I have a friend of mine that owns a Alis Chalmers HD4. He has gone through the tractor and all he has left to replace or repair is the Track Assembly on both side. After numerous trip to bone yards and endless searching on line we have come to realize that the D3 and the 931 Track Assembly will bolt on to the HD4 with only minor changes     The Idler Assembly and Track will bolt up with little or no modifications. The track center pins are a little bigger so the Sprocket on the HD4 has to be modified. We want to use 931 Segment Sprocket section, and machine the HD4 Sprocket down to except the 931 Sprocket Segments. The only problem we have is that we do not know the 25 bolt hole diameter for the 931 Sprocket Segments. Is there anyone out there that would know the,  “ 25 Bolt Hole Diameter for the 931 Sprocket Segments “.  Thank you and as always, Semper Fi. Hank

I can find lots of dimensions for the undercarriage but that bolt circle is not one of them. I'd say you need to get to a Cat Engineering drawing to find that info.

Where would I start to look to get a Caterpillar Engineering Drawing?

Try the parts dept at the Cat dealer.

Aye aye.

If it's a discontinued part, they usually can get a print.