Although the FROG model engine range produced by International Model Aircraft (IMA) of Merton in Surrey, England is perhaps best remembered for its larger models such as the FROG 2.49 and 3.49 diesels as well as the FROG 500 glow-plug model and the earlier FROG "bicycle-spoke" designs, the company did manufacture a number of engines in the smaller-displacement category below 1 cc. This series was initiated by the delightful little FROG 50 diesel of 0.5 cc displacement, which appeared in February 1952.
After passing through several relatively minor design evolutions, the FROG 50 was replaced in January 1957 by the first variant of the FROG 80 diesel of 0.8 cc. This model too was to pass through a series of design evolutions, remaining in production right up to the end of IMA's model engine manufacturing program in early 1962. Even then it was not done, since its manufacture was continued for some years by Davies-Charlton Ltd., working under contract to the A. A. Hales organization which had taken over the FROG brand from IMA. A glow-plug version of this engine called the FROG .049 was also produced.
The full story of the FROG under-1 cc models may be found in my earlier article on Ron Chernich's "Model Engine News" (MEN) web-site.