What year did Chevy start making a 350 with 4 bolt main?

Chevrolet's 350 small-block engine with a four-bolt main bearing cap setup wasn't introduced as a standard across the board for all 350s in a single year. The four-bolt main was used in higher-performance applications *before* it became more common. The transition was gradual, depending on the specific model and year.

You'll find that some high-performance 350s (like those in certain Corvettes or other performance models) had four-bolt mains earlier than others. But to say definitively when *all* 350s were four-bolt is inaccurate, as it wasn't the case. Many 350s continued to use a two-bolt main setup even later into the production years of the small block.