1998/08/10 Caterpillar

777B (4YC),
777C (4XJ),
777D (3PR),
783B (8YM),
785 (8GB),
785B (6HK),
787B (9TM),
789 (9ZC),
789B (7EK),
793 (3SJ),
793B (1HL),
793C (4AR) Off-Highway Trucks;
776B (6JC),
776C (2TK),
776D (5ER),
784B (5RK) Off-Highway Tractors;
854G (1JW) Wheel Tractors;
D11N (74Z, 4HK),
D11R (8ZR, 9TR, 9XR) Tractors;
992G (7HR),
994 (9YF) Wheel Loaders;
5130 (7TJ, 5ZL),
5130B (4CS),
5230 (7LL) Excavators

Reference: Service Magazine; September 22, 1997; Page 38; "Improved Piston Crown Assembly Used In The Rod And Piston Group". Service Magazine; September 22, 1997; Page 39; "New Piston Crown Improves Service Life". Service Magazine; April 28, 1997; Page 27; "Improved Cylinder Liner Has Longer Service Life". Service Magazine; July 28, 1996; Page 19; "New Oil Jet Tube Assemblies Replace Oil Cooling Jet Assemblies". Service Magazine; This issue; "Piston And Piston Ring Reference Chart For 3500 And 3500B Engines". Service Magazine; This issue; "New Piston Rings Provide Increased Service Life".

3500B and 3500 machine engines with two piece pistons now use new piston rings. See the Reference Article, "New Piston Rings Provide Increased Service Life" in this issue.

The new piston rings are more robust and will have a longer service life. The new piston rings are also less susceptible to corrosive and abrasive wear. But engines with a history of abrasive or corrosive wear problems will continue see these problems, even with the new piston rings. Therefore, it is important to find the source of these problems so they can be corrected.

The following information reviews the possible causes and corrections for excessive wear to pistons, rings, and liners.

The prime cause of broken piston rings is excessive wear. The two main causes of wear that lead to shortened piston ring life and shortened engine life are Abrasive Wearand Corrosive Wear. Engines with abrasive and/or corrosive wear will have a high wear rate between the piston rings and piston ring grooves, and also between the piston ring face and cylinder liner bore. Excessive piston, ring, and liner wear will result in high oil consumption, high blowby, and sometimes broken piston rings.

Abrasive Wear

Abrasive wear comes from dirt entering the air intake system. This can be the result of one or more of the following:

* Very dusty applications.

* Separation of air piping or loose clamps between the air filters and turbochargers.

* Too frequent air filter servicing (service the filter elements as per the indicator).

* Too little air filter servicing (filter elements are plugged).

* Inadequate filter element maintenance (rough cleaning or insufficient checking for holes in the filter element media before reuse).

Abrasive wear can be identified by the presence of silicone from the dirt in the abrasive scratches on worn parts. Unfortunately, this can only be seen with an electron microscope. Testing engine oil [Scheduled Oil Sampling oil analysis (S·O·S)] for the presence of silicon is another way to identify the likelihood of abrasive wear. But, a significant number of engines with abrasive wear problems do not have high silicone in the S·O·S.

Abrasive Wear Prevention

The best way to prevent abrasive wear is proper maintenance and servicing of the air inlet system. This includes:

* Proper monitoring of the air filter restriction.

* Replacing the air filter elements when the restriction is at 20 to 25 inches of water.

* If cleaning the filter elements, strictly following the guidelines in "Procedure To Inspect And Clean Air Filters", SEBF8062.

Corrosive Wear

Corrosive wear occurs when the following conditions exist:

1. Low engine jacket water temperature [the temperature should be maintained above 79°C (175°F)].

2. Low engine oil Total Base Number (TBN).

3. Sulfur in the fuel (a higher fuel sulfur content can accelerate corrosive wear).

4. High humidity (can accelerate corrosive wear).

5. Plugged crankcase breathers (the breathers need to be maintained in order to help prevent moisture condensation in the crankcase which causes rapid TBN depletion).

Magnified view of the bottom surface of a top piston ring that shows evidence of corrosive wear.

Corrosive wear can be most easily identified by the presence of hundreds of small black pits on the bottom surface of the top piston ring. These pits can be seen with a 10X or 20X eye piece.

Corrosive Wear Prevention

The best way to prevent corrosive wear is to minimize the occurrences of an over cooled engine and limit used oil TBN to 50 percent of new oil. The number one way to have an over cooled engine is for one or more of the four jacket water thermostats to be stuck open. A thermostat will be come stuck open as a result of a cooling system that has been poorly maintained, or having only raw water (no inhibitor or antifreeze). Raw water or poorly maintained coolant will allow the cast iron cylinder block to oxidize and deposit small iron particles into the coolant. These iron particles will get into the moving parts of the thermostats and cause them to stick open.

NOTE: The recommended interval for thermostats to be changed is every 6000 hours.

NOTE: If a application has lots of idle time and the ambient temperature is cold, even with properly operating thermostats, the engine jacket water temperature may drop below 79°C (175°F). Given enough time at cold enough temperatures, the jacket water temperature can drop to 59°C (130°F) or lower. In these cases, fan clutches should be used instead of the standard fan drive to help keep the jacket water temperature at or above 79°C (175°F).

Also, to minimize corrosive wear, the engine's oil must be changed so the oil's TBN does not drop below 50 percent of the new oil's TBN as per ASTM D2896. If the oil's TBN gets too low it will not neutralize the acids that form during combustion. If these acids are not neutralized, corrosive wear will occur.