</font><font color="blue" class="small">( and is it done for engineering considerations or cost cutting? )</font>
Both. John Deere is forward thinking. You have to be to remain at the top of the heap for 168 years. John Deere isn't content to play "follow the leader" or continue using the same processes and materials because it's the cheap or easy way out. In order to be a leader it's necessary to innovate. an example is Boeing's new 787. It is primarily made of carbon fiber reinforced plastics or "composites". Think about it; the fuselage and the wings are made of plastic. All large planes up to this point have been aluminum, but now there's a better way. An recent except from the magazine "BusinessWeek" about the 787:
One big plus: Jets made of composites require far fewer parts, so there's less to bolt together. And since these plastics weigh less than aluminum, the planes should burn less fuel. Boeing says the Dreamliner will also improve passenger comfort. Why? The superior strength of the composite fuselage will allow the passenger cabin to withstand higher pressurization -- equal to the air pressure at an altitude of 6,000 feet instead of the usual 8,000 feet. So it's easier to control cabin temperature, humidity, and ventilation.
In particular, pay attention to the first sentence, "far fewer parts". By using plastics Deere's tractors have less parts than the models they replaced resulting in less up front tooling costs, less labor in assembly, and less warehouse space to maintain (for fewer spares). That adds up to a cost savings for Deere (or cost cutting as its called in the initial quote). That cost cutting frees up capital for future research and product improvements while not jeopardizing the integrity of the current product.
As far as plastic engine parts, automobiles are running plastic intakes, throttle bodies, water pumps, oil pans, valve covers, and the tank portion of the radiators (and other stuff you can't see) and few people are even aware of their non-metallic engine parts. Performance is as good or better and maintainance is reduced. As the EU phases in mandatory recycling for automobile manufacturers look for more and more plastics with no degradation of performance or longevity.
