1987/11/01 Caterpillar

Reference: Truck Engine News, September 1986, Page 1, "Indications Of Engine Over-speeding When Only One Cam Lobe/Lifter Has Failed ".

As shown in the chart, 3406B Truck Engines have used and still use two different camshafts. Both camshafts provide improved performance, fuel economy, and emission control compared to 3406 Truck Engines, 92U1-Up. Each camshaft is easily identified by the part number stamped on the front of the camshaft. (This is the number listed in the chart.)

As shown in the chart, the 4W1765 Camshaft (4W2430 Camshaft Assembly) was used in all 3406B Truck Engines in 1984. It continues to be used in all JWAC 260/300 kW (350/400 hp) engines for 1985, 1986 and 1987 models. The user must give more attention to speed control of the engine with the 4W1765 Camshaft than with the 7W3796 Camshaft (7W3797 Camshaft Assembly) because the high lift and extended dwell (flatter top) of the exhaust lobes on the 4W1765 Camshaft produce rapid acceleration and deceleration of the cam lifters. See Illustration 1 for comparison of the two camshaft exhaust lobe profiles. The cam lobe and lifter stresses with the 4W1765 Camshaft are increased considerably with an increase in engine speed.

Illustration 1. Comparison of the exhaust lobes on the two camshafts.

Extended operation at "moderate" engine over-speed (above high idle speed) with the 4W1765 Camshaft can fatigue (spall) the roller surface in the lifters. See Illustration 2. Such damage is not a material defect of the lifter roller, but could likely be an indicator of high speed operation. Under normal operating conditions, spalled lifter rollers will not cause a cam lobe to fail. But, do not reuse lifters with spalled rollers. If operating conditions have been severe, a mid-life inspection of the rollers is recommended. Also, check rotocoil operation every 62 500 mi (100,000 km) as explained in the Operation and Maintenance Guide.

Illustration 2. Spalling on a lifter roller used with the 4W2430 Camshaft.

Extreme overspeeding of the engine will cause an instantaneous separation of the valve train and can result in severe engine damage, including single "unexplainable" cam lobe failures, loose roller pins, and/or broken rotocoils. (Valve-to-piston contact and bent push rods may or may NOT occur.) See reference article.

When valve train separation is suspected, do the following:

... Install new lifters for all valves that had contact with the pistons.

... Check all other lifters by hand for a loose pin or missing locating wire.

... Retighten the five retaining bolts that hold the idler gear stub shaft to the block. See Illustration 3. Use 9S3263 Thread Compound and tighten the bolts to 45 ±7 N·m (32 ±5 lb ft).

Illustration 3. Retighten these bolts if the engine has had separation of the valve train. See reference article.

The 7W3796 Camshaft (7W3797 Camshaft Assembly) is used in all 230 kW (310 hp) engines since 1985, and in all ATAAC engines for 1986 and 1987. This camshaft is more forgiving to high speed operation because of the improved exhaust lobe profile. But, the same service recommendations for the 4W1765 Camshaft apply to the 7W3796 Camshaft. Due to the significantly different positions of the cam lobes for valve timing, the two camshafts are NOT interchangeable.