For the first time in its 160-year history, the entire mechanism was removed from its home in London, and taken to a workshop in the village of Dacre, near Penrith.
For three years, experts from the Cumbria Clock Company worked to clean, repair and restore what is arguably the most famous timepiece in the world.
They kept their task secret for security reasons, before their role was finally made public in November. Now, they are involved in reassembling and reinstalling more than 1,000 components at the Palace of Westminster.
Mark Crangle, - horologist for The Cumbria Clock Company, said: "To be honest it's been one of these projects you think you'd never be able to do, you'd never dream of working on it.
"I first visited the clock when I was 22, I've got my photograph taken in front of it and back then - did I ever think I'd be working on it? No, I wouldn't. But here I am and it's nice to think you've touched every part of the clock, you've had an involvement in it - and it will be sad to leave it but I'm just glad it will be in better condition when I left than it was when I arrived and that's all you can ask for."
Since 2017, a temporary electric device has helped to mark big occasions, like Brexit and the New Year.
The clock will be connected up to Big Ben (the famous name actually refers to the biggest of the bells in the Elizabeth Tower), so its bongs can be heard again permanently from sometime this spring.
The ticking mechanism which the clockmakers like to describe as the "heartbeat of the nation" is already back in place.
The team are currently testing the pendulum beat within the clock mechanism to within thousandths of a second."It's capable of keeping very, very accurate time, within a second a week, perhaps even better if you can really work on it - and that's where the little pennies come in, the little regulating weights, said Keith Scobie-Youngs, director of The Cumbria Clock Company.
"One thing people don't realise is that this was the smartphone of 1859, so if the satellites fall out of the sky and we don't have GPS, and you're in London, listen to the strike and you'd have accurate time as good as the GPS can give you."The team also repainted the clock hands and dials. They have been black in recent decades, but are now restored to their original blue.
Visitors will be allowed back inside Big Ben's tower by the end of this year.
It should be decades until similar restoration work is needed again - a lasting legacy for some high-level Cumbrian craftsmanship.