Because walk-behind concrete planers are expensive and built to take a beating, buying a used machine from a rental fleet or auction is a common practice. However, rental machines are subjected to horrifying abuse by inexperienced users. If you don't know exactly what to inspect, you will buy a machine that requires a complete, costly rebuild.
When I evaluate a used scarifier, I completely ignore the paint and look straight at the drivetrain and the drum assembly. First, I tip the machine back and manually rotate the drum. It should spin smoothly. If I hear a grinding noise or feel resistance, the massive main drum bearings are shot. Replacing these bearings requires a heavy hydraulic press and hours of labor.
Next, I pull the drum out and inspect the hex or round drive shaft. If the shaft is twisted or deeply grooved, it means the previous operators ran the machine without proper spacers, allowing the cutters to eat into the core steel. Then, I check the engine mount vibration isolators. These heavy rubber blocks absorb the shock between the chassis and the engine block. If they are torn or dry-rotted, the extreme vibration of milling will transfer directly into the carburetor and snap the throttle linkages. Finally, I do a compression test on the gasoline or diesel engine. If the machine was run dry without a HEPA vacuum, silica dust likely bypassed the air filter and scored the cylinder walls, leading to a loss of compression and an impending engine death. Buying used iron requires a cynical, forensic approach to mechanical wear.
