10-22-2025, 12:03 AM
A clunking sound during rotation on a 1989 CAT E120B excavator often points to wear in the swing bearing, misalignment in the turntable gear, or damage to the grease seal. While the issue may resolve temporarily, underlying mechanical fatigue should be addressed before committing to restoration work.
CAT E120B Excavator Overview
The Caterpillar E120B was introduced in the late 1980s as part of Caterpillar’s expansion into compact and mid-size excavators. Built primarily for export markets, many E120Bs are considered “grey market” machines—units originally sold outside North America and later imported. The E120B features a Mitsubishi diesel engine, hydraulic pilot controls, and a swing system mounted on a large turntable bearing. With thousands of units sold globally, the E120B remains a popular choice for land clearing, trenching, and farm work due to its mechanical simplicity and robust steel frame.
Terminology Note
The excavator exhibited a distinct clunk or knock when rotating the upper structure past a certain point. This sound was felt through the cab floor and occurred consistently at the same rotational position. Over time, the clunking ceased, but the grease seal was visibly damaged, allowing lubricant to escape and dirt to enter the bearing cavity.
The machine had been stored in the field and was scheduled for use in constructing a riding ring, stump removal, and topsoil loading. Before investing time and money into repairs, the operator sought to determine whether the swing system was structurally sound.
Possible Causes of Clunking
In Pennsylvania, a farm operator used an E120B to clear pine stumps and build a riding arena. Despite the initial clunking, the machine performed well after seal replacement and bolt inspection. The swing bearing showed no excessive play, and the gear teeth were intact. The clunk was likely caused by a temporary misalignment or hardened grease buildup.
In Australia, a contractor reported similar symptoms on a Komatsu PC120. The issue was traced to a cracked bearing race, requiring full disassembly and bearing replacement—a costly repair that justified early inspection.
Conclusion
A clunking sound in the turntable of a 1989 CAT E120B excavator may not indicate catastrophic failure, but it should be investigated thoroughly before restoration. Gear backlash, bearing wear, and seal damage are common culprits. With careful inspection and targeted repairs, the machine can be returned to reliable service. Given the grey market status of many E120Bs, sourcing parts may require creativity—but the mechanical design remains accessible to skilled hands.
CAT E120B Excavator Overview
The Caterpillar E120B was introduced in the late 1980s as part of Caterpillar’s expansion into compact and mid-size excavators. Built primarily for export markets, many E120Bs are considered “grey market” machines—units originally sold outside North America and later imported. The E120B features a Mitsubishi diesel engine, hydraulic pilot controls, and a swing system mounted on a large turntable bearing. With thousands of units sold globally, the E120B remains a popular choice for land clearing, trenching, and farm work due to its mechanical simplicity and robust steel frame.
Terminology Note
- Swing Bearing: A large ring bearing that supports the upper structure and allows 360° rotation.
- Turntable Gear: A ring gear mounted to the swing bearing, driven by the swing motor pinion.
- Grease Seal: A flexible seal that retains lubrication within the swing bearing and prevents contamination.
- Clunking Sound: A mechanical knock or thud felt during rotation, often caused by gear backlash or bearing play.
- Grey Market Machine: Equipment imported from non-domestic markets, often with different specifications and limited parts support.
The excavator exhibited a distinct clunk or knock when rotating the upper structure past a certain point. This sound was felt through the cab floor and occurred consistently at the same rotational position. Over time, the clunking ceased, but the grease seal was visibly damaged, allowing lubricant to escape and dirt to enter the bearing cavity.
The machine had been stored in the field and was scheduled for use in constructing a riding ring, stump removal, and topsoil loading. Before investing time and money into repairs, the operator sought to determine whether the swing system was structurally sound.
Possible Causes of Clunking
- Gear backlash between the swing motor pinion and the turntable gear. Excessive clearance can cause a knock when load shifts during rotation.
- Flat spot or wear zone on the swing bearing raceway. This can create uneven rotation and audible clunks.
- Loose or missing bolts on the bearing flange or gear ring. These can allow movement under load.
- Contaminated grease or lack of lubrication due to seal failure, leading to metal-on-metal contact.
- Debris intrusion into the bearing cavity, causing intermittent binding.
- Remove the circular inspection cap near the boom cylinder mount to access the swing gear and bearing interface.
- Check for metal shavings, water intrusion, or hardened grease—signs of bearing wear.
- Rotate the house slowly and observe gear engagement through the access port.
- Measure gear backlash with feeler gauges and compare to factory specs (typically under 0.5 mm).
- Replace the grease seal using aftermarket kits if available. Some seals can be installed without removing the upper structure.
- Torque all swing bearing bolts to spec and apply thread locker to prevent loosening.
In Pennsylvania, a farm operator used an E120B to clear pine stumps and build a riding arena. Despite the initial clunking, the machine performed well after seal replacement and bolt inspection. The swing bearing showed no excessive play, and the gear teeth were intact. The clunk was likely caused by a temporary misalignment or hardened grease buildup.
In Australia, a contractor reported similar symptoms on a Komatsu PC120. The issue was traced to a cracked bearing race, requiring full disassembly and bearing replacement—a costly repair that justified early inspection.
Conclusion
A clunking sound in the turntable of a 1989 CAT E120B excavator may not indicate catastrophic failure, but it should be investigated thoroughly before restoration. Gear backlash, bearing wear, and seal damage are common culprits. With careful inspection and targeted repairs, the machine can be returned to reliable service. Given the grey market status of many E120Bs, sourcing parts may require creativity—but the mechanical design remains accessible to skilled hands.
