I am surprised. Usually by now you would have had 14 people jumping up and down going "NO! NO! NO!" When the 8630 and 40 were made they were state of the art. Fortunately the state of art has improved. The idiot who had the bright idea of sticking a man sized muffler in the midle of the hood for you to look around should have been beaten and thrown in the pond.
We had an 8640 we ran for several years with minimal trouble. But we baby'd it and didn't work it hard. Dad bought it when a cousin lost the ground he was renting and had to get out of farming. We were going to the sale and Dad said "That tractor is going to bring $12,000" It did ... I had to go after it. We pulled a 9 shank Soil Saver (as fast as we wanted to drive), a 28 ft disk, anhydrous toolbar, and a grain cart. It was a great grain cart tractor.
When we were talking about trading it for a newer tractor salesman would look at it and go "You've not had any trouble the engine?" "You've not done anything to the transmission?" "The final drives haven't been touched?" and the tone of their voice would get more incredulous as it went on. We ran it about 10 years and put a couple hoses on it, a hyd pump coupler (I found a spare in the parts drawer the other day), a u-joint, overhauled the A/C, replaced the muffler (it was cheaper from JD than aftermarket!) but basically did nothing major to it. I've been told not to buy a lottery ticket because I've used up all my luck.
It's a topic that has been discussed frequently on various forums. On here look at http://community.agriculture.com/t5/Machinery-Talk/JD-8630-vs-8640/m-p/2351 . There are several hits on AgTalk such as http://talk.newagtalk.com/forums/thread-view.asp?tid=194889 and http://talk.newagtalk.com/forums/thread-view.asp?tid=195755
I tried searching AgWeb and remembered why I quit going there.
You will find responses like "You could buy a more expensive to own old 4 wd, but, you'd have to work at it" "stay away from them ,Kinze made a lot of money on the way they were built." "Got two at the farm....I wouldn't take one as a gift." "Id stay away from either one and go to a 50 series" "Run away...far and fast. Deere did not make an articulated 4WD until the 60 series tractors." "Walk on by them Dan. A Panther III Steiger can be bought in the same ballpark and for drawbar use, there's no comparison."
The other issue with them is time required to work on them. I recall being told one time it took 12 hours to get to the cluctch to replace it. I have no doubt that is true after working on one. Our Case 9270 had a major engine problem (that's off topic so I won't go there right now) and took to a local heavy equipment shop. The guys who did the work kept going on and on about how much easier it was to work on than the 8640 they had just done the same thing to.
So, in answer to your question, look for one the engine has been replaced with a 50 series. It would be better if the transmission has been rebuilt recently. Check for loose pivot pins. Crank it hard side to side and moving with the PTO on and listen for noise in the drive line. Then go look for a good 9130 or 9230 Case
