the engine's retirement in 2001, nearly 40 years after the basic block design debuted. Two additional displacements were produced during the engine's history. A model was offered beginning in 1969 and continuing until 1996. The 351W (so identified to distinguish from the 335-series Cleveland 351C) uses a taller block than the other engines in the series to avoid excessively short connecting rods. And for a brief time in the early 1980s a version with a smaller bore diameter that displaced was produced as Ford struggled with emissions and fuel economy. In response to the Chevrolet Camaro's success in the SCCA Trans-Am Series, Ford engineers developed a new racing engine from the small block. The first attempt mated a tunnel-port head to a 289 cu in block, but the displacement proved to be too small to deliver the desired power. The next iteration of the engine mated an improved head design to the 302 cu in block, producing the famous "Boss 302". The heads from the Boss 302 became the production heads on the 335-series Cleveland engines, which used the same bore spacing and head bolt configuration as the small block engines. As the 1980s drew to a close, Ford began the design of a new OHC V8 to replace the venerable small block design. The Modular 4.6 L OHC V8 debuted in the 1991 Lincoln Town Car, signaling the eventual demise of the OHV Ford Small Block. Through the rest of the decade, Ford gradually shifted V8 applications to the Modular engine, with the Mustang transitioning in 1996. Even as the small block neared the end of its life, development continued, with new cylinder heads introduced for the Ford Explorer in 1997. American sales in new vehicles ended with the 2001 Ford Explorer, but the engine continues to be offered for sale as a crate engine from Ford Racing and Performance Parts. Design changes All of the July 1961 through August 1964 221-260-289 engines used a five-bolt bell housing, with all 221s and 260s being of this configuration, but the 289 changed to the six-bolt arrangement at this time - the change was made due to transmission utilization issues i.e. the need for larger-diameter clutches, for example. The block mount pads and the cylinder wall contour of the 221 and 260 engines changed in January–February 1963 with the introduction of the 289 variant – all 221 and 260 engine blocks