There should also be a plate behind the magneto on the pony motor on the block. Minus the rust looks pretty good undercarriage looks nice overall. Wonder how tippy it is without anything on the back!

the serial number tag is also on the left side of the engine block behind the pony motor levers

The old U series D6's where liked by many for balance and the ability to get around on slopes. But no idea of what a front cable unit and the tree pusher too will do to that.

Nice looking under carriage, but pictures are not able to tell the whole story.

Exactly where you said it would be Kracked1 - thank you! - 8U 1040 as best I can make out.

Does anyone know if there is there a document or register somewhere that I can look up this serial number to work out what year it was manufactured?

I couldn't find a plate behind the magneto, but thankfully the number was stamped on the back, so all good.

Yes the rust seems to be mostly superficial (apart from the cab roof, so that will require a bit of attention in due course). Yes I was quite surprised by how good the undercarriage looked. I was fully prepared for it to be flogged out. I think it must have had work done to it before the current owner bought it as the top rollers on the right side clearly weren't replaced and they are pretty rough.

Yes I may have to put a block of concrete on the back!

Yes photos can only get you so far, but it's definitely better than I expected it to be and possibly good enough to do the work that I want to do with it.

That's interesting to hear about balance. I'm sure you are right that the tree pusher will make a difference. I will probably remove it in due course.  I may have some clearing to do, but most of the work will be pushing in erosion gullies on open ground - unless I get some more jobs for it.

Update: so the current owner tells me that the problem they had last time they tried to start it was that the pony motor - while it ran - "didn't have enough power to start the diesel".

So I had another look over it this morning and notice that when I was fiddling around the spark plug lead pulled straight out of the end cap on the right side of the pony motor! So it's possible that the last time they tried to start it that there was no spark, or a weak spark on the right hand cylinder of the pony motor, which would explain the low power issue. So a new set of spark plug leads is on the list.

I found a company that does heavy diesel maintenance close (ish) to where the dozer is currently sitting, so I'm going to give them a call tomorrow. See if I can get one of their mechanics and his service truck up there in the new year some time to go over the machine and do all the basics and see if we can get it started. I think it will be worth the expense as we can probably do the job in one or two days where as it might take me three or four as I work out what needs doing (and I'm a four hour drive away from it!).

The only thing about it that is definitely badly worn are the ends of the pusher arms (see photos) and the blade angling arms. They are pretty knackered.

But I don't think that's too big a problem. At worst they can be cut off and replaced, so I think that it's worth attempting to recover this old girl.

8U1040 would be a 1948 built machine

Thank you for that. Wow, 1948. Only three years younger than my *father*...