Transport officials have found no deficiency in the maintenance work on three double-decker buses that emitted smoke and burst into flames two weeks ago - a finding that a veteran engineer says he finds hard to believe. The Transport Department blamed a broken alternator bearing for the most severe blaze, which reduced a KMB vehicle to charred debris in Wan Chai on December 10. Failed mounting brackets and broken axle casings were to blame for the other two incidents, involving Citybus in Wan Chai and Mid-Levels, the officials reported to the Legislative Council yesterday on the department's preliminary investigation. Edmund Leung Kwong-ho, a former Institute of Engineers president who led an investigation into a Kowloon Canton Railway equipment blunder two years ago, said that if the drive belts that tied the alternator and the engine were wheeled too tightly, the friction would induce a high temperature that could cause a fire. 'I don't see how this is not a maintenance issue; maintenance workers should control the amount of force applied on the drive belts,' he said. 'They should cap the tension at a certain level.' Mr Leung said fans that prevented overheating might be switched off in cold weather, which might also raise the temperature inside the bus. KMB principal engineer Kane Shum Yuet-hung said the operator was studying whether it could renew the alternator bearings every two years instead of every 21/2 years. 'Most bearings have a lifespan of up to three years, but we are planning to replace the part more frequently anyway to enhance service reliability,' he said. KMB and Citybus have completed inspections on about one-third of the 748 buses that are of the same model. The Transport Department said no irregularities had been spotted so far. The inspections will be completed by the end of next month.