"In the past, at least four to five men were needed to rewind the clock springs and the process could take several hours," said Wei, pointing to the clock's three springs, of which the heaviest weighs more than 1 ton.

But thanks to the installation of a motor, "it now only takes 15 minutes for the motor to get it done", Wei continued.

But Wei still has to operate the motor to make sure nothing goes wrong.

White spiral staircases soar straight up the clock tower leading to Wei's exclusive work space occupying the top three floors of the Customs House.

"There are 69 stairs on the way to the machine room, where there are hundreds of big and small gears that I take care of," he said.

"Another 48 twisting steps will guide you to the bronze bells on top of the clock tower."

It used to play the chimes of Westminster Quarters every quarter of an hour - like every clock made in UK - until 1966. At that time it began to play The East is Red, a famous Chinese revolutionary song created during Chairman Mao's era.

In 1986, when Queen Elizabeth II visited Shanghai, the clock resumed the mechanical chimes of Westminster Quarters at the request of the queen.

"But after some time it turned out that Chinese still prefer Chinese songs," Wei said, adding that the clock has again been playing The East is Red since July 1, 1997, when Hong Kong returned to the motherland.

Wei's only trip longer than three days during the past 21 years was to Xi'an, Shaanxi province.

"It was a reward from my company," he said.

"My teacher Zhang Hejian, who was also the tower's third clock keeper, volunteered to cover my shifts in those five days, so I could make the trip," he said.

Zhang took care of the clock from 1977 to 1992.

"I'm living in the same neighborhood as Wei so we have a chance to talk about the clock all the time - things like the updated acoustics - even after I left," said Zhang.

"The biggest requirement for this job is a sense of responsibility - you've got to know nothing should go wrong with Shanghai's landmark."

Wei said he owes much to his wife, Bao Yunju, but promises that he will take her with him to visit the clocks in London and Moscow after he retires.

"For that, we'll have to wait for another eight years. But who knows, probably longer," Bao said. "He has been talking about trying to find an apprentice as conscientious and responsible as possible."