I've seem enough Pontiac balancers with broken key ways to know a press fit works better than a loose fit. They use a 5/8"-18 bolt torqued to 180 ft lbs plus a very snug fit, not a beat in on hammer fit, more of a use the bolt to pull it on fit. Since the inline is pretty well balanced by design anyway, a bit looser fit may work as long as it stays in a reasonable rpm range. Green retainer Loc-tite on the shaft, and red on the bolt would be a good practice.
