In an attempt to improve reliability, 20 locomotives (D6100–03, D6106-D6108, D6112–D6114, D6116, D6119, D6121, D6123, D6124, D6129, D6130, D6133, D6134 and D6137) were re-engined between 1963 and 1967 with 1350 horsepower Paxman Ventura engines to form Class 29.