marks on the piston.
Scuffs and seizure marks on the skirt on the exhaust side of the piston, indicates the upper portion of the piston was larger than the lower portion of the piston. The seize marks being on the upper portion of the piston indicates the piston operating temperature was higher than what the piston was designed for. A properly designed and well-developed piston operating under full load and operating in the temperature range for which it was designed, will be round and have approximately the same diameter from top to bottom. At room temperature most pistons diameters are oval and tapered from top to bottom. Piston diameters are typically .005” to .015” smaller at the top than at the lower skirt. The oval/tapered shape we see on pistons at room temperature is the shape they need to have in order to move toward a shape that make full contact with the cylinder wall when the piston is up to normal operating temperature.
A piston will NEVER transfer aluminum to the cylinder wall or produce seizure marks on the piston if there is a working lubrication film. I have had numerous times in the controlled environment of the dyno room, using OEM pistons and plated cylinders, where the piston is over-heated and expands enough to have a press-fit for clearance. With the press-fit for piston clearance there is enough friction to stop the engine under full load and RPM. After such an occurrence, a tear down will show a polished bore without any aluminum on the cylinder wall or any seize marks on the piston. This is another reason that I believe in running a lot of good oil in the fuel.
Most lubricants cannot hold up to the tremendous heat generated in the areas where the piston to cylinder wall clearance has become a press fit, especially in iron sleeve cylinders. The surface temperature on the cylinder wall of an iron sleeve is considerably hotter than the surface of a plated aluminum cylinder wall with the same water jacket design and coolant flow rates. It may only take a second or less to generate enough heat to kill the oil film in these tight areas when the engine is under full load and high RPM and the piston clearance has become a press-fit.
The melted edge on the exhaust side of the piston indicates high exhaust temperatures. Ignition timing that is too retarded or an air/fuel mixture that is too lean in the RPM range where the engine was operating when the failure occurred will cause high exhaust temperatures. In this situation I would guess that the high exhaust temp. was due to a lean condition.
The under side of the piston shows that the piston crown has been operating near it’s thermal limit for some time. Oil and fuel vapors in the crankcase are in constant contact with the underside of the piston. Coking is a process that occurs when the temperature of a hydrocarbon liquid is raised to the point that a carbon like substance is formed. Anytime the underside of the piston reaches the coking temperature of the fuel/oil vapors, some of the vapors coke and start forming a carbon like substance on the underside of the piston crown. When the underside temperatures decrease the coking stops but previous carbon or discoloration remain.
The discoloration occurs where the temperatures are the highest. The discoloration usually starts in the center of the piston and moves toward the outer edge of the piston as the piston crown temperature increases. The bottom side of the piston will not instantly turn black if the piston crown temperature is raised high enough for a few seconds to cause the piston crown to “sag”. The discoloration takes a while to form as very thin layers form on top of each other until the color is black.
Engines that are only used for 300 ft. drag racing will not usually have the underside of the piston colored black. The underside of the piston is not usually hot enough long enough for the much discoloration to occur. Drag racers that only race the sand hills and spend 10 to 15 seconds under full throttle will often see the center of the piston black but the black will seldom reach the outer edge of the piston.
Engines (Dune bikes, Desert racers, Short track engines, and Moto-Cross) that are well tuned and run under prolonged ½ to full throttle with short periods of off throttle will usually have more discoloration on the under side of the piston than high horse power engines that are only at full throttle for 5 seconds or less then shut down.
In summary for the pistons in the Photos:The piston crown temperature has been knocking on deaths door for some time. The piston temperature the last 5 to 10 seconds of life finally reached point that the piston clearance turned into a press-fit. The heat generated in the areas where the press-fit occurred created enough friction to melt the surface of the piston making the seize marks on the piston and stopping engine rotation. This engine/piston has approached death many times but the operating conditions changed in the nick of time to let it continue to live. I would estimate that the piston crown temperature has been on deaths door for an accumulated time of at least 15 minutes or more.
Things to do to reduce piston crown temperatures:1. Reduce the amount of time the engine is held at wide-open throttle at any one time.
2. Eliminate ANY detonation the engine may be experiencing.
3. Increase the main jet size and or richen the needle.
4. Decrease the pipe restriction by improving the exhaust flow through the stinger and muffler.
5. Give the engine a few more seconds rest between periods of full throttle.
6. Increase the efficiency of the radiator to lower the average coolant temperatures.
7. Operate the engine in a manner for which the engine is designed. This means not to operate the engine in the RPM range after the power peak for more than a second or two. It is Ok to rev the engine into the RPM after the power peak when going through the gears but avoid operating the engine in this RPM range when the gearing is wrong and you are over-revved on a top speed pass or running it up a long steep sand hill and needing to shift to the next higher gear. Prolonged operation of the engine in the RPM range past the power peak can cause piston crown overheating due to insufficient exhaust port flow and insufficient scavenging inside the cylinder.
