Rivets and Sprockets — Alexander Key

Sprockets and his brother race across space to solve a martian mystery

Life for Sprockets, the little robot with the giant brain, has been very exciting since he came off the assembly line. Backed up by Dr. Bailey and his son Jim, Sprockets has zipped across the stars, having fun all the way. Now it's time for his greatest adventure yet, and he has someone new to help him out: his little brother, Rivets, who's pretty handy in a pinch -- even if he does have one or two screws loose.
 
Dr. Bailey is working day and night to perfect an interplanetary radio, but Sprockets and Jim build one in a single night. When their new contraption picks up a signal coming from Mars, Dr. Bailey sends Sprockets and Rivets to investigate. Pursued by a mysterious gang of Mongolian researchers led by the fearsome Vladimir Katz, Rivets and Sprockets must escape the red planet with all their gears intact.

Rivets and Sprockets is the 2nd book in the Sprockets series, which also includes Sprockets and Bolts.