caterpillar diesel 40 buy it or stay away?

buy it and appreciate it...make it a runner and enjoy..

The 40 Became the RD6. From what I have heard from the old timers years ago is that the 40's would drop rod bearings. The later RD6 engine was better with its upgrades. Without it running you have no idea what a can of worms it is.

How is the parts availability?

Dandy Dave, just curious whether the Diesel Thirty Five suffered from the same issue. I believe the Thirty Five and the Forty have the same bore and stroke. My Thirty Five runs and my Forty doesn't (but that's because it has a frozen cylinder, not a thrown rod bearing... at least I hope it doesn't have a thrown rod bearing.)

I would think parts are not that easy to come by. I do know that a head gasket was still available a while back for an RD-6 as it was the same as an RD8. The RD8 used the same one. It just added 3 more cylinders. You are buying a can of worms. No telling how good or bad the engine is without taking it apart. If the engine is seized it is only worth the price of scrap in my opinion. And yes, The Diesel 35 was before the Diesel 40.

What were the key differences between a Diesel Thirty Five and a Forty, other than the name? Was it just a matter of "opening up the pump" to make more power or was a materially different design?

What were the key differences between a Diesel Thirty Five and a Forty, other than the name? Was it just a matter of "opening up the pump" to make more power or was a materially different design?

I can not tell you exactly. Cat always had up grades on each new model. Also many changes in the parts book for many models during production. 

The key difference between the Thirty Five and the Forty is the undercarriage. The Thirty Five used the same chains as the S/PS Thirties and L/PL Twenties. Same chain different number of links. This change also changed the sprocket, track frames, rollers, idler, and a couple of different parts on the carrier roller. As someone said much earlier, most of the Thirty Five and Forty blocks have been rebored at this point to take the 5 3/4 bore sleeves and pistons. If I had my parts books here, I would look and see what differences there were in the crankshafts between the D40 and the RD6. Any of these old machines can have problems, and you will never know about them until you open them up and see what is inside. If it looks good, buy it and slowly start gathering parts for it. I have a couple of projects I have been gathering parts for for twenty years. I have the remains of a Diesel Thirty Five that will be going to scrap as soon as I pull a couple of more things off of it. You can have it if you get it out of my yard. I also have a Diesel Forty engine, but I have already given it away.