The Palace of Westminster is seen in London October 12, 2009. REUTERS/Kieran Doherty

LONDON (Reuters Life!) - A pile of old coins which have helped keep the clock mechanism of London’s Big Ben accurate for the last 150-years were replaced on Thursday by a new five pound ($8.29) coin to mark the 2012 Olympics.

The pre-decimal pennies are stacked on the pendulum of the clock and have acted as weights to help regulate it since 1859 when the clock tower was completed and the first strikes of its 13.7 ton bell, nicknamed “Big Ben,” were heard.

Adding or taking away coins effects the pendulum’s center of mass and the rate at which it swings, said Mike McCann, the clock’s keeper. Adding one penny causes the clock to gain two-fifths of a second in 24 hours.

“There is a long and historic relationship between Big Ben and the UK’s coins,” he said. “Few people realize the technical role the old pennies have played inside the clock.”

Three of the 10 coins have now been replaced with a 5 pound crown -- produced as part of the Royal Mint’s collection to celebrate the London 2012 Olympics -- which features Big Ben’s clock face.

But McCann will keep hold of the old pennies in case they need to be returned to the pendulum at a future date.

The changeover is part of Big Ben’s 150th anniversary celebrations. The famously accurate clock sits at one end of the Palace of Westminster -- the seat of the houses of Parliament.