What criteria should we use to decide if redesigns are significant enough to not claim it is the same engine?

When Honda changes the letter on the end of the motor code, that’s a good indication: They started with K20A and now it’s K20C, there was a K20Z and so on.

It’s actually very common for engine series to span decades, even though this article is presenting it like it’s an unusual achievement. Chevy has the LS, Subaru with the EJ, Mitsubishi with the 4G.

Parts compatibility?

Engine of Theseus