1986/09/01 Caterpillar

3406, 3406B, 3408, 3408B Truck Engines

Over the years there have been some "unexplained " failures of ONLY one engine cam lobe and/or lifter on the 3400 Family of engines. As a result of a thorough analysis of the failed part(s) and associated parts, Caterpillar now believes that many of these failures could have been caused by overspeeding the engine. This article lists the possible clues to engine overspeeding.

If you suspect an engine has overspeeded, inspect the RELATED engine parts carefully at all locations in the engine. The signs of overspeeding can be as follows:

1. The timing gear bolts have loosened.

2. Bent push rod(s).

3. The coil surfaces of the valve spring(s) are polished, indicating the spring had been operated in a solid condition.

4. Cracked or broken valve rotator(s). This can result from impact loads due to the instantaneous separation of the valve train. Contact between the valve rotator and valve guide can also occur during overspeed.

5. Contact between piston(s) and valve(s).

6. Loose lifter roller pin(s). See illustration. This will result in roller scuffing and/or seizure because the oil passage in the pin will not be in alignment to lubricate the roller. Such a condition will also break the clip-type spring at the top of the lifter. If the roller failure is gradual, you may notice an abnormal position of the valve adjusting nut when the valves are adjusted.

The roller-type lifter used in the 3400 Family of engines.

NOTE: The roller pin locating wire only locates the roller pin in the lifter body. The wire does NOT retain the pin in the body. A loose locating wire might indicate a loose pin, but will not cause a loose pin.

7. Damage to the descending slope of the cam lobe(s). This is caused by the downward impact of the lifter roller after the valve train and lifter had instantaneously separated from the lobe near the top of the lobe.

8. Damage to valve faces caused by the impact of very rapid closing.

9. Damage to the upper halves of the crankshaft main bearings. (Only the upper half of the bearing has the lubrication groove and this reduces the amounts of loads that the upper half can handle.) On the 3406 and 3408 Engines, check especially the front and rear mains. Check the front, rear, and center mains on the 3412 Engines.