We have been involved with the Mercury Mark 20H racing engine since 1955 when Alan purchased one brand new and we started racing.  This article shows and describes the exterior and the power head of an unmolested Mark 20H. Included are many photos with a running explanation as the cowlings were removed for the first time.  Part numbers, a full parts list, covers of the owners manual, owners envelope and a 1954 Price List are included.  A history of the 1959 conversion kits with a photo of the prototype exhaust, together with an explanation of the development by Charles Strang, an interview with Jack Leek and the patent issued are part of the article.  Additional information about the factory assemble of already converted engines has been provided.  In all there are 84 pages with over 80 color photos.