On initial completion, the first locomotives were found to weigh 3 long tons (3.05 t; 3.36 short tons) over the specification weight of 72 long tons (73.2 t; 80.6 short tons).
A programme of lightening was begun: some of this involved cutting circular lightening holes into the bogie frames, and replacing steel buffer beams or roof panels with aluminium.
Much of the over-weight was due to ancillary components, particularly the train-heating steam generators, being supplied over weight.
To avoid waste, these components were replaced by lighter versions from other makers, but the heavier originals were then used in the Class 20 and Class 40 locomotives.
The lightened locomotives eventually met British Rail's approval, but only after a painstaking weighing that involved specifying the amount of sand in the sandboxes and other precise details.

After acceptance trials at Doncaster, they entered service between April and June 1959.
They were based at Hornsey, although at weekends were usually located at Hitchin engine shed.
It had been British Rail's original intention to work the locos across London on the widened lines but the locomotives were found to be too heavy.
The first of the class was held at Vulcan Foundry whilst EE tried to reduce the weight but this could not be completed to a satisfactory standard.
On initial completion, the first locomotives were found to weigh 3 long tons (3.05 t; 3.36 short tons) over the specification weight of 72 long tons (73.2 t; 80.6 short tons). A programme of lightening was begun: some of this involved cutting circular lightening holes into the bogie frames, and replacing steel buffer beams or roof panels with aluminium. Much of the over-weight was due to ancillary components, particularly the train-heating steam generators, being supplied over weight. To avoid waste, these components were replaced by lighter versions from other makers, but the heavier originals were then used in the Class 20 and Class 40 locomotives. The lightened locomotives eventually met British Rail's approval, but only after a painstaking weighing that involved specifying the amount of sand in the sandboxes and other precise details.
After acceptance trials at Doncaster, they entered service between April and June 1959. They were based at Hornsey, although at weekends were usually located at Hitchin engine shed. It had been British Rail's original intention to work the locos across London on the widened lines but the locomotives were found to be too heavy. The first of the class was held at Vulcan Foundry whilst EE tried to reduce the weight but this could not be completed to a satisfactory standard.
British Rail Class 23