I have worked on Caterpillar diesels for many years, and I can definitely tell you that is not a Cat diesel, not even an old one. The part numbers make it very obvious. CAT has always been very particular about the way that they number their parts, and this def doesnt fit the bill. Other than that, I have no idea what it might be. Cool truck for a grand though. The axles are worth more than that by themselves. Portals are killer offroad, Hummers use a similar design to achieve the ground clearance that they have as well.