New Engine - 725EX B&S, replaced 3.5hp

I'm a new poster, just changed the engine on my Sensation after more than 40 years with the original 3.5hp.
It runs fine with the new engine. The mower shop where I bought it rigged up a kill switch.
The problem is the blade adapter fails when the engine is shutting down. Lost 2 so far.
I'm using the original blade, no disk.

Sensation Model 11G3.

Any ideas on how to fix the problem?

Use a steel hub, no nubs. Thats what i use with my souped up engines. The cast junk just shatters usually.

By Rocco