In a jiffy, lads as young as 14 had the new leads and spark plugs out, removing anything else they thought necessary as they toiled in the back streets of south London.
They did so with the full blessing of local magistrates, Lambeth social services and South Thames College as the venture aims to cut the appalling crime figures.
The Lambeth Auto Project encourages young men to develop their skills for more socially-acceptable careers. Launched two years ago, most trainees were sent by magistrates under supervision and probation orders.
The youths are set to emerge with City and Guilds certificates - the first step towards a level 2 national vocational qualificatio n and status as a trained mechanic.
Many had stolen cars, some had committed violent crimes or burglaries, some joined the project to avoid appearing in court after being cautioned by police.
In the first year of the project attendance was poor, but after changes to the timetable and teaching styles, attendance and results improved. Many become so interested in their studies they volunteered for extra lessons.
Last summer, eight out of 12 students gained a certificate and some have returned this year for further qualifications. Unlike most college-based courses - which mix theory and practical demonstratio ns - the project runs a working garage and carries out real "not-for-profit" repairs under expert supervision for Lambeth Council and staff.
Most jobs are fairly simple: servicing, checking faulty brakes and electrical problems. Students will take the initiative, repairing cars towed in after breaking down nearby roads. They are also building a Mini from scratch which will serve as an extra practice vehicle and be entered for grass-track racing.
"It's no good lecturing the lads for three or four hours because they would be bored after 10 minutes," said course leader Robert Stanger. "You have to work with them at street level and help them to find the problem."
The social worker's Metro was repaired in two hours after trainees diagnosed a cracked plug and discovered that the owner had put new leads on the wrong way around. They threw in a pre-MOT check for good measure.
Gavin, an 18-year-old who is in his third year at the project after being sent under a supervision order, said: "You don't get the same practical work in college. Here we find out what cars are all about."
Some of the law-breakers, who also have been excluded from school, have not had formal lessons for up to three years.
It is difficult to get them back into a classroom and classes are kept to the minimum.
Lou Haughton, project co-ordinator at South Thames, described it as a "masked" education. "By the time they find out they are back in college they have got a certificate in their hands," he said.
Manneh Elliott, project co-ordinator with Lambeth social services, said students had behaved better since they had the chance to gain qualifications and more chance of finding jobs. And most had turned their backs on crime. A social services survey recently showed that reoffending dropped 70 per cent among those on the course.
Steven, also 18 who originally attended under a community service order, said he had been offered a couple of jobs but wanted further training first. "I've been interested in cars since I was eight," he said. "I've learnt nearly everything about the inside and outside of a car."