Lost bottom pulley > again ?

2 different engines, 2 different balancers
pulley center cracks and tosses the pulley
all bolts are still tight
only running single belt > alt and water pump
Why ?

would going to an aluminum (thicker flange ) help
I'm running out of spare pulleys
only shift @ 5800

eng #1 pulley lasted 62 passes
eng #2 lasted 48 passes

both engines are strokers / balanced
other than tossing the pulley still run great