Some owners of the early 4.5 litre cars wanted the increased power and performance that was available from the larger 4.9 litre engine. Therefore, when the new engine was introduced an engine exchange programme was established to replace the earlier 35/8-bore engines without excessive delay. While most exchanges took place shortly after the introduction of the larger engine, the program continued for over two decades with some cars receiving exchange engines well into the 1970s.
The first exchange was made on the 13th of December 1954. BC-24-A received a new 3¾-bore engine from stock and its original engine was converted to the new production specification and placed in the exchange program. Most but not all conversions were done on an exchange basis. Several cars did have their original engines converted.
Most engine exchanges were done at the Hythe Road or at Crewe. Records were kept of the exchanges and replacement engines were meant to be re-numbered to correspond with the original engine number of the chassis.
However, because some of the exchanges were done at regional service depots, the original engine number was not always removed and the new number was not always stamped. Therefore some exchange engines still have their original number, or possibly both engine numbers. There is no evidence of any exchange engines having no numbers. Several new engines used in the exchange program, such as BCD 75 and BCD 77, had original D series engine numbers that are higher than those of any engine fitted to a D series chassis. It is believed that this was the result of overproduction and the surplus engines were placed in company stock.
A total of 37 4.5 litre cars with 35/8-bore engines were refitted with 3 ¾-bore exchanges engines. Some cars have had more than one engine replacement to date. These were spread amongst the cars as follows, 9 in the A series, 6 in the B series and 22 in the C series. In the D series, 4 cars received exchange 3 ¾-bore engines as a service replacement for their original 3 ¾-bore engines.