Cracked crankshaft

Could be a number of things. From simply fatigued metal to possible misalignment of the main bearing journals, causing slight deflection of the crankshaft during each revolution.

Was this on a fresh rebuild or on a high hour engine?

 

Is the truck turned up.

 

not a cheap date being a Cat.

 

One reason could be is the mains/big ends haven't been replaced regularly as they should. ("bearing roll")
The 3406B you had to do them about every 300,000 miles but the 3406E and C15 they had a different design, but you've still got to do bearing rolls. Like NorthernMechanic said, the crank will misalign and flex if the bearings are worn.

 

Was the starter ever bad and truck roll started?

 

A bad harmonic balancer.