Introduced in late 1949, the FROG 500 was one of Britain's most successful mid-sized general-purpose glow-plug motors of the classic era. Designed by 1936 Wakefield Trophy winner Bert Judge, this plain-bearing engine was loosely based upon the 1948 K&B Glo-Torp 29 from America. In May 1951 the FROG 500 appeared in a retrospective spark-ignition variant, thus reversing the usual trend. Soon thereafter, a stronger crankcase was introduced, which was followed in May 1955 by a final variant featuring a ball thrust race at the rear of the crankshaft. This latter modification was not a success, giving rise to reliability issues. 

The FROG 500 remained in production until 1958, although New-in-Box examples remained available from wholesale and retail sources for some years thereafter. The full story of the FROG 500 may be found elsewhere on this website.