These three pistons have piston-crown burn patterns linked to carb jetting and fuel choice. The piston in the center is a great example of a perfect carbon pattern; the color looks like hot-chocolate and the flow pattern resembles a leaf pattern. Looking under the piston crown a dark carbon spot should be visible taking up approximately 25% of the area of the underside of the piston crown center.
The piston on the right is dark stained with no carbon spot under the crown. This means that the jetting is rich, and perhaps the pre-mix ratio or oil type is too rich as well. The transmission-side crankshaft seal may be worn, allowing transmission oil to enter the crankcase and be burned in the combustion chamber, producing smoke at the tail-pipe, spooge at the header, and excess carbon on the piston crown.
The piston on the left suffered from too-lean carb jetting, too hot of a spark plug heat range or too low of an octane fuel for the engine’s compression ratio. The piston crown got so hot that it melted. The melted blow-out spot usually occurs on the exhaust side and up to the center where the heat and pressure is the greatest.
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Most pump gas contains ethanol which works to separate pre-mix oils from gas. Piston to cylinder seizures from lack of lubrication is the most common piston failure. The piston in this photo is dry to the finger touch and the piston is deeply scoured and melted. The gas tank has globs of oil and straight volumes of gasoline.
Millennium Technolodies does not offer cylinder warranties when the piston and cylinder wall are dry with little or no oil and seizure marks. In some cases the cylinder can be repaired with light diamond honing for a small cost. Millennium also offers competitive prices on brand name piston kits like Wiseco, Wossner, and Vertex.
MULTI-POINT CLEARANCE SEIZURE
This piston has multiple seizure streaks equidistant around the periphery of the piston face. The clearance between the piston to cylinder wall was too small considering the piston manufacturer’s recommended specifications. Our technicians can advise you on the proper clearance spec for all sorts of fuel types, tuning, and usage patterns. For example, grass-drag snowmobiles and youth mini-cycle racers need more piston to cylinder wall clearance because there is a short time between cold and full-throttle.
4-POINT COLD SEIZURE
The term “cold seizure” refers to starting your engine and putting a big load on it before the temperature of the piston and cylinder normalize. The piston heats up faster than the cool cylinder and expands so fast it seizes the piston on 4 equidistant points around the outside of the piston.
Air leaks can come from a variety of sources such as the intake joints, seals, and even splits in the carburetor. An air leak is a dangerous condition because it can lean-out the air/fuel mixture and cause the piston to heat up and eventually melt. Most air leaks occur at gasket joints and crankshaft seals on 2-cycle engines.
The piston in this photo suffered a massive meltdown on the exhaust side of the piston. The cause of the failure was traced to a dry-rotted rubber intake manifold.
The piston ring end-gap must be checked prior to use. The general rule is .004 inches for every inch of bore diameter. The piston in this photo has seizure marks over the ring end and 180 degrees from the gap.
CAST SHATTERED VS. FORGE HAMMERED PISTONS
The pistons in these photos have broken or shattered to pieces from either over-revving or having been run for too many engine hours. Generally speaking, cast pistons offer advantages of low friction and better resistance to high thermal loads whereas forged pistons are lighter and more durable.
When cast pistons reach their wear limit they can shatter into pieces. Forged pistons get hammered at key wear points and make a lot of noise when the engine is running.
BLACK SPOT HOT SEIZURE
The pistons in this photo suffered seizures related to the carbon build-up that accumulates on the exhaust side of the piston. This engine had a faulty silencer. The core screen was broke and partially blocked the exhaust allowing heat and burnt gasses to pulse back to the exhaust face of the piston.
2-STROKE PISTON TO HEAD COLLISION
The piston in this photo has a distinct ring pattern near the outside edges that indicates the piston contacted the head. This can happen when the crankshaft bearings fail and the connecting rod gets loose allowing the piston to contact the head near the squish-band.
CIRCLIP FLIP SEIZURE
Circlips are funny little squiggly things that we loathe, yet depend on. When they’re installed right they secure the piston pin from tearing a trench in the cylinder wall. They’re frustrating to install. When they’re installed wrong, they flip out of the piston clip groove and cause a piston seizure by wedging the spring wire against the cylinder wall. Seizures like this require welding repairs to return the cylinder bore to stock size.
RING PIN BACK-OUT SEIZURE
When pistons are manufactured the ring centering pins are pressed in with interference clearance and an adhesive. Unfortunately sometimes these little pins back-out of the piston allowing the ring to spin and break while the pins jam in between the piston and cylinder, or get smashed into the head.
EXHAUST BRIDGE SEIZURE
Piston seizures can occur at the center exhaust-bridge on a delta-shaped port, or the sub-exhaust bridges on a triple port design. The exhaust bridge is the hottest are of a 2-stroke cylinder and the ring pressure is at its peak because the bridge’s surface area is so small. Seizures happen for many reasons but the most common are 1) Lack of lubrication 2) Too lean jetting or air leak 3) Coolant over-heating 4) Not enough bridge relief 5) No oiling holes in the piston over-lapping the exhaust bridge.
The piston in this photo was damaged because the air filter leaked, allowing dirt to enter the cylinder. The dirt scoured the intake side of the piston, wearing it thin enough to produce a crack between the boost port and the bottom of the skirt.
COOLANT LEAKS VS. DETONATION, PRE-INTION, PINGING & PINKING
This photo shows two pistons, the small one on the right suffered a coolant/lube seizure. A head gasket leak enabled coolant to spill into the bore and wipe the oil off the cylinder causing a lack of lubrication.
The big piston on the left suffered a detonation seizure, which is evident because of the melting of the crown on the exhaust side. Detonation can often be confused with coolant leaks because the top edge of the cylinder and head can become eroded, which isn’t due to combustion pressure but rather the pressurized coolant flowing back and forth across the surfaces like a water-jet.
Detonation can be an American slang word for a variety of engine problems. Technically speaking combustion problems can be divided between detonation and pre-ignition. Detonation is an abnormal combustion condition linked to everything from too lean jetting, too much turbulence in the squish-band, too much spark advance, ethanol additives in pump gas, and coolant leak contaminates trapped in the mixture. The “sounds” of detonation include used terms like “pinging” in America and “pinking” in the UK. Dieseling is another good term; abnormal combustions sounds like shaking ball bearings in a coffee can.
All of this detonation occurs after the spark plugs ignite during ignition.
Pre-ignition happens before the timed spark when the edge of the spark plug threads or a protruding steel head gasket heats up to auto-ignition temperature and causes the mixture to ignite before the spark occurs. The sound of pre-ignition is a loud, almost scary type of sound from the motor. Auto ignition and stuck throttle conditions make it impossible to shut-off the engine until it suffers a catastrophic failure.
Some interesting reading can be found also on Eric Gorr’s page:
Piston problem troubleshooting
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