Macks usually have Mack engines. Not sure how old a truck you are interested in, but... The B model came with a Thermodyne gas or diesel. These are tough old engines and some seem to run forever. Parts can be hard to find, but it depends what you need and where you look. Later R models use the Maxidyne diesel. These are still fairly easy to get parts for. The 237 engine is close to bullet proof, and can be retrofitted into a B model easily. Later 6 cylinder diesels like the 285 or 300 plus engines take up more space, and are not usually put into the B model, but they are very good engines and if you find one in an R model, I would recommend it.