A C172 CFI WITH A STUDENT EXPERIENCE AN ENG FAILURE RESULTING IN AN OFF ARPT LNDG ON A ROAD.

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THE STUDENT AND I DEPARTED MYF TO CONDUCT TOUCH AND GO OPS AT SDM. THE STUDENT PLT WAS PRE-SOLO. OUR PLAN WAS TO DO SEVERAL TOUCH AND GOES AT SDM; THEN PROCEED TO SEE FOR MORE TOUCH AND GO OPS; AND THEN RETURN TO MYF FOR SEVERAL MORE. WE ARRIVED AT SDM; DID 3 TOUCH AND GO OPS AND WERE DEPARTING TO THE NE ENRTE TO SEE. AT APPROX 2000 FT MSL AND APPROX 2.5 NM NE OF SDM; THE ENG QUIT WITHOUT ANY WARNING. IT WAS AS IF THE IGNITION HAD BEEN TURNED OFF. I QUICKLY WENT THROUGH COCKPIT CHKS AND DETERMINED NOTHING WAS OUT OF PLACE OR HAD BEEN TURNED OFF. I CALLED A MAYDAY TO SDM AND ADVISED THEM I WOULD BE MAKING AN OFF ARPT LNDG ON A DIRT ROAD ABOUT .5 NM N OF THE PGY VOR. I SUCCESSFULLY LANDED THE AIRPLANE ON THE DIRT ROAD. THE STUDENT AND I WERE UNHARMED AND THE ACFT SUSTAINED NO DAMAGE. UPON EXITING THE ACFT; WE OBSERVED SIGNIFICANT AMOUNTS OF OIL ON THE BELLY STRETCHING ALL THE WAY BACK TO THE TAIL AND OIL WAS ALSO NOW RUNNING DOWN ONTO THE NOSEWHEEL PANT. IN A SUBSEQUENT CONVERSATION WITH THE MECH WHO MAINTAINS THIS ACFT; HE SURMISED THAT A MAGNETO SEAL HAD FAILED; ALLOWING ENG OIL TO ENTER THE MAGNETOS WHICH SHORTED THEM OUT. APPARENTLY THIS CESSNA 172 HAS ONE OF THE LYCOMING ENGS THAT HAS 2 MAGNETOS THAT ARE DRIVEN OFF ONE DRIVE; UNLIKE A STANDARD ACFT ENG WHERE EACH MAGNETO HAS ITS OWN DRIVE. IN MY OPINION; THE LYCOMING SINGLE MAGNETO DRIVE ENG IS A TICKING TIME BOMB. IT SHOULD BE RECALLED. IT IS A MYSTERY ON HOW THIS ENG GOT CERTIFIED; THERE IS NO IGNITION REDUNDANCY.