D2 5U sprocket nut loose

Hi, btsclan.
It's not quite as simple as just pressing the sprocket back on. If it has been running like that for a while, there may be too much wear in the splines for that to work. The nut was only ever designed to stop the sprocket falling right off on the ground. It is the press fit that is designed to hold it on. Excessive wear in the splines will make that a non-event so you need to examine the splines closely before pressing the sprocket back on - which may well mean breaking the track. On the (slightly) plus side, at least you don't need to remove the track frames to get the sprocket off to examine those splines.

OR, you could just try pressing the sprocket back on and hope for the best. Depends on whether you want a working tractor or 'toy'.

Just from your photos - and with my limited mechanical knowledge and judgement - I would say that the nut and the axle thread will be O.K.. It might be a good idea to 'acquire' a new lock washer.

Just my 0.02.

depending how bad it is worn you may be able to repair the splines use loctite quick metal or devcon metal epoxy

I thought you checked this back when you were asking why there's an oil leak. As you see, here is where the oil is coming from.
Hopefully the tractor hasn't too many hours running and the tapered splines are not trashed because the sprocket wasn't pressed on correctly. You have said that the previous owner put new steering clutches in not long ago and it appears it wasn't put back together properly.
I would get someone with the proper hydraulic press and try pushing it to the recommended 15-17ton, clean off as much oil you can with some Break Kleen put some sealer under the washer and nut. Tighten it up and give it a try. If it gets loose again, splines may be junk. Remove the track and remove the sprocket. If bad, you may get away with what Trainkid said.

Thanks, @side-seat, I have to admit I was planning on sorting out the axle nut leak but ended up slipping my mind to check 😞

Looking at the nut and wear on the sprocket Im guessing it had been loose for some time. Im thinking that the previous owner didnt install the sprocket correctly when they rebuild the steering clutches. It also lived most of its life on a flat farm discing between orchard trees. My property has some steep hills so I was probably working the tractor harder than it has worked in a long while which could have caused the sprocket to slide out. 

I want to do it right and keep this tractor running for a long time. How much of a PITA is it to remove the track? My biggest reservation on tearing into it is finding the right tools and presses to get the job done. Where would one find a press and tools to remove the track and pull/push the sprocket back on?

Interesting, thats good to know you can remove the sprocket without breaking the track apart. Im going to try and find a hydraulic ram and press the sprocket back on and see how it holds, if it comes loose again then I'll pull the sprocket and hope the splines on the shaft arent damaged.

That said, if you have the plug-type master pins, then splitting the track is relatively easy and only requires a 10# hammer. I had the same problem on my 5U - the sprocket splines were worn to the point that the sprocket pushed further on the shaft than it should, over-compressing the seal but also meaning that the nut wasn't able to bear on the sprocket face. My solution was to just pick up a couple of sprockets with worn teeth so that I can transfer my teeth over to the worn sprocket (but with good splines). I have a 20t hollow ram from ebay and the adaptors already made up from when I did the clutches so it's a pretty easy job to swap the sprockets out. After that, all that needs to be done is to adjust the track frames to suit the new sprocket alignment

There is a specification for how far the sprocket should be pressed on . Squatch's video should have it. If the shaft is worn you will be in for a more complicated repair. Parts tractor time.