Zac Efron spotted with butt double on 'Dirty Grandpa' set

TYBEE ISLAND, Ga., May 7 (UPI) -- Zac Efron and his butt double were spotted on the Dirty Grandpa set this week.

The 27-year-old actor was seen exchanging places with an equally buff stunt double while filming a motorcycle scene. The part calls for the character to strip down to a thong, and Efron is reportedly uncomfortable with baring his behind.

Viewers will see plenty of the star's body, however, as evidenced on set last week. Efron and co-star Robert De Niro were photographed as they flexed their muscles and appeared to engage in a shirtless competition for a spring break scene.

Zac Efron is riding a motorcycle half naked. Happy Thursday! http://t.co/gxKEBxUWaw pic.twitter.com/YyKBAa67HB

— Daily Mail Celebrity (@DailyMailCeleb) May 7, 2015

Dirty Grandpa stars Efron as an uptight groom-to-be whose perverted, former Army general grandfather (De Niro) tricks him into driving to Florida days before his wedding. The movie is directed by Da Ali G Show writer Dan Mazer, and opens in theaters August 12, 2016.