The most useful manual is the operation and maintenance instructions(OMI). It has the instructions for adjustments to the clutch, interlock, and more.
I found this one on eBay cheap:
https://www.ebay.com/itm/405145501601
Craig
All D4Cs has D330 engine as far as I know
PDF DOWNLOAD FILES $20EA -- https://www.jensales.com/categories/caterpillar/d4c.html
How do I know what manual to get for the engine by itself? I had a 5u d2 and ended up getting the wrong manual for the motor so I'd like to be sure before I do that again. Is there an I'd tag that is different from the tractor serial number? I'd like to find all of the necessary books for it eventually.
https://www.jensales.com/content/ppp/caterpillar-crawler-service-manual-ct-s-d4-39a1.pdf