Andy Samberg - Did the hammer stunt himself

"In Hot Rod, Andy Samberg plays Rod Kimble, an Evel Knievel-esque daredevil preparing for a major motorcycle stunt. "I can tell you, 100 percent, no one but me did the 'bang on the engine block' [stunt]," the funnyman boasts to PEOPLE. And not only did he come out unscathed, he showed his stunt prop who was boss. "Well, the hammer took a beating," Samberg jokes. "I mean, I was really, really putting my all into it.""

From:
http://www.people.com/people/gallery/0,,20048671,00.html