When launched at the 1955 Earls Court show, the BSA Dandy was advertised as using a chrome plated aluminium cylinder. Aluminium was needed to better dissipate the heat from the rear facing cylinder since cast iron has a much lower thermal conductivity. However, by the time it went into production in 1957, the accepted view was that the aluminium cylinder had been replaced by a cast iron one since BSA had failed to fully develop the aluminium chrome plating process by that time. Unfortunately, this change then led to cooling problems with the engine.
Intriguingly, I have Dandy engine DSE 365 which is fitted with a chrome plated aluminium cylinder. I'm not sure where the engine originally came from but it's engine number relates to a BSA Dandy produced in 1957 which seems to be at variance with the normally understood development history of the bike. It seems to suggest that the Dandy originally went into production with the chrome plated aluminium cylinder but was quickly changed to a cast iron one, presumably, due to premature wear of the chrome.
It would be good to hear from anyone with an early Dandy who might be able to shed further light on this issue or who has an engine with an aluminium cylinder?