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[snip] The F-1 after Apollo — There was an uprating redevelopment of the F-1 undertaken by Rocketdyne during the 1960s which resulted in a new engine specification known as the F-1A. While outwardly very similar to the F-1, the F-1A was actually lighter yet 33% more powerful (2 million lbf compared to F-1’s 1.5 million) and would have been used on future Saturn V vehicles in the post-Apollo era. However, the Saturn V production line was closed prior to the end of Project Apollo and no F-1A engine ever flew on a launch vehicle. [end]
http://en.wikipedia.org/wiki/F-1_(rocket_engine)#The_F-1_after_Apollo