How do you tell a 4 bolt main Chevy 350?

You can't definitively tell a 4-bolt main Chevy 350 just by looking at the outside of the engine. The 4-bolt main bearing caps are *internal* to the engine block. You need to look at the engine block itself, specifically the main bearing caps.

Here's how to identify a 4-bolt main 350:

* Remove the oil pan: This is the most reliable way. Once the pan is off, you'll see the main bearing caps. A 4-bolt main will have four bolts securing each of the main bearing caps (except sometimes the rear cap which might have only two). A 2-bolt main will have only two bolts per cap (except possibly the front).

* Check the casting numbers: While not foolproof, some casting numbers on the block itself might correlate with 4-bolt main blocks. You'll need a reliable source like a Chevy engine casting number database to interpret this. This is less reliable than inspecting the main caps directly.

* Look for markings on the block: Some (but not all) 4-bolt main blocks may have markings indicating their configuration, though this is uncommon and not a reliable method.

Important Note: Simply looking at the outside of the engine or at the valve covers will *not* tell you if it's a 4-bolt main or not. The distinction is entirely internal.