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 in my earlier article on the late Ron Chernich's "Model Engine News" (MEN) web-site.