Rebuilding an E40D and I bought a thrust washer kit to replace all of them. The kit comes with plastic replacements for a couple of the brass thrust washers. Is there a difference as far as durability? Should I use old metal thrust washers? Or am I fine using the plastic?

 

Plastic is fine but as long as the washer is good it really doesnt matter.

 

The main reason for plastic is, less cost to manufacture. We all know how not so well plastic holds up from years of use and heat. I would always take the metal over plastic any day. Also most always for heavy duty and racing applications
all cheesy weak parts are replaced with stronger parts. I don't think there would be one professional racing transmission going down a drag strip with a plastic thrust washer in it, I could be wrong but don't think so.
Makes me think about the corrugated and welded tin cans in modern transmissions used for clutch drums, very cheap to make versus the good old parts from the old early 1950's transmissions, I don't think a big off road heavy machine transmission has that kind of junk in it, they sure didn't 30 years ago.

 

There are some places in the E4OD where there's an option of plastic or bronze but other areas like on the overdrive planetary and the back of the pump it only ever had plastic. Low reverse planetary only used bronze.