SAN FRANCISCO >> San Francisco’s iconic cable cars will stop running for 10 days starting Friday while they undergo the final repairs in a three-year restoration project of the gearboxes that propel the world-famous system up the city’s notoriously steep hills.

The San Francisco Municipal Transportation Agency said Wednesday that shuttle buses will run along the three cable car routes.

The agency says it needs to get the manually operated cable cars off the streets to rehabilitate the gearboxes that power the system that started in the 1890s.

The gearboxes spin the 30-foot tall wheels that pull the 12 miles of steel cables under San Francisco to lift the cable cars up steep city hills.

The work is part of an upgrade project that started in 2017 to repair the four heavy equipment gearboxes that have been in service since 1984. The fourth gearbox controls the Hyde Line, the master cable that controls all cable cars going in and out of the cable car barn. Without it operating, cable cars can’t enter or return from service.

The overall project is estimated to cost about $6 million.