Looked at a D5 for sale and the serial number plate had a Z prefix(z98J997) and 1z311 stamped below that. Does any one know what the Z means?  Also have z98j893 1z296. My other D5’s do dot have the Z designation. Both of these tractors have D333C engines I believe and are turboed. 6 speed direct drive in all of them. Just mostly curious as I do not see any obvious differences. Thanks, Mike     

I cannot answer your question; I had inquired about some of my D-5s as well with the "Z" prefix as well; Bumping this up. Maybe we stumped the experts!
We had a fellow named Zoller around here that ran a lot of crawlers and made much of their own farm equipment. Perhaps something he touched?
Go ahead and throw some guesses at least. Hah! JM

Nothing in my stash of stuff has any info on a "Z" prefix. Perhaps a dealer marking????

???? - www.zieglercat.com/about/

Not likely Ziegler cat here in eastern Washington. I looked this afternoon and see that one of my other 98j tractors has a z prefix also. It is 1176 so a 3306 engine. Possibly to denote factory turbo? The z number is not noted on the original foil delivery sticker stuck on the fuel tank of the 893 tractor thinking it must be some internal difference as I see nothing obvious walking around the tractors. Would love to hear some more ideas, Mike

it may have been added later as its a different font and size the the other lettering.

Anybody know what year they started putting 3306s in 98Js

3306 came in  @ s/n 98J1134

1973 was the first year of the 3306 I think. I don’t really have any big loads to pull so I can’t really tell any difference between the late D333C tractors and the 3306 powered 98J’s. Was the 3306 considered a performance improvement back in ‘73 or a reliability improvement? Thanks, Mike

That's correct, D330C/D333C came in in 1968
The 3304/3306 came in in 1973
I'd say there were more reliability improvements than performance except they kept cranking up the rpm/s.