International Model Aircraft (IMA) of Merton in Surrey, England were the manufacturers of the FROG (Flies Right Off Ground) model engine range between 1946 and 1962. They manufactured both diesel and glow-plug engines in a wide range of displacements from 0.5 cc up to 5 cc. An article descrbing their early years may be found elsewhere on this website.

During the 1950's, IMA produced a short series of 2.5 cc diesel engines. They began in August 1850 with the introduction of the FROG 250. This was a very useable engine, but as time went on it fell increasingly behind developing designs from competing manufacturers. In 1955, the company finally decided to address this issue, resulting in the December 1955 release of the FROG 249 BB to replace the 250. This was a far more sophisticated design built along generally similar lines to the all-conquering Oliver Tiger Mk. III.

IMA's American distributor, World Engines of Cincinnati, Ohio, asked IMA to develop an improved version of the engine to make it more competitive in he US market.  By mid 1956 IMA had developed the FROG 249 BB "Modified" version of the engine. Initially this was marketed only in the USA, but in mid 1957 it was made available on the home and world markets. 

Although it did perform at a higher level, the 249 BB "Modified" did so by adopting the earlier "sawn slot" form of transfer porting, abandoning the more advanced Oliver concept. This rendered it far less amenable to further development. As things turned out, it was the final FROG 2.5 cc model.Tests have shown that the earlier Oliver-ported version had far greater potential as a contest engine.

A detailed article on the FROG 2.5 cc diesel series appears elsewhere on this website.