The Kubota ZD21 has been, overall, a highly reliable and high performing zero turn mower for small acreage. The 18HP engine has not needed repair or tune up.
The claimed 21 horsepower is the advertising invention of the company. The real power is stated in KW on the engine data plate. The 18 HP figure I use is the correct conversion of the KW rating.
The hydrostatic transmission did suffer a failed steel hydraulic line early in its life. This was a design or material error, and Kubota improved the material. We, however, paid for the repair part and labor I recently changed the mower deck drive belt, but it could have gone much longer. Belt changing is difficult relative to many more conventional machines but the double-v belt is uncommonly durable.