** 1/2 Various artists, “Loud Rocks,” Loud.

When rockers play hip-hop, they often can’t make their guitar-bass-drums power as colorful and varied as the best rap acts’ library of samples, and they can get tripped up trying to sound natural with rap jargon and cadence. And when rappers tackle rock, they frequently fail to make their delivery stand out against the sonic crunch.

It’s hit and miss on those counts in this project, which pairs rockers with Loud Records’ roster of hip-hop stars on some of the rappers’ hits.

The album opens on a high note, with System of a Down and the Wu-Tang Clan combining the best of both worlds on the furious “Shame,” produced by Rick Rubin--who pretty much invented the rap-rock hybrid. A teaming of Rage Against the Machine guitarist Tom Morello and Red Hot Chili Peppers drummer Chad Smith with the Wu-Tang isn’t as fruitful, despite Morello’s virtuosity. And while Sugar Ray’s collaboration with Tha Alkaholics sounds natural, the band Crazytown hammers the playfulness out of the latter’s “Only When I’m Drunk.”

Everlast with Mobb Deep and the surprisingly effective work of Ozzy Osbourne and Black Sabbath guitarist Tony Iommi on another Wu-Tang session affirm Loud founder Steve Rifkind’s observation that rock and rap are “the same street, just different blocks.” But some of the rockers here steamrollered the asphalt too flat.

*

Albums are rated on a scale of one star (poor), two stars (fair), three stars (good) and four stars (excellent). The albums are already released unless otherwise noted.