Cat RD7 Worth?

Serviceman's reference book would be a good place to start reading, also a serviceman's reference book for the D-8800 engine used in other applications, like generator sets, marine etc., seems to have a little more information on the engine than the one for the crawler, Cat Legendary literature decent reprint is probably available, and ebay you may find an original, just check the serial number of your tractor, left side of engine as if you are sitting in the seat and left side just above the final drive on the back of the tractor. If you find a serviceman's reference book for the engine only, may want to make sure it's for an early D-8800, that engine did have upgrades throughout production.