GEAR PUMP GROUP 3P4855 - Caterpillar

All 3300 Series Engines With 8N3181, 8N3183, 9N5251 Or 9N5404 Basic Piston GroupsPistons with offset (not in the center) pin bores are now used in all 3300 Series Engines.
Because of the offset pin bore design, the piston will have carbon cutting on the thrust side of the piston top land. See the Illustration. The area of carbon cutting will be approximately 38.2 mm (1.5") wide but will not extend all the way to the top piston ring groove.
This carbon cutting, which comes during the first 100 hours of operation, is a normal condition for pistons with an offset pin bore, and it will not have an effect on the performance of the engine. Use the Illustration as a "use again" guide. Carbon cutting will be shown in a future revision of the "Guideline For Reusable Parts-Pistons And Cylinder Liners", Form No. SEBF8001-01.
The offset pin bore design decreases vibration of the cylinder liner caused by piston slap (side to side movement) when the piston moves through the top center position. The offset pin bore permits the piston to move more smoothly through top center. Vibration of the cylinder liner can cause cavitation erosion. The result is pitting on the outside of the liner.
High frequency vibrations can some times cause pitting of the outside of the cylinder liner when there is no corrosion on the liner.
For this reason, regular cooling system maintenance is necessary. The coolant must prevent rust, corrosion and pitting.
Water is not a good coolant because water, without cooling system conditioner, becomes an acid solution very rapidly. See the June 23, 1980 Service Magazine article, "Do Not Add Caterpillar Cooling System Conditioner (Corrosion Inhibitor) To New Machines Before Delivery; Cooling System Conditioner Service Interval Changed," for the correct amount of cooling system conditioner and service interval. (Do not add cooling system conditioner to Dowtherm 209 or Dowtherm 209 Full-Fill.)
Carbon-cutting on a piston after the engine was operated eight hours at full load. This carbon-cutting does not have an effect on the engine's performance, and the piston can be used again.