Is that a 3406 ? 

That is what I would like to know. Is it a Caterpillar 3406 engine? Anyone know for sure? 

Depending on the year of the truck. Caterpillar only put Cat engines in commercial trucks until 2010 then it was either Detroit Series 60, Cummins.

If you are wondering about the 3406 Cat engine it could be the 3406B or 3406E inline 6 cylinder.

It is a 3406 Cat. Definitely not a 3406E. It can’t be determined otherwise because the timing cover nor pump is visible. However, if this engine is original it is a 3406 or 3406A.