PILOT SCHEME DIESEL locomotive designs didn’t always go to plan and in some cases production orders were placed before type testing could be completed. The North British built Class 21 Bo-Bo diesel electrics were one such class.
The first 10 pilot scheme engines were built in 1958 and while they were under construction BR placed an order for a further 48. What they couldn’t have expected was the power unit problems with the British built MAN L12V18/21 engines which prevailed and led to the class being rebuilt with new Paxman Ventura engines from 1963. This saw the ‘21s’ reclassified Class 29 which also had four-character headcode boxes in place of the original disc headcodes.
