Not all. I think it changed to steel around 1998. Failure of the plastic one largely depends on how well the tractor is cared for. I've read they can fail at about 700 hours or so. Mine failed 997 hours. But, I did take care to change the oil every 25 hours, always let it warm up and cool down as one is supposed to, and kept the valves adjusted.
Not too bad to change one out. Some have done it with the motor in the tractor. I removed the motor, and replace water pump, oil pump, governor gear, push rods, seals and gaskets. Removing the motor was the hardest part - alot of parts to get out of the way, and I had a stuck motor mount bolt.
