From 1991-1998, Chevy/GM medium and heavy-duty trucks included the 366. This engine had a long run. It was manufactured without fail from 1968 until 2004, with only minor changes throughout its life.
The 366 is Chevy/GM’s tallest engine–one half of an inch taller than other Chevy big blocks.
The 1991-1998 366s eliminated the 2-piece main seal, making it easier to identify than both the 1968-1985 and 1985-1990 models.