The Mk. I version of the Mills 1.3 cc (0.079 cuin.) sideport diesel was manufactured by Mills Brothers (Model Engineers) Ltd. of London, England. The engine was designed by Arnold L. Hardinge, who had obtained his first patent with respect to model diesel engine design in May of 1946. There have been credible suggestions that the design of the Mills 1.3 was based in large part upon that of the very similar but earlier Allouchery 1.25 model from France. 

The Mk. I Mills 1.3 was distinguished by its machined plain aluminium crankcase. It was first advertised in July 1946, making it among the first British model diesel engines to be offered commercially. Thanks to its excellent running characteristics, easy starting and durability, it proved to be an overwhelmingly popular model right from the outset.

Based upon reported serial numbers, between 13,000 and 15,000 examples of this model appear to have been produced between mid 1946 and June 1948, when the engine was replaced by a comprehensively revised Mk. II version featuring a chemically-blackened crankcase cast in magnesium alloy. This version was to remain in production with minor variations all the way up to 1964, when all Mills production ceased. 

Further information on the different versions of the Mills 1.3 may be found in a separate article on this website. In addition, a full production history of the entire Mills model engine range may also be found here.