I recently read that the 300 I6 was in production longer than the Ford flathead v8. I suspect that the 300 I6 is the longest production run of all Ford engines (1965 through 1996, inclusive, or 32 years!) For that matter, has any engine had a longer production run? Possibly the VW?
The small-block V-8, both Ford's ('62), and the "other", 1955? (Are either one still in production for 2000?
A case could be made for the flathead Mopar sixes, 1933-'71 (odr thereabouts).
I guess I should add that in my original post above, I only counted the years the 300 I6 was offered in Ford vehicles. I believe the engine is still made as Pitt Electric offers it on their web site as a power source for industrial and other commercial applications.
Chevy small block, 1955-present. Chevy big block, 1965-present. Buick V6, 1962-present. (Jeep owned-1968-1974). Chrysler LA small block, 1964-present. Ford Winsor small blocks, 1962-present. VW flat four is still in production in Mexico and Brazil. Some of the longest running same displacement engines are the 350 Chevy, 1967-present. 454 Chevy-1970-2000. Chrysler 318 (modern LA engines) 1967-1999. Ford 302, 1968-present. 460 Ford, 1970-1997. The 300 six, 1965-1997. Buick 3.8 (231) 1975-present.