The BMC B engine was built for nearly 30 years, but its little brother remained in front-line service for close to 50 years. That's up there with the legendary Toyota R!
Not only was the A- which was such a frugal engine that it needed but three main bearings instead of the five that squander-prone fools prefer for their four-bangers- built from 1954 through 2000 and installed in millions of Minis, Midgets, Minors and the like, it served as the basis for many of Nissan's engines of the 1950s and 1960s. Available in displacements from 803cc all the way up to 1,275cc, the A put postwar Britain- which was still under food rationing well into the 1950s- on wheels and kept it there into the new millenium, winning plenty of racing glory along the way.
[Wikipedia]