air brake speed regulator of the chiming mechanism broke from torsional fatigue after more than 100 years of use, causing the fully wound 4-ton weight to spin the winding drum out of the movement, causing much damage. The Great Clock was shut down for a total of 26 days over nine months – it was reactivated on 9 May 1977. This was the longest break in operation since its construction. During this time BBC Radio 4 broadcast the pips instead. Although there were minor stoppages from 1977 to 2002, when maintenance of the clock was carried out by the old firm of clockmakers Thwaites & Reed, these were often repaired within the permitted two-hour downtime and not recorded as stoppages. Before 1970, maintenance was carried out by the original firm of Dents; since 2002, by parliamentary staff. March 1986 and January 1987: The problem of the rubber bushings on the quarter bell chimes freezing recurred, muffling the chimes. 30 April 1997: The clock stopped 24 hours before the general election, and stopped again three weeks later. 21st century 27 May 2005: The clock stopped at 10:07 pm, possibly because of hot weather; temperatures in London had reached an unseasonable . It resumed, but stopped again at 10:20 pm, and remained still for about 90 minutes before resuming. 29 October 2005: The mechanism was stopped for about 33 hours to allow maintenance work on the clock and its chimes. It was the lengthiest maintenance shutdown in 22 years. 7:00 a.m on 5 June 2006: The clock tower's "Quarter Bells" were taken out of commission for four weeks as a bearing holding one of the quarter bells was worn and needed to be removed for repairs. During this period, BBC Radio 4 broadcast recordings of British bird song followed by the pips in place of the usual chimes. 11 August 2007: Start of six-week stoppage for maintenance. Bearings in the clock's chime train and the "great bell" striker were replaced, for the first time since installation. During the maintenance the clock was driven by an electric motor. Once again, BBC Radio 4 broadcast the pips during this time. The intention was that the clock should run accurately for a further 200 years before major maintenance is again required; in fact the repairs sufficed for ten years. 17 April 2013: The bells were silenced as a mark of "profound dignity and deep respect"