the british rail class 29 consisted of 20 diesel locomotives which were rebuilt from the north british class 21. the original licence-built man engines of the class 21s had proved unreliable, so in 1963 locomotive d6123 was sent to paxman's colchester works to receive a new paxman ventura v12 engine. a further 19 were re-engined in 1964–1965 at polmadie works, with other changes made at the same time. this included the fitting of four-character headcode displays in the nose ends. after rebuilding, they returned to service from eastfield depot in glasgow.