FROG 249 BB Mk II modified - Produced by I.M.A. (1957)

The FROG 249 BB Mk II Modified Red Head engine displayed here was a factory semi-tuned iteration of the George Fletcher-designed FROG 249 BB of 1955. This modification was prompted by the necessity to cater to the American markets demand for increased power to allow the engine to compete with American glow-plug engines it was was introduced in July 1957.

The FROG 249 BB Mk II Modified boasted a 10% power boost over the standard 1955 FROG 249 BB Mk II primarily achieved through alterations to the cylinder porting alongside other minor internal adjustments. the FROG 249 BB Mk II Modified aircraft version can be easily recognized by its very distinctive red anodised cooling fins.

Notably the rubber front bearing seal was in fact an oildust seal sourced from a Morris Minor drum brake slave cylinder. Production of the engine was subsequently transferred to Davies-Charlton Ltd. on the Isle of Man in 1964 where production continued until 1972.

The needle valve can serve as a good indicator of the manufacturer and production date range engines manufactured by IMA between 1957 and 1962 had a brass thimble needle valve as depicted here those produced between 1964 and 1972 by Davies Charlton had a needle valve with an aluminium thimble.

Capacity 2.494 cc (0.152 cu in) - Bore 0.581 in (14.76 mm) - Stroke 0.574 in (18.8 mm) - Ignition Diesel (compression ignition) - Induction Front rotary valve - Power 0.285 BHP 14500 RPM (0.213 KW) - Weight 6.15 oz (175 g)