Because gasoline and diesel plate compactors are expensive, many contractors look to the secondary market, buying used machines from rental yard auctions. I do this frequently, but I approach every used machine with extreme cynicism. Rental equipment is tortured by inexperienced users who run it off the back of trucks, run it without oil, and smash it into concrete curbs.
When I walk up to a used plate compactor, I completely ignore the engine first. Commercial engines are cheap and relatively easy to swap. I look directly at the base plate. A steel base plate should be perfectly flat. I take a straightedge across the bottom. If the plate is "cupped" (bowed inward in the center) or ground paper-thin at the rear edge, the machine is junk. A cupped plate will not transfer energy evenly into the soil and will leave permanent ruts in the grade.
Next, I check the engine mounting plate and the shock isolators. If the rubber mounts are torn, the machine has likely been dropped. Finally, I run the machine. I listen closely to the exciter housing as it spins up. A healthy eccentric bearing makes a smooth, high-pitched hum. If I hear a grinding, rattling, or metallic knocking sound over the roar of the engine, the main bearings are in the final stages of catastrophic failure. Rebuilding a seized exciter housing often costs more than the machine is worth. Buying used iron requires you to look past the fresh coat of paint and listen to what the mechanical components are telling you.
