"Sticky Pins" Can Wreck Pistons - Part 1

When pistons fail, one of the most common causes has been dubbed "sticky pin." In other words, when pins are not able to turn freely, serious damage to the piston skirt occurs.

What happens?

Although even the stickiest of pins can be made to rotate because fully developed engine power is brought to bear on the piston, the piston itself will not be able to rotate freely on the pin. As friction forces increase, the resulting heat created causes the piston to expand in the transition area between the skirt and the pin boss wall.

Inevitably, the expanded area of the piston comes into contact with the cylinder wall, wiping the lubricant film and increasing the friction and heat. At this point, damage begins.

The first test result is scuffing of the piston in the four transition zones. Called the "four corners" signature, it's one of the clearest signals of a sticky pin problem. Other signs include: burned oil on the pin indicating high temperatures, burned oil near the pin boss also indicating high temperatures, and scuffing of the pin boss bearing surface indicating high frictional forces.

If operation continues, damage to the piston skirt will increase until expansion causes the piston to jam in the cylinder. The crankshaft will then trash a rod bearing or even pull the connecting rod completely through the pin boss.

What makes pins stick?

Since a properly fitted pin will have only .0001" - .0002" of clearance when installed, any error in manufacturing or assembly is naturally magnified. If the piston and pin do meet manufacturing specifications, lack of pin lubrication or pin damage/distortion are the most likely causes.

Lubrication of the pin is critical because the limited oscillating motion prevents the ready distribution of lubricant in the manner of rod and main bearings. The lack of pin lubrication will contribute to sticking pins.

Pins and boss bearing surfaces should be lubricated prior to assembly. On occasion, high temperatures encountered during rod/piston assembly will cause a loss of lubricant. This should be checked prior to assembly into the cylinder. OEMs recommend the use of an Extreme Pressure (EP) lubricant. This will minimize loss of lubrication due to creep before start up. These lubricants will be identified as EP and are usually further identified as engine assembly lube.

Pins can also be damaged or distorted during the assembly process. Damage often occurs when the pin is pressed into the connecting rod without benefit of a heater. Grooves are cut into the pin or burs are raised. Either will be a certain source of excessive friction. The obvious solution is to use a rod heater.

Even when a heater is used, a less than perfect rod pin hole will also cause problems. If the rod pin hole is off axis, the installed pin will try to force the rod from an attitude perpendicular to the crankshaft. As a result, the cylinder wall and the crankshaft will place enough pressure on the pin/pin boss interface to result in a sticky pin.

Pin distortion, on the other hand, can occur when the connecting rod cools and shrinks around the pin because stresses are applied to the pin. These stresses are not evenly applied due to the differing rod cross sections. If the shrink interference is to great, the pin will actually be forced into an oval shape. This will reduce the oil film to zero at the oval lobes, again resulting in a sticky pin.